Biology

Enzymes

⚙️ Chapter 6 · MDCAT Biology
📅 Updated Jan 2026 ⏱️ 25 min read 🎯 High-yield

Enzymes — Characteristics, Mechanism & Inhibitors

Complete MDCAT Biology Chapter 6 on enzymes: their characteristics, mechanism of action (lock-and-key vs induced fit), factors affecting enzyme activity (temperature, pH, concentration), and enzyme inhibitors (competitive vs non-competitive). Includes 155 solved MCQs with explanations and year-wise past papers.

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⚡Quick Answers to Common Searches

Direct, snippet-optimized answers to the top questions students search about enzymes.

What are enzymes?

Enzymes are biological catalysts — mostly globular proteins (except ribozymes) — that speed up reactions by lowering activation energy without being consumed.

What is an active site?

The active site is a 3D cleft on the enzyme where the substrate binds. It has a binding site (holds substrate) and a catalytic site (performs reaction).

What is apoenzyme vs holoenzyme?

Apoenzyme = inactive protein portion. Holoenzyme = active enzyme (apoenzyme + cofactor).

What is lock and key model?

Proposed by Emil Fischer (1894). The active site is rigid and pre-shaped like a lock fitting a specific key (substrate).

What is induced fit model?

Proposed by Koshland (1959). The active site is flexible and changes shape upon substrate binding for optimal fit.

What is Km?

Km is the substrate concentration at half Vmax. Lower Km = higher affinity between enzyme and substrate.

What is optimum temperature?

For human enzymes, ~37°C. Above optimum, enzymes denature (lose shape). Below optimum, they become inactive but not denatured.

What is optimum pH for pepsin?

Pepsin (stomach) = pH ~2. Trypsin (small intestine) = pH ~8.

Competitive vs non-competitive?

Competitive: binds active site, Vmax unchanged, Km increases. Non-competitive: binds allosteric site, Vmax decreases, Km unchanged.

What is feedback inhibition?

End product of a pathway binds to an allosteric site on an earlier enzyme, inhibiting the pathway. A key regulatory mechanism.

What are zymogens?

Zymogens (proenzymes) are inactive enzyme precursors activated by proteolytic cleavage. Example: pepsinogen → pepsin.

What are ribozymes?

Ribozymes are RNA molecules with catalytic activity — the exception to "all enzymes are proteins".

What is a cofactor?

A cofactor is a non-protein helper required for enzyme activity. Types: inorganic ions (Zn²⁺, Mg²⁺), coenzymes (NAD⁺, FAD), prosthetic groups (heme).

Why do enzymes denature?

High temperature or extreme pH breaks hydrogen bonds and hydrophobic interactions maintaining the enzyme's 3D shape. The active site is distorted.

What is a suicide inhibitor?

An irreversible inhibitor that covalently binds the active site. Example: penicillin (transpeptidase), aspirin (COX), nerve gases (acetylcholinesterase).

❓People Also Ask — Enzymes

Questions students most frequently search about this chapter. Answers are direct and exam-focused.

QAre all enzymes proteins?
Almost all — except ribozymes, which are RNA molecules with catalytic activity (e.g., in ribosomes and self-splicing RNAs).
QWhat is the difference between a coenzyme and a prosthetic group?
Coenzymes are organic cofactors that bind loosely and transiently (e.g., NAD⁺, FAD). Prosthetic groups are tightly/covalently bound to the enzyme (e.g., heme in catalase).
QHow does temperature affect enzyme activity?
Rate increases with temperature up to optimum (~37°C for humans, Q₁₀ ≈ 2). Above optimum → denaturation. Below optimum → inactive but reversible.
QHow does pH affect enzymes?
Each enzyme has an optimum pH. Extreme pH changes the ionization of active-site amino acids, disrupting substrate binding and catalysis.
QWhy does reaction rate plateau at high substrate concentration?
Because all active sites are saturated. Rate reaches Vmax — no further increase until more enzyme is added.
QWhat is the difference between Vmax and Km?
Vmax is the maximum reaction rate (all active sites full). Km is the substrate concentration at half-Vmax; low Km = high affinity.
QWhat is an allosteric enzyme?
An enzyme with a regulatory site (allosteric site) distinct from the active site. Binding of modulators changes activity. Usually multi-subunit with sigmoidal kinetics.
QWhat is a competitive inhibitor?
An inhibitor that resembles the substrate and binds to the active site. Can be outcompeted by increasing substrate concentration. Vmax unchanged, Km increased.
QWhat is a non-competitive inhibitor?
An inhibitor that binds to an allosteric site, changing the enzyme's shape. Not overcome by substrate. Vmax decreases, Km unchanged.
QWhat is malonate an example of?
Malonate is a classic competitive inhibitor of succinate dehydrogenase — it resembles the normal substrate succinate.
QWhat does cyanide do to enzymes?
Cyanide is an irreversible non-competitive inhibitor of cytochrome c oxidase, blocking cellular respiration. Highly toxic.
QWhat is the Q₁₀ value?
Q₁₀ (temperature coefficient) = 2 for most enzymes. Rate doubles for each 10°C rise within the physiological range.
QWhat is the Lineweaver-Burk plot?
A double-reciprocal plot (1/V vs 1/[S]) that yields a straight line. y-intercept = 1/Vmax, x-intercept = -1/Km.
QWhat are the industrial uses of enzymes?
Proteases in detergents, amylases in food processing, rennin in cheese, streptokinase in dissolving clots, Taq polymerase in PCR.
QHow do enzymes differ from inorganic catalysts?
Enzymes are biological, protein-based, highly specific, work at mild pH/temperature, and are regulated. Inorganic catalysts work at extreme conditions and are less specific.
QWhat is the transition state?
The high-energy, unstable intermediate state during a reaction. Enzymes stabilize it to lower activation energy.
QWhat are metalloenzymes?
Enzymes with a tightly bound metal ion as prosthetic group. Examples: carbonic anhydrase (Zn²⁺), cytochrome oxidase (Fe), arginase (Mn²⁺).
QHow are enzymes named?
Usually by the substrate + "-ase" (e.g., lactase, maltase). Some have historical names (pepsin, trypsin). Modern system uses EC numbers.
QWhat are the most frequently asked enzymes questions in MDCAT?
Apoenzyme/holoenzyme, competitive vs non-competitive inhibitors, optimum pH, lock-and-key vs induced fit, Km and Vmax, feedback inhibition.
QHow many MCQs come from Enzymes in MDCAT?
About 3–5 MCQs typically, but the concepts are used throughout biology so mastering them is essential.

📋Syllabus & Learning Outcomes

According to PMDC/PMC MDCAT syllabus, Enzymes cover:

📝Full Notes — Characteristics, Mechanism & Inhibitors

6.1 Characteristics of Enzymes

  • Chemical nature: Globular proteins (except ribozymes – RNA enzymes).
  • Catalytic power: Accelerate reactions by 10⁶–10¹² times without being consumed.
  • Specificity: Absolute (one substrate) or group (similar substrates). Due to specific 3D active site.
  • Reusability: Remain unchanged after reaction; tiny amounts suffice.
  • Lower activation energy (Eₐ): Do not change ΔG; only lower the energy barrier.
  • Heat labile: Denatured at high temperature (except thermostable enzymes like Taq polymerase).
  • pH sensitive: Each enzyme has an optimum pH (e.g., pepsin ~2, trypsin ~8).
  • Regulatable: Can be activated or inhibited.
  • Co-factor requirement: Some require non-protein helpers:
    • Inorganic cofactors: Zn²⁺, Mg²⁺, Fe²⁺.
    • Coenzymes (organic): NAD⁺, FAD, coenzyme A (often vitamin-derived).
    • Prosthetic groups: tightly bound cofactors (heme in catalase).
  • Apoenzyme (protein part) + cofactor = Holoenzyme (active form).
Key: Holoenzyme = apoenzyme + cofactor. Apoenzyme is inactive; holoenzyme is active.

6.2 Mechanism of Enzyme Action

  • Enzymes work by lowering activation energy (Eₐ) of a reaction.
  • Active site: Specific cleft/crevice on enzyme surface where substrate binds. Formed by folding of polypeptide chain.
  • Lock-and-Key Model (Emil Fischer, 1894): Active site rigidly matches substrate shape.
  • Induced Fit Model (Koshland, 1959): Active site adjusts around substrate upon binding — more accurate.
  1. E + S ⇌ ES complex (transient intermediate).
  2. Bond rearrangement occurs → transition state stabilised.
  3. Products formed and released; enzyme unchanged.
Trap: Enzyme does NOT provide energy or change the nature of products. It does NOT affect equilibrium or ΔG — only increases rate.

6.3 Factors Affecting Enzyme Action

Temperature

  • Rate increases with temperature (Q₁₀ ≈ 2; rate doubles per 10°C) up to optimum.
  • Optimum temperature: Human enzymes ~37°C; thermophilic bacteria ~70°C+.
  • Beyond optimum → denaturation (loss of 3D structure, H-bonds break → active site distorted).
  • At very low temperature: enzyme inactive but NOT denatured (reversible).

pH

  • Each enzyme has an optimum pH where activity is maximal.
  • pH affects ionisation of active-site residues → alters substrate binding and catalysis.
  • Extreme pH denatures enzyme by breaking ionic and H-bonds.
  • Examples: Pepsin (pH 1.5–2), Salivary amylase (pH 6.8), Trypsin (pH 8), Pancreatic lipase (pH 9), Arginase (pH 9.7).

Substrate Concentration

  • At fixed [E], velocity increases with [S] until Vmax (all active sites saturated).
  • Michaelis constant (Km): [S] at which rate = ½ Vmax. Lower Km = higher affinity.
  • Graph: rectangular hyperbola (Michaelis-Menten kinetics).

Enzyme Concentration

  • At excess substrate, rate ∝ [E] (linear relationship).

6.4 Enzyme Inhibitors

TypeBinds toEffect on VmaxEffect on Km
CompetitiveActive site (resembles substrate)UnchangedIncreased
Non-competitiveAllosteric siteDecreasedUnchanged
IrreversibleCovalent bond (destroys active site)Decreased permanently—
  • Competitive: e.g., statins (HMG-CoA reductase), malonate inhibits succinate dehydrogenase. Reversible by increasing [S].
  • Non-competitive: e.g., cyanide inhibits cytochrome oxidase. Not overcome by [S].
  • Irreversible: e.g., penicillin (transpeptidase), aspirin (COX), organophosphates (acetylcholinesterase).
  • Feedback inhibition: End-product inhibits first enzyme of pathway (allosteric).

High-Yield Points & Common Traps

  • Enzymes are proteins (except ribozymes); lower Eₐ; remain unchanged.
  • Induced fit model > lock-and-key; active site is a 3D cleft.
  • Holoenzyme = apoenzyme + cofactor; coenzymes are often vitamins.
  • Optimum temp: human ~37°C; above → denaturation; low → inactivation (reversible).
  • pH optimum: pepsin = acidic (2), trypsin = alkaline (8).
  • Km: lower value = higher affinity for substrate.
  • Competitive inhibitor: can be reversed by ↑ [S]; Vmax unchanged, Km increases.
  • Non-competitive: Vmax decreases, Km unchanged.
  • Feedback inhibition: end-product regulates pathway.
  • Traps: "Enzyme provides energy" → False. "Enzymes change equilibrium" → False. "Denaturation at low temp" → False (only high temp/extreme pH).

✅Practice MCQs — Test 1

🧪 Basic Concepts

50 MCQs
1Chemically, almost all enzymes are composed of:
  • A. Primary structure proteins
  • B. Fibrous proteins
  • C. Globular proteins
  • D. Conjugated lipids
Answer: C. Globular proteins
Enzymes are typically globular proteins with a specific three-dimensional shape.
2The non-protein part of an enzyme that is essential for its catalytic activity is known as:
  • A. Apoenzyme
  • B. Holoenzyme
  • C. Cofactor
  • D. Epienzyme
Answer: C. Cofactor
Cofactors (inorganic ions or organic molecules) are required for enzyme activity.
3An organic cofactor that is loosely or transiently attached to the protein part is called a/an:
  • A. Prosthetic group
  • B. Co-enzyme
  • C. Activator
  • D. Apoenzyme
Answer: B. Co-enzyme
Co-enzymes are organic cofactors that bind reversibly and transiently to the enzyme.
4Which of the following is a characteristic of a 'Prosthetic Group'?
  • A. It is always a metal ion
  • B. It is covalently and permanently bound to the enzyme
  • C. It is easily dissociable
  • D. It is only found in digestive enzymes
Answer: B
Prosthetic groups are tightly bound, often covalently, to the apoenzyme.
5An enzyme without its required cofactor is termed as:
  • A. Holoenzyme
  • B. Zymogen
  • C. Apoenzyme
  • D. Co-enzyme
Answer: C. Apoenzyme
Apoenzyme is the inactive protein part; holoenzyme = apoenzyme + cofactor.
6Enzymes increase the rate of a reaction by:
  • A. Increasing the temperature of the reactants
  • B. Lowering the activation energy
  • C. Increasing the activation energy
  • D. Changing the equilibrium constant
Answer: B
Enzymes lower the activation energy barrier, speeding up the reaction without changing equilibrium.
7Metal ions like Mg²⁺, Fe²⁺, and Cu²⁺ that help in enzyme activity are specifically called:
  • A. Co-enzymes
  • B. Activators
  • C. Prosthetic groups
  • D. Apoenzymes
Answer: B. Activators
Inorganic metal ions that assist enzyme function are called activators.
8Which vitamin is often used as a precursor for co-enzymes?
  • A. Vitamin A
  • B. Vitamin B complex
  • C. Vitamin D
  • D. Vitamin K
Answer: B
B vitamins (e.g., B1, B2, B3, B6) are precursors to many co-enzymes like NAD, FAD, TPP.
9The specific site on the enzyme where the substrate binds is the:
  • A. Allosteric site
  • B. Active site
  • C. Binding site only
  • D. Catalytic site only
Answer: B. Active site
The active site consists of binding and catalytic regions.
10The active site of an enzyme is composed of only a few:
  • A. Fatty acids
  • B. Nucleotides
  • C. Amino acids
  • D. Monosaccharides
Answer: C. Amino acids
The active site is a small crevice formed by specific amino acid residues.
11The 'Lock and Key' model of enzyme action was proposed by:
  • A. Daniel Koshland
  • B. Emil Fischer
  • C. Robert Brown
  • D. Louis Pasteur
Answer: B. Emil Fischer
Emil Fischer (1894) proposed the lock and key model.
12According to the Induced Fit hypothesis, the active site of an enzyme is:
  • A. A rigid structure
  • B. A flexible structure that molds around the substrate
  • C. Independent of the substrate shape
  • D. Only active in the presence of inhibitors
Answer: B
Induced fit: substrate binding induces conformational change in the active site for optimal fit.
13In the Lock and Key model, the 'Key' represents the:
  • A. Enzyme
  • B. Substrate
  • C. Active site
  • D. Co-enzyme
Answer: B. Substrate
The substrate is the key that fits into the enzyme (lock).
14The two functional regions of an active site are:
  • A. Attachment site and Allosteric site
  • B. Binding site and Catalytic site
  • C. Inhibitory site and Active site
  • D. Primary site and Secondary site
Answer: B
The binding site holds the substrate, and the catalytic site performs the chemical transformation.
15What is the function of the 'Binding Site' of an enzyme?
  • A. To break chemical bonds
  • B. To hold the substrate in the correct orientation
  • C. To change the pH of the environment
  • D. To release the product
Answer: B
The binding site ensures substrate specificity and proper positioning for catalysis.
16The transition state of a reaction occurs when:
  • A. The substrate is first added
  • B. The Enzyme-Substrate (ES) complex is formed
  • C. The product is released
  • D. The enzyme is denatured
Answer: B
The ES complex represents the transition state where bonds are strained and ready to break/form.
17Induced Fit model was proposed by Daniel Koshland in:
  • A. 1894
  • B. 1959
  • C. 1920
  • D. 1905
Answer: B. 1959
Koshland proposed the induced fit model in 1959.
18The 'Lock and Key' model fails to explain:
  • A. Enzyme specificity
  • B. The dynamic nature of enzyme-substrate interaction
  • C. Protein structure
  • D. Substrate binding
Answer: B
It is static and doesn't account for conformational changes upon binding.
19The temperature at which an enzyme works at its maximum rate is called:
  • A. Maximum temperature
  • B. Optimum temperature
  • C. Absolute temperature
  • D. Threshold temperature
Answer: B
Optimum temperature is where enzyme activity is highest (e.g., 37°C for human enzymes).
20In humans, the optimum temperature for most enzymes is approximately:
  • A. 25°C
  • B. 37°C
  • C. 45°C
  • D. 98°C
Answer: B. 37°C
Body temperature (37°C) is the optimum for human enzymes.
21What happens to enzyme activity when the temperature is increased significantly above the optimum?
  • A. It increases exponentially
  • B. It remains constant
  • C. The enzyme is denatured
  • D. The substrate concentration decreases
Answer: C
High temperatures disrupt hydrogen bonds and tertiary structure, causing denaturation.
22The Q₁₀ value (temperature coefficient) for most biological reactions is approximately:
  • A. 1
  • B. 2
  • C. 5
  • D. 10
Answer: B. 2
Q₁₀ ≈ 2 means reaction rate doubles for every 10°C rise (within physiological range).
23Pepsin, a protein-digesting enzyme in the stomach, has an optimum pH of:
  • A. 2.0
  • B. 7.0
  • C. 8.5
  • D. 12.0
Answer: A. 2.0
Pepsin works best in acidic pH ~2.0 due to stomach environment.
24Which enzyme has an optimum pH near 9.0?
  • A. Salivary Amylase
  • B. Pepsin
  • C. Arginase
  • D. Catalase
Answer: C. Arginase
Arginase has an alkaline optimum pH around 9.7.
25Enzymes are sensitive to pH changes because amino acids at the active site:
  • A. Change their color
  • B. Undergo ionization
  • C. Evaporate
  • D. Turn into lipids
Answer: B
pH alters the ionization state of active site residues, affecting substrate binding and catalysis.
26At high substrate concentrations, the rate of reaction becomes constant because:
  • A. The enzyme is denatured
  • B. All active sites are saturated
  • C. The substrate becomes toxic
  • D. No more ATP is available
Answer: B
Vmax is reached when all active sites are occupied; adding more substrate doesn't increase rate.
27If the enzyme concentration is doubled while substrate is in excess, the rate of reaction will:
  • A. Stay the same
  • B. Double
  • C. Decrease by half
  • D. Stop
Answer: B. Double
With excess substrate, rate is directly proportional to enzyme concentration.
28The enzyme 'Trypsin' works best in which environment?
  • A. Highly acidic
  • B. Highly alkaline
  • C. Slightly alkaline
  • D. Neutral
Answer: C. Slightly alkaline
Trypsin has an optimum pH around 7.5–8.0 (slightly alkaline).
29Which factor does NOT affect the rate of enzyme action?
  • A. Substrate concentration
  • B. Light intensity (unless photosynthetic)
  • C. Enzyme concentration
  • D. pH
Answer: B
Light intensity generally does not affect non-photosynthetic enzymes.
30Maximum velocity (Vmax) is reached when:
  • A. pH is 7
  • B. Substrate concentration is at saturation level
  • C. Temperature is 0°C
  • D. The inhibitor is added
Answer: B
Vmax occurs when all enzyme active sites are occupied by substrate.
31An inhibitor that competes with the substrate for the active site is called a/an:
  • A. Non-competitive inhibitor
  • B. Competitive inhibitor
  • C. Allosteric inhibitor
  • D. Irreversible inhibitor
Answer: B. Competitive inhibitor
Competitive inhibitors bind reversibly to the active site, competing with substrate.
32Competitive inhibitors often have a structural similarity with the:
  • A. Enzyme
  • B. Substrate
  • C. Product
  • D. Co-enzyme
Answer: B. Substrate
They resemble the substrate and thus fit into the active site.
33Which of the following is a classic example of a competitive inhibitor?
  • A. Cyanide
  • B. Malonate
  • C. Penicillin
  • D. Heavy metal ions
Answer: B. Malonate
Malonate resembles succinate and inhibits succinate dehydrogenase.
34Malonate inhibits the enzyme succinic dehydrogenase because it resembles:
  • A. Fumarate
  • B. Succinate
  • C. Citrate
  • D. Malate
Answer: B. Succinate
Malonate is a structural analog of succinate.
35Non-competitive inhibitors bind to the enzyme at:
  • A. The active site
  • B. A site other than the active site (Allosteric site)
  • C. The substrate
  • D. The product
Answer: B
Non-competitive inhibitors bind to an allosteric site, altering enzyme shape.
36Which of the following inhibitors causes permanent damage to the enzyme by forming strong covalent bonds?
  • A. Reversible inhibitors
  • B. Irreversible inhibitors
  • C. Competitive inhibitors
  • D. Allosteric activators
Answer: B. Irreversible inhibitors
Irreversible inhibitors (e.g., cyanide, nerve gases) covalently modify the enzyme.
37The effect of a competitive inhibitor can be overcome by:
  • A. Increasing the enzyme concentration
  • B. Increasing the substrate concentration
  • C. Decreasing the temperature
  • D. Changing the pH
Answer: B
High substrate concentration outcompetes the inhibitor for active sites.
38Cyanide is a deadly poison because it acts as an irreversible inhibitor of:
  • A. Salivary Amylase
  • B. Cytochrome oxidase
  • C. DNA Polymerase
  • D. Pepsin
Answer: B. Cytochrome oxidase
Cyanide binds to cytochrome c oxidase (Complex IV), blocking electron transport.
39Penicillin acts as an inhibitor for enzymes involved in:
  • A. Human digestion
  • B. Bacterial cell wall synthesis
  • C. Viral replication
  • D. Fungal respiration
Answer: B
Penicillin inhibits transpeptidase, preventing bacterial cell wall cross-linking.
40In non-competitive inhibition, the Vmax of the reaction:
  • A. Increases
  • B. Decreases
  • C. Remains the same
  • D. Becomes zero immediately
Answer: B. Decreases
Non-competitive inhibition lowers Vmax because fewer active enzymes are available.
41Enzymes that are secreted in an inactive form are called:
  • A. Co-enzymes
  • B. Pro-enzymes (Zymogens)
  • C. Holoenzymes
  • D. Apoenzymes
Answer: B. Zymogens
Zymogens are inactive precursors activated by proteolytic cleavage (e.g., pepsinogen).
42Salivary amylase works best at a pH of:
  • A. 2.0
  • B. 6.8
  • C. 8.0
  • D. 9.5
Answer: B. 6.8
Salivary amylase has an optimum pH around 6.8 (slightly acidic).
43The study of the rate of enzyme-catalyzed reactions is called:
  • A. Enzymology
  • B. Enzyme kinetics
  • C. Bioenergetics
  • D. Proteomics
Answer: B. Enzyme kinetics
Enzyme kinetics studies reaction rates and factors affecting them.
44Which of the following is NOT an enzyme?
  • A. Pepsin
  • B. Cellulase
  • C. Lactose
  • D. Maltase
Answer: C. Lactose
Lactose is a disaccharide sugar, not an enzyme.
45Which of the following bonds is NOT typically involved in maintaining the tertiary structure of an enzyme?
  • A. Hydrogen bonds
  • B. Disulfide bridges
  • C. Peptide bonds
  • D. Hydrophobic interactions
Answer: C. Peptide bonds
Peptide bonds form the primary structure, not tertiary.
46Allosteric enzymes have:
  • A. Only one binding site
  • B. Multiple subunits and regulatory sites
  • C. No active site
  • D. Only inorganic cofactors
Answer: B
Allosteric enzymes have multiple subunits and regulatory sites for modulators.
47Feedback inhibition is a process where the ______ inhibits the first enzyme of the pathway.
  • A. Substrate
  • B. Intermediate
  • C. End product
  • D. Co-enzyme
Answer: C. End product
The final product binds to an allosteric site, inhibiting the first enzyme (negative feedback).
48Which of these enzymes is found in the lysosome?
  • A. ATP Synthase
  • B. Acid Phosphatase
  • C. DNA Ligase
  • D. RNA Polymerase
Answer: B. Acid Phosphatase
Acid phosphatase is a lysosomal enzyme active at acidic pH.
49Heavy metals like Mercury (Hg²⁺) and Lead (Pb²⁺) usually act as:
  • A. Competitive inhibitors
  • B. Non-competitive irreversible inhibitors
  • C. Co-enzymes
  • D. Activators
Answer: B
Heavy metals bind to sulfhydryl (-SH) groups, irreversibly inhibiting enzymes.
50The activation energy is defined as the minimum energy required to:
  • A. Stop a reaction
  • B. Start a chemical reaction
  • C. Cool down a reaction
  • D. Dissolve the enzyme
Answer: B
Activation energy is the energy needed to initiate a chemical reaction.

✅Practice MCQs — Test 2

🧪 Advanced Concepts

25 MCQs
1Enzymes are considered 'Biological Catalysts' because they shift the equilibrium of a reaction:
  • A. Toward the products
  • B. Toward the reactants
  • C. They do not shift equilibrium; they only increase the rate to reach it
  • D. By increasing the internal energy of the substrate
Answer: C
Enzymes do not alter equilibrium; they only speed up the attainment of equilibrium.
2The 'Activation Energy' (Ea) of a reaction is effectively lowered by enzymes through:
  • A. Increasing the thermal motion of molecules
  • B. Stabilizing the transition state complex
  • C. Decreasing the entropy of the products
  • D. Changing the ΔG (Gibbs Free Energy)
Answer: B
Enzymes stabilize the transition state, reducing the energy barrier.
3Which of the following is a 'Metalloenzyme' where the metal ion is an integral part of the protein structure (Prosthetic group)?
  • A. Salivary Amylase (Cl⁻)
  • B. Carbonic Anhydrase (Zn²⁺)
  • C. Hexokinase (Mg²⁺)
  • D. Arginase (Mn²⁺)
Answer: B
Carbonic anhydrase contains tightly bound Zn²⁺ as a prosthetic group.
4In the absence of a cofactor, the protein portion of a conjugated enzyme is catalytically inactive because:
  • A. The primary structure is destroyed
  • B. The active site cannot achieve the correct three-dimensional conformation
  • C. The enzyme undergoes immediate denaturation
  • D. The peptide bonds are hydrolyzed
Answer: B
Cofactors help maintain the active site's proper shape.
5A 'Ribozyme' is a unique biological catalyst because it is composed of:
  • A. Globular proteins
  • B. RNA molecules
  • C. Steroid derivatives
  • D. Conjugated polysaccharides
Answer: B. RNA molecules
Ribozymes are RNA molecules with catalytic activity (e.g., ribosome rRNA).
6In the Michaelis-Menten equation, Km represents:
  • A. The maximum velocity
  • B. The substrate concentration at half Vmax
  • C. The total enzyme concentration
  • D. The energy required to start the reaction
Answer: B
Km is the substrate concentration at half Vmax, indicating enzyme-substrate affinity.
7A low Km value for an enzyme-substrate pair indicates:
  • A. Low affinity
  • B. High affinity
  • C. The reaction is irreversible
  • D. The enzyme is pH sensitive
Answer: B. High affinity
Low Km means high affinity; less substrate needed to reach half Vmax.
8According to the 'Induced Fit Model,' the binding of the substrate induces a conformational change that:
  • A. Breaks the enzyme into subunits
  • B. Brings specific catalytic groups into correct orientation to stress substrate bonds
  • C. Permanently denatures the active site
  • D. Increases the Km of the reaction
Answer: B
Conformational change aligns catalytic residues for bond strain and catalysis.
9The 'Binding Site' of an enzyme is primarily responsible for:
  • A. Breaking the covalent bonds of the substrate
  • B. Determining the specificity of the enzyme for a particular substrate
  • C. Providing the prosthetic group
  • D. Lowering the temperature of the reaction
Answer: B
Binding site structure dictates which substrate can fit.
10During the formation of the Enzyme-Substrate (ES) complex, which type of bonds are typically involved?
  • A. Permanent covalent bonds
  • B. Weak non-covalent interactions
  • C. Only disulfide bridges
  • D. Metallic bonds
Answer: B
ES complex is stabilized by reversible non-covalent interactions.
11Extreme changes in pH decrease enzyme activity primarily by:
  • A. Breaking the peptide bonds of the primary structure
  • B. Altering the ionization state of amino acids at the active site
  • C. Decreasing the collision frequency between molecules
  • D. Increasing the Vmax beyond limits
Answer: B
pH changes affect the charge of active site residues, disrupting substrate binding.
12At very low temperatures, enzymes become 'Inert.' This is because:
  • A. Their tertiary structure is permanently lost
  • B. Molecules lack sufficient kinetic energy to overcome the activation energy barrier
  • C. The active site is filled with ice crystals
  • D. The cofactors dissociate from the apoenzyme
Answer: B
Low temperature reduces molecular motion, slowing reactions, but does not denature.
13The Q₁₀ rule states that for every 10°C rise in temperature, the rate of reaction doubles, but this only holds true:
  • A. Until the enzyme reaches its Vmax
  • B. Within the physiological range before denaturation begins
  • C. If the pH is kept at 14.0
  • D. For inorganic catalysts only
Answer: B
Beyond optimum temperature, denaturation causes rate to drop.
14Which enzyme would remain functional in the highly alkaline environment of the small intestine?
  • A. Pepsin
  • B. Trypsin
  • C. Carbonic Anhydrase
  • D. Salivary Amylase
Answer: B. Trypsin
Trypsin has an alkaline pH optimum (~8.0).
15The saturation effect observed in a 'Rate vs. Substrate Concentration' graph proves that:
  • A. Enzymes are consumed in the reaction
  • B. There is a finite number of active sites available
  • C. Substrates are inhibitors at high concentrations
  • D. Vmax is independent of enzyme concentration
Answer: B
At saturation, all active sites are occupied, so rate plateaus.
16In 'Competitive Inhibition,' how are the kinetic parameters Km and Vmax affected?
  • A. Vmax decreases, Km stays the same
  • B. Vmax stays the same, Km increases
  • C. Both Vmax and Km decrease
  • D. Vmax increases, Km decreases
Answer: B
Competitive inhibitor increases apparent Km (lower affinity) but does not change Vmax.
17A 'Non-competitive' inhibitor decreases the rate of reaction by:
  • A. Blocking the substrate from entering the active site
  • B. Changing the shape of the enzyme so the catalytic step is less efficient
  • C. Competing for the binding site
  • D. Decreasing the substrate concentration
Answer: B
Non-competitive inhibitor binds elsewhere, altering active site shape and reducing Vmax.
18The inhibition of 'Succinate Dehydrogenase' by Malonate is a classic example of:
  • A. Non-competitive reversible inhibition
  • B. Competitive reversible inhibition
  • C. Irreversible inhibition
  • D. Feedback inhibition
Answer: B
Malonate is a structural analog of succinate, competing for the active site.
19'Allosteric Enzymes' are regulated by molecules that bind to a site other than the active site. This site is called the:
  • A. Prosthetic site
  • B. Regulatory/Allosteric site
  • C. Apoenzyme site
  • D. Zymogen site
Answer: B. Regulatory/Allosteric site
Allosteric sites bind effectors that modulate enzyme activity.
20Irreversible inhibitors, like organophosphates (nerve gas), usually function by:
  • A. Forming weak hydrogen bonds with the substrate
  • B. Forming strong covalent bonds with a key amino acid at the active site
  • C. Increasing the Km to infinity
  • D. Acting as a co-enzyme
Answer: B
Organophosphates covalently modify serine in acetylcholinesterase, permanently inhibiting it.
21Feedback inhibition (End-product inhibition) is an example of:
  • A. Competitive inhibition
  • B. Allosteric regulation
  • C. Irreversible denaturation
  • D. Co-enzyme activation
Answer: B. Allosteric regulation
End product binds to an allosteric site on the first enzyme, inhibiting the pathway.
22What is the effect of a Non-competitive inhibitor on the Vmax and Km of an enzyme?
  • A. Vmax stays the same, Km increases
  • B. Vmax decreases, Km stays the same
  • C. Both increase
  • D. Both decrease
Answer: B
Non-competitive inhibitor reduces Vmax but does not affect substrate binding (Km unchanged).
23Sulfa drugs work as antibiotics by acting as competitive inhibitors for the bacterial enzyme that produces:
  • A. ATP
  • B. Folic acid
  • C. Peptidoglycan
  • D. RNA
Answer: B. Folic acid
Sulfonamides inhibit dihydropteroate synthase, blocking folic acid synthesis.
24Which of the following is an example of an 'Irreversible Inhibitor' used as a medicine?
  • A. Malonate
  • B. Penicillin (inhibits transpeptidase)
  • C. Citrate
  • D. Ethanol
Answer: B
Penicillin irreversibly inhibits bacterial transpeptidase via covalent binding.
25The active site of an enzyme is usually a 'Cleft' or 'Pocket.' This environment is often:
  • A. Hydrophilic to attract water
  • B. Hydrophobic to promote specific substrate orientations and reactions
  • C. Made of purely acidic amino acids
  • D. Lacking any tertiary structure
Answer: B
Hydrophobic environment excludes water and favors substrate binding.

✅Practice MCQs — Test 3

🧪 Exam-Level Questions

30 MCQs
1According to the transition state theory, enzymes accelerate reactions by:
  • A. Decreasing the Gibbs free energy (ΔG)
  • B. Increasing the entropy of the substrate-enzyme complex
  • C. Decreasing the enthalpy of the products
  • D. Lowering the energy of the 'Activated Complex' or transition state
Answer: D
Enzymes stabilize the transition state, reducing activation energy.
2If an enzyme-catalyzed reaction has a negative ΔG, the reaction is:
  • A. Endergonic and non-spontaneous
  • B. Exergonic and spontaneous
  • C. At equilibrium
  • D. Faster than a reaction with a positive ΔG
Answer: B
Negative ΔG means products have lower free energy; reaction is spontaneous.
3The 'Induced Fit' mechanism implies that binding energy is used to:
  • A. Synthesis ATP
  • B. Lower the activation energy by straining specific substrate bonds
  • C. Increase the Km of the enzyme
  • D. Permanently alter the primary structure of the apoenzyme
Answer: B
Binding energy induces conformational changes that strain substrate bonds toward transition state.
4Which describes a 'Metalloenzyme' correctly?
  • A. The metal ion is loosely attached and acts as an activator
  • B. The metal ion is a prosthetic group covalently bound to the protein
  • C. The metal ion is only used to maintain pH
  • D. The metal ion is always Na⁺ or K⁺
Answer: B
Metalloenzymes contain tightly bound metal ions (e.g., Zn²⁺ in carbonic anhydrase).
5In an Arrhenius plot, the slope of the line is used to calculate:
  • A. Vmax
  • B. Km
  • C. Activation Energy (Ea)
  • D. Optimum pH
Answer: C. Activation Energy
Slope = -Ea/R in the Arrhenius plot (ln k vs 1/T).
6The Michaelis constant (Km) is a measure of:
  • A. The catalytic efficiency of the enzyme (kcat)
  • B. The affinity of the enzyme for its substrate
  • C. The concentration of the enzyme in the cell
  • D. The time taken for the reaction to reach equilibrium
Answer: B
Km is inversely related to affinity; low Km = high affinity.
7An enzyme with a high Km value requires:
  • A. Less substrate to reach Vmax
  • B. More substrate to reach Vmax (lower affinity)
  • C. No cofactors
  • D. A temperature of exactly 37°C
Answer: B
High Km means more substrate needed to achieve half Vmax, indicating lower affinity.
8If the enzyme concentration is doubled in a reaction where the substrate is already at saturation level:
  • A. Vmax doubles and Km remains the same
  • B. Vmax remains the same and Km doubles
  • C. Both Vmax and Km double
  • D. The rate of reaction decreases
Answer: A
Vmax is proportional to enzyme concentration; Km is unaffected.
9The Lineweaver-Burk plot is a double-reciprocal plot used to determine Vmax and Km. The x-intercept represents:
  • A. 1/Vmax
  • B. -1/Km
  • C. Vmax/2
  • D. [S]
Answer: B. -1/Km
x-intercept = -1/Km.
10The 'Turnover Number' (kcat) of an enzyme represents:
  • A. Number of substrate molecules converted per active site per unit time
  • B. The total time the enzyme remains active
  • C. The pH at which the enzyme is denatured
  • D. The inhibitor concentration required to stop the reaction
Answer: A
kcat is the catalytic constant (molecules per second per active site).
11How does a 'Competitive Inhibitor' affect the Lineweaver-Burk plot?
  • A. It shifts the y-intercept upward
  • B. It shifts the x-intercept closer to zero (increasing Km) while the y-intercept is unchanged
  • C. It moves the entire line downward
  • D. It has no effect on the plot
Answer: B
Competitive inhibition increases slope (Km/Vmax) but same y-intercept (1/Vmax).
12A 'Non-competitive Inhibitor' binds to the ES complex or free enzyme with equal affinity. This results in:
  • A. Increased Km and decreased Vmax
  • B. Unchanged Km and decreased Vmax
  • C. Decreased Km and decreased Vmax
  • D. Increased Km and unchanged Vmax
Answer: B
Non-competitive inhibitor lowers Vmax; Km unchanged because binding not affected.
13'Uncompetitive Inhibition' is a rare type where the inhibitor binds only to the ES complex. This leads to:
  • A. A decrease in both Vmax and Km
  • B. An increase in both Vmax and Km
  • C. No change in Vmax
  • D. A shift in optimum pH
Answer: A
Uncompetitive inhibitor decreases both Vmax and apparent Km (parallel lines in Lineweaver-Burk).
14In the inhibition of the bacterial cell wall, Penicillin acts as a 'Suicide Inhibitor' because it:
  • A. Competes for the active site and then dissociates
  • B. Forms a stable, covalent transition-state analog with the enzyme transpeptidase
  • C. Destroys the bacterial DNA
  • D. Acts as an allosteric activator
Answer: B
Penicillin covalently binds to transpeptidase, irreversibly inhibiting it.
15Which is an example of 'Reversible Non-competitive' inhibition used in metabolic regulation?
  • A. Cyanide on Cytochrome oxidase
  • B. Feedback inhibition by ATP on Citrate Synthase
  • C. Malonate on Succinate Dehydrogenase
  • D. Acetylcholine on its receptor
Answer: B
ATP binds allosterically to citrate synthase, inhibiting it reversibly.
16Nerve gases like DIPF are potent toxins because they:
  • A. Bind reversibly to the Km site
  • B. Covalently modify the Serine residue in the active site of Acetylcholinesterase
  • C. Increase the Vmax of heart contractions to fatal levels
  • D. Act as co-enzymes for viral replication
Answer: B
DIPF phosphorylates the serine in acetylcholinesterase, causing irreversible inhibition.
17Which mechanism allows a cell to increase the rate of an enzymatic reaction beyond its Vmax?
  • A. Increasing the substrate concentration
  • B. Enzyme Induction (synthesizing more enzyme molecules)
  • C. Decreasing the temperature
  • D. Adding a competitive inhibitor
Answer: B
Vmax is limited by enzyme concentration; synthesizing more enzymes increases Vmax.
18Allosteric enzymes typically exhibit a ______ curve when plotting velocity against substrate concentration.
  • A. Hyperbolic
  • B. Sigmoidal (S-shaped)
  • C. Linear
  • D. Exponential
Answer: B. Sigmoidal
Allosteric enzymes show sigmoidal kinetics due to cooperativity.
19The 'Cooperativity' seen in multi-subunit enzymes means:
  • A. All subunits work independently
  • B. Binding of a substrate to one active site increases the affinity of other active sites
  • C. The enzyme requires multiple different cofactors
  • D. The enzyme only works at high pH
Answer: B
Positive cooperativity: binding of one substrate enhances binding at other subunits.
20In a metabolic pathway A → B → C → D, if 'D' inhibits the enzyme converting A → B, this is:
  • A. Positive feedback
  • B. Negative feedback / End-product inhibition
  • C. Irreversible denaturation
  • D. Competitive activation
Answer: B
End-product inhibition is a negative feedback mechanism.
21Which molecule acts as an 'Allosteric Effector' for Phosphofructokinase in Glycolysis?
  • A. Glucose
  • B. AMP (Activator) and ATP (Inhibitor)
  • C. Water
  • D. Carbon dioxide
Answer: B
AMP activates PFK; ATP inhibits it, regulating glycolysis.
22The 'Optimum pH' of an enzyme is the point where:
  • A. All amino acids are positively charged
  • B. The ionization state of active site residues is most favorable for substrate binding and catalysis
  • C. The enzyme has the highest possible kinetic energy
  • D. The peptide bonds are most rigid
Answer: B
At optimum pH, active site residues are in their correct ionization state for catalysis.
23Heavy metal poisoning (e.g., Lead) usually involves the metal binding to ______ groups in enzymes.
  • A. Carboxyl (-COOH)
  • B. Sulfhydryl (-SH)
  • C. Methyl (-CH₃)
  • D. Phosphate (-PO₄)
Answer: B. Sulfhydryl (-SH)
Heavy metals bind to cysteine -SH groups, inhibiting enzyme function.
24When an enzyme is denatured by heat, which level of protein structure is typically preserved?
  • A. Primary
  • B. Secondary
  • C. Tertiary
  • D. Quaternary
Answer: A. Primary
Denaturation disrupts secondary, tertiary, and quaternary structure but primary structure (peptide bonds) remains intact.
25Enzymes found in 'Extremophiles' (like Taq Polymerase) have high thermal stability due to:
  • A. Fewer disulfide bridges
  • B. Increased number of ionic bonds and hydrophobic interactions in the core
  • C. Being made of RNA instead of protein
  • D. Lacking an active site
Answer: B
More ionic and hydrophobic interactions increase thermal stability.
26Which enzyme is used clinically as a marker for a Myocardial Infarction (Heart Attack)?
  • A. Salivary Amylase
  • B. Creatine Kinase (CK-MB)
  • C. Pepsinogen
  • D. DNA Polymerase
Answer: B. CK-MB
CK-MB is elevated in blood after heart attack.
27'Zymogens' (Pro-enzymes) like Pepsinogen are activated by:
  • A. The addition of a phosphate group
  • B. Proteolytic cleavage (removal of a masking peptide)
  • C. Cooling the solution
  • D. Binding to a competitive inhibitor
Answer: B
Zymogens are activated by irreversible proteolytic cleavage.
28Carbonic Anhydrase is one of the fastest enzymes known. Its efficiency is due to its ability to:
  • A. Work without any water
  • B. Use a Zn²⁺ ion to polarize a water molecule for the reaction
  • C. Function at 100°C
  • D. Change its primary structure during every cycle
Answer: B
Zn²⁺ in the active site activates water for nucleophilic attack on CO₂.
29Which cofactor is a derivative of Vitamin B₃ (Niacin)?
  • A. FAD
  • B. NAD⁺
  • C. TPP
  • D. Biotin
Answer: B. NAD⁺
Niacin (B₃) is a precursor to NAD⁺ and NADP⁺.
30The lock-and-key model is essentially a 'Static' model, whereas the induced-fit model is a ______ model.
  • A. Chemical
  • B. Dynamic
  • C. Mathematical
  • D. Geological
Answer: B. Dynamic
Induced fit describes conformational changes, making it dynamic.

📄Past Paper MCQs

📅 Actual MDCAT / NUMS Questions

50 MCQs
1The non-protein part of a conjugated enzyme which is covalently and permanently bound to the apoenzyme is called:
MDCAT 2017
  • A. Co-enzyme
  • B. Prosthetic group
  • C. Activator
  • D. Holoenzyme
Answer: B. Prosthetic group
Prosthetic groups are tightly bound, often covalently.
2Which of the following is an example of an organic co-factor?
NUMS 2024
  • A. Fe²⁺
  • B. Mg²⁺
  • C. NAD⁺
  • D. Zn²⁺
Answer: C. NAD⁺
NAD⁺ is an organic coenzyme derived from niacin.
3In the absence of a cofactor, the protein part of an enzyme is known as:
MDCAT 2021
  • A. Holoenzyme
  • B. Apoenzyme
  • C. Zymogen
  • D. Co-enzyme
Answer: B. Apoenzyme
Apoenzyme is the inactive protein portion.
4All enzymes are globular proteins EXCEPT:
MDCAT 2019
  • A. Pepsin
  • B. Ribozymes
  • C. Amylase
  • D. Polymerase
Answer: B. Ribozymes
Ribozymes are RNA molecules with catalytic activity.
5Which statement is correct about the active site of an enzyme?
MDCAT 2023
  • A. It is a rigid structure
  • B. It is composed of a large number of amino acids
  • C. It is a three-dimensional crevice or pocket
  • D. It is only found in apoenzymes
Answer: C
The active site is a 3D cleft formed by a few amino acids.
6The 'Induced Fit Model' of enzyme action was proposed by:
MDCAT 2016
  • A. Emil Fischer
  • B. Koshland
  • C. Robert Brown
  • D. Singer and Nicholson
Answer: B. Koshland
Daniel Koshland proposed the induced fit model in 1959.
7According to the Lock and Key model, the active site is:
NUMS 2020
  • A. Flexible
  • B. Rigid
  • C. Dynamic
  • D. Non-specific
Answer: B. Rigid
The lock and key model assumes a rigid, pre-shaped active site.
8The 'Catalytic Site' of an active site is responsible for:
MDCAT 2022
  • A. Binding the substrate
  • B. Transformation of substrate into product
  • C. Inhibition of the reaction
  • D. Maintaining pH
Answer: B
The catalytic site performs the chemical reaction.
9The Q₁₀ value for most enzymes is 2, which means the rate of reaction doubles for every:
NUMS 2021
  • A. 1°C rise
  • B. 10°C rise
  • C. 5°C rise
  • D. 10°C fall
Answer: B. 10°C rise
Q₁₀ = 2 means rate doubles per 10°C increase (within range).
10The optimum pH for the enzyme Arginase is:
MDCAT 2018
  • A. 2.0
  • B. 7.6
  • C. 9.7
  • D. 8.0
Answer: C. 9.7
Arginase has an optimum pH around 9.7.
11Excessive increase in temperature causes denaturation of enzymes by:
MDCAT 2024
  • A. Increasing kinetic energy too much
  • B. Breaking the peptide bonds
  • C. Disrupting the hydrogen bonds and tertiary structure
  • D. Decreasing the Vmax
Answer: C
Heat disrupts hydrogen bonds and hydrophobic interactions, unfolding the enzyme.
12At high substrate concentration, the rate of enzyme action:
MDCAT 2025
  • A. Increases linearly
  • B. Decreases
  • C. Becomes constant (Saturation)
  • D. Stops completely
Answer: C. Saturation
Rate reaches Vmax when all active sites are saturated.
13Pepsin works most effectively in which part of the human body?
NUMS 2019
  • A. Mouth
  • B. Stomach
  • C. Small intestine
  • D. Pancreas
Answer: B. Stomach
Pepsin is active in the acidic environment of the stomach.
14Cyanide is an example of which type of inhibitor?
MDCAT 2017
  • A. Competitive
  • B. Non-competitive irreversible
  • C. Allosteric
  • D. Feedback
Answer: B. Non-competitive irreversible
Cyanide irreversibly binds to cytochrome c oxidase.
15Malonate is a competitive inhibitor of the enzyme:
MDCAT 2020
  • A. Cytochrome oxidase
  • B. Succinic dehydrogenase
  • C. Salivary amylase
  • D. Hexokinase
Answer: B. Succinic dehydrogenase
Malonate resembles succinate and inhibits succinate dehydrogenase.
16Competitive inhibitors can be overcome by:
NUMS 2023
  • A. Decreasing substrate concentration
  • B. Increasing substrate concentration
  • C. Increasing temperature
  • D. Adding more inhibitors
Answer: B. Increasing substrate concentration
High substrate concentration outcompetes the inhibitor for active sites.
17Which inhibitor binds to a site other than the active site?
MDCAT 2021
  • A. Competitive
  • B. Non-competitive
  • C. Substrate analog
  • D. Prosthetic group
Answer: B. Non-competitive
Non-competitive inhibitors bind to allosteric sites.
18Antibiotics like Penicillin act as inhibitors for bacterial:
NUMS 2022
  • A. DNA replication
  • B. Cell wall synthesis enzymes
  • C. Protein synthesis
  • D. Respiration
Answer: B
Penicillin inhibits transpeptidase involved in cell wall synthesis.
19If the y-intercept of a Lineweaver-Burk plot increases but the x-intercept stays the same, the inhibition is:
MDCAT 2025
  • A. Competitive
  • B. Non-competitive
  • C. Uncompetitive
  • D. Mixed
Answer: B. Non-competitive
Increased y-intercept (1/Vmax) indicates non-competitive inhibition.
20Which of the following enzymes is used to dissolve blood clots?
MDCAT 2023
  • A. Amylase
  • B. Streptokinase
  • C. Lipase
  • D. Protease
Answer: B. Streptokinase
Streptokinase activates plasminogen to dissolve clots.
21In 'Feedback Inhibition,' the end product usually binds to the:
NUMS 2018
  • A. Active site
  • B. Allosteric site of the first enzyme
  • C. Substrate
  • D. Second enzyme in the pathway
Answer: B
End product binds to an allosteric site on the first enzyme, inhibiting the pathway.
22The optimum pH for Salivary Amylase is:
MDCAT 2016
  • A. 2.0
  • B. 6.8
  • C. 8.0
  • D. 9.0
Answer: B. 6.8
Salivary amylase has an optimum pH around 6.8.
23The transition state of an enzyme-substrate complex is:
MDCAT 2022
  • A. Stable and long-lived
  • B. Unstable and high energy
  • C. Low energy and stable
  • D. Found only in irreversible reactions
Answer: B
The transition state is a high-energy, unstable intermediate.
24Enzymes are 'Sensitive' to pH because:
MDCAT 2020
  • A. They are made of lipids
  • B. pH changes the ionization of amino acids at the active site
  • C. pH changes the temperature
  • D. pH only affects the co-enzyme
Answer: B
Active site residues have specific pKa values; pH changes alter their charge.
25Succinate is converted into Fumarate by succinic dehydrogenase. In this reaction, Malonate acts as a:
NUMS 2025
  • A. Co-enzyme
  • B. Competitive inhibitor
  • C. Non-competitive inhibitor
  • D. Activator
Answer: B. Competitive inhibitor
Malonate is a classic competitive inhibitor of succinate dehydrogenase.
26Which of the following is an example of an inorganic activator?
MDCAT 2017
  • A. Cl⁻ for salivary amylase
  • B. NAD for dehydrogenase
  • C. FAD for succinic dehydrogenase
  • D. Vitamin B complex
Answer: A. Cl⁻ for salivary amylase
Chloride ion activates salivary amylase.
27Enzymes are specific in their action due to:
NUMS 2018
  • A. High molecular weight
  • B. Their chemical nature as proteins
  • C. Specific configuration and chemistry of the active site
  • D. Presence of peptide bonds
Answer: C
The unique shape and chemistry of the active site confer specificity.
28In a conjugated enzyme, the 'Holoenzyme' is formed by:
MDCAT 2020
  • A. Apoenzyme + Inhibitor
  • B. Apoenzyme + Cofactor
  • C. Co-enzyme + Substrate
  • D. Activator + Product
Answer: B. Apoenzyme + Cofactor
Holoenzyme = apoenzyme + cofactor.
29The non-protein part of an enzyme that is a derivative of vitamins is called:
MDCAT 2024
  • A. Activator
  • B. Co-enzyme
  • C. Prosthetic group
  • D. Zymogen
Answer: B. Co-enzyme
Co-enzymes are often vitamin derivatives.
30According to the 'Lock and Key' hypothesis, the enzyme is represented by the:
MDCAT 2016
  • A. Key
  • B. Lock
  • C. Handle
  • D. Metal
Answer: B. Lock
Enzyme is the lock; substrate is the key.
31The energy required to destabilize existing chemical bonds and initiate a reaction is:
NUMS 2021
  • A. Kinetic energy
  • B. Activation energy
  • C. Potential energy
  • D. Thermal energy
Answer: B. Activation energy
Activation energy is the minimum energy to start a reaction.
32Enzymes increase the rate of reaction by:
MDCAT 2022
  • A. Decreasing the energy of the products
  • B. Decreasing the activation energy
  • C. Increasing the energy of the reactants
  • D. Increasing the temperature
Answer: B
Enzymes lower the activation energy barrier.
33The 'Induced Fit' model suggests that:
MDCAT 2019
  • A. The active site is pre-shaped to the substrate
  • B. The substrate changes shape to fit the enzyme
  • C. The enzyme changes its shape slightly when the substrate binds
  • D. The enzyme remains rigid throughout the process
Answer: C
Induced fit involves conformational change in the enzyme upon substrate binding.
34At what temperature do human enzymes generally start to denature?
NUMS 2020
  • A. Above 37°C
  • B. Above 40°C–45°C
  • C. At 100°C only
  • D. Below 0°C
Answer: B
Human enzymes denature above ~40-45°C.
35The optimum pH for 'Pancreatic Lipase' is:
MDCAT 2023
  • A. 2.0
  • B. 7.0
  • C. 8.0
  • D. 9.7
Answer: C. 8.0
Pancreatic lipase has an optimum pH around 8.0.
36Which of the following enzymes works best at a pH of 9.7?
MDCAT 2018
  • A. Pepsin
  • B. Arginase
  • C. Sucrase
  • D. Catalase
Answer: B. Arginase
Arginase has an optimum pH of 9.7.
37Enzymes are sensitive to pH changes because they affect:
NUMS 2025
  • A. The solubility of the enzyme
  • B. The ionization of amino acids at the active site
  • C. The speed of the co-enzyme
  • D. The temperature of the cell
Answer: B
pH alters the charge state of active site residues.
38The optimum pH for the enzyme Catalase is:
MDCAT 2021
  • A. 2.0
  • B. 6.8
  • C. 7.6
  • D. 9.0
Answer: C. 7.6
Catalase has an optimum pH around 7.6.
39Inhibition of Cytochrome oxidase by Cyanide is an example of:
MDCAT 2017
  • A. Competitive inhibition
  • B. Non-competitive irreversible inhibition
  • C. Feedback inhibition
  • D. Allosteric activation
Answer: B
Cyanide irreversibly binds to heme a3 in cytochrome c oxidase.
40Which of the following describes a 'Competitive Inhibitor'?
NUMS 2022
  • A. It binds to the allosteric site
  • B. It has a structural similarity to the substrate
  • C. It permanently denatures the enzyme
  • D. It increases the Vmax
Answer: B
Competitive inhibitors resemble the substrate and bind the active site.
41Heavy metals like Hg²⁺ and Pb²⁺ inhibit enzymes by:
MDCAT 2025
  • A. Occupying the active site
  • B. Disrupting the tertiary structure (Irreversible inhibition)
  • C. Acting as co-enzymes
  • D. Increasing Km
Answer: B
Heavy metals bind to -SH groups, causing denaturation.
42Feedback inhibition is the most common way of regulating:
MDCAT 2024
  • A. Single step reactions
  • B. Multi-step metabolic pathways
  • C. Only digestive enzymes
  • D. Photosynthesis
Answer: B
End-product inhibition regulates entire biosynthetic pathways.
43A reversible inhibitor that does not compete with the substrate for the active site is:
NUMS 2019
  • A. Competitive inhibitor
  • B. Non-competitive inhibitor
  • C. Suicide inhibitor
  • D. Activator
Answer: B. Non-competitive inhibitor
Non-competitive inhibitors bind to allosteric sites.
44What is the effect of increasing enzyme concentration on the rate of reaction if substrate is in excess?
MDCAT 2016
  • A. Rate decreases
  • B. Rate increases proportionally
  • C. Rate remains constant
  • D. Enzyme becomes denatured
Answer: B
Rate is directly proportional to enzyme concentration when substrate is saturating.
45Enzymes used in biological detergents to remove protein stains are:
MDCAT 2021
  • A. Amylases
  • B. Proteases
  • C. Lipases
  • D. Cellulases
Answer: B. Proteases
Proteases break down protein-based stains (e.g., blood, egg).
46'Streptokinase' is an enzyme used to:
NUMS 2023
  • A. Digest starch
  • B. Dissolve blood clots
  • C. Synthesize DNA
  • D. Produce antibiotics
Answer: B. Dissolve blood clots
Streptokinase is a thrombolytic agent.
47The saturation point in an enzyme-catalyzed reaction occurs when:
MDCAT 2020
  • A. The substrate is depleted
  • B. All active sites are occupied
  • C. The temperature reaches 100°C
  • D. The pH is neutral
Answer: B
At saturation, all enzymes are bound to substrate.
48In the PTB textbook, the enzyme used for making cheese is:
MDCAT 2018
  • A. Amylase
  • B. Rennin
  • C. Pepsin
  • D. Lipase
Answer: B. Rennin
Rennin (chymosin) coagulates milk for cheese production.
49Which of the following is true for Km?
NUMS 2024
  • A. It is the maximum velocity
  • B. It is the substrate concentration at 1/2 Vmax
  • C. It is independent of enzyme affinity
  • D. It is measured in degrees Celsius
Answer: B
Km is the substrate concentration at half Vmax.
50The enzyme used to remove 'Starch' stains from clothes is:
MDCAT 2019
  • A. Protease
  • B. Amylase
  • C. Lipase
  • D. Cellulase
Answer: B. Amylase
Amylase breaks down starch into sugars.

🔍Direct Search Queries — Enzymes

Exact search terms students use on Google for this chapter. Click any to jump to the relevant section:

enzymes MDCAT notes characteristics of enzymes apoenzyme vs holoenzyme cofactor vs coenzyme prosthetic group example active site function lock and key model enzymes induced fit model Koshland Km and Vmax explained Michaelis-Menten equation optimum pH pepsin trypsin temperature effect on enzymes competitive vs non-competitive inhibitor non-competitive inhibitor Km Vmax feedback inhibition example allosteric enzyme definition zymogen activation pepsinogen ribozyme examples metalloenzyme carbonic anhydrase enzyme denaturation process what are enzymes in biology how do enzymes work why are enzymes specific difference between enzyme and catalyst why are enzymes proteins enzymes MCQs with answers enzymes MCQs pdf enzyme inhibitors MCQs MDCAT past papers enzymes UHS MDCAT enzymes MCQs NUMS enzyme past papers enzymes chapter 6 class 11 enzymes class 11 biology notes first year enzymes notes enzyme kinetics formulas

❔Frequently Asked Questions

What are enzymes?
Enzymes are biological catalysts — mostly globular proteins (except ribozymes) — that speed up reactions by lowering activation energy without being consumed.
What is the difference between apoenzyme and holoenzyme?
Apoenzyme = inactive protein portion. Holoenzyme = active enzyme (apoenzyme + cofactor).
What is the difference between competitive and non-competitive inhibitors?
Competitive: binds active site, Vmax unchanged, Km increases. Non-competitive: binds allosteric site, Vmax decreases, Km unchanged.
What is the optimum pH for pepsin and trypsin?
Pepsin works best at pH ~2 (acidic, stomach). Trypsin works best at pH ~8 (alkaline, small intestine).
What is the lock-and-key model?
Proposed by Emil Fischer (1894) — the active site has a rigid, pre-shaped structure that fits the substrate like a lock fits a key.
What is the induced fit model?
Proposed by Koshland (1959) — the active site is flexible and changes shape upon substrate binding for optimal fit.
What is Km in enzyme kinetics?
Km is the substrate concentration at half Vmax. Lower Km = higher affinity between enzyme and substrate.
What is feedback inhibition?
Feedback inhibition: the end product of a pathway binds to an allosteric site on an earlier enzyme, inhibiting the pathway.
What are zymogens?
Zymogens (proenzymes) are inactive enzyme precursors activated by proteolytic cleavage. Example: pepsinogen → pepsin.
Why do enzymes denature at high temperature?
High temperatures break hydrogen bonds and hydrophobic interactions maintaining the enzyme's 3D shape. The active site is distorted and the enzyme loses activity.
What are ribozymes?
Ribozymes are RNA molecules with catalytic activity — the exception to "all enzymes are proteins".
What is a prosthetic group?
A prosthetic group is a cofactor that is tightly bound to the enzyme (covalent). Examples: heme in catalase, Zn²⁺ in carbonic anhydrase.
What is the Q₁₀ value for enzymes?
Q₁₀ ≈ 2 — reaction rate doubles for every 10°C rise in temperature within the physiological range.
What is the active site?
The active site is a 3D cleft on the enzyme where the substrate binds. It has two regions: the binding site (holds substrate) and the catalytic site (performs reaction).
How does pH affect enzyme activity?
Each enzyme has an optimum pH. Extreme pH alters the ionization state of amino acid residues at the active site, disrupting substrate binding and catalysis.
What are metalloenzymes?
Enzymes with a tightly bound metal ion as prosthetic group. Examples: carbonic anhydrase (Zn²⁺), cytochrome oxidase (Fe).
What is the Lineweaver-Burk plot?
A double-reciprocal plot (1/V vs 1/[S]) — a straight line. y-intercept = 1/Vmax, x-intercept = -1/Km.
What is allosteric regulation?
Allosteric regulation: a molecule binds to a site other than the active site, changing the enzyme's shape and activity.
What are the most commonly tested enzymes topics in MDCAT?
Apoenzyme/holoenzyme, competitive vs non-competitive inhibitors, optimum pH, lock-and-key vs induced fit, Km and Vmax, zymogens.
How many MCQs come from Enzymes in MDCAT?
About 3–5 MCQs typically, but the concepts are used throughout biology so mastering them is essential.

📚Other Biology Chapters

1
Biodiversity
2
Bioenergetics
3
Biological Molecules
4
Cell Structure
5
Coordination & Control
8
Digestive System
14
Variation & Genetics
16
Biotechnology

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