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Enzymes — Questions and Answers

Enzymes are the most frequently examined topic in biochemistry because they connect structure, kinetics and clinical application in one place. Expect questions on what enzymes do to activation energy, the meaning of Km and Vmax, how competitive and non-competitive inhibitors differ, and which coenzymes derive from which vitamins.

8 solved questionsBiochemistryFree · no signup

Enzymes— Concepts, Formulas & Shortcuts

  • Almost all enzymes are proteins (ribozymes are catalytic RNA); they lower activation energy without being consumed.
  • Enzyme activity depends on temperature, pH, substrate concentration and inhibitors; each has an optimum pH (pepsin ≈ 2, trypsin ≈ 8).
  • Km is the substrate concentration at half Vmax and measures affinity — a low Km means high affinity.
  • Competitive inhibitor: raises apparent Km, Vmax unchanged (can be overcome by more substrate).
  • Non-competitive inhibitor: lowers Vmax, Km unchanged.
  • Vitamin-derived coenzymes: NAD⁺ from niacin (B3), FAD from riboflavin (B2), CoA from pantothenic acid (B5), TPP from thiamine (B1).

Enzymes Practice Questions with Answers

Attempt each question first, then open the explanation. All 8 questions below are free to read and require no signup.

  1. Q1.Enzymes are chemically:

    Easy
    • ACarbohydrates
    • BLipids
    • CProteins (mostly)
    • DNucleotides
    +Show Answer & Explanation

    Answer: C. Proteins (mostly)

    Explanation: Nearly all enzymes are proteins; the exceptions are ribozymes, which are catalytic RNA molecules.

    Enzymes question 1 of 8

  2. Q2.Which enzyme breaks down starch into maltose?

    Easy
    • ALipase
    • BPepsin
    • CAmylase
    • DTrypsin
    +Show Answer & Explanation

    Answer: C. Amylase

    Explanation: Salivary and pancreatic amylase hydrolyse the α-1,4 bonds of starch.

    Enzymes question 2 of 8

  3. Q3.The Michaelis constant Km represents:

    Moderate
    • AMaximum reaction velocity
    • BSubstrate concentration at half Vmax
    • CEnzyme concentration
    • DActivation energy
    +Show Answer & Explanation

    Answer: B. Substrate concentration at half Vmax

    Explanation: Km is the substrate concentration giving half-maximal velocity and is inversely related to enzyme–substrate affinity.

    Enzymes question 3 of 8

  4. Q4.A competitive inhibitor affects enzyme kinetics by:

    Difficult
    • AIncreasing Km with Vmax unchanged
    • BDecreasing Vmax with Km unchanged
    • CDecreasing both Km and Vmax
    • DIncreasing Vmax
    +Show Answer & Explanation

    Answer: A. Increasing Km with Vmax unchanged

    Explanation: It competes for the active site, so more substrate is needed for half-maximal rate (higher apparent Km), but the maximum rate is still reachable.

    Enzymes question 4 of 8

  5. Q5.Enzymes accelerate reactions by:

    Easy
    • ARaising the activation energy
    • BLowering the activation energy
    • CChanging the equilibrium constant
    • DIncreasing the temperature
    +Show Answer & Explanation

    Answer: B. Lowering the activation energy

    Explanation: Enzymes stabilise the transition state, lowering activation energy. They do not shift the equilibrium position.

    Enzymes question 5 of 8

  6. Q6.The coenzyme NAD⁺ is derived from which vitamin?

    Moderate
    • AThiamine (B1)
    • BRiboflavin (B2)
    • CNiacin (B3)
    • DPyridoxine (B6)
    +Show Answer & Explanation

    Answer: C. Niacin (B3)

    Explanation: NAD⁺ and NADP⁺ derive from niacin; FAD derives from riboflavin.

    Enzymes question 6 of 8

  7. Q7.Which enzyme in gastric juice digests proteins?

    Easy
    • APepsin
    • BAmylase
    • CLipase
    • DLactase
    +Show Answer & Explanation

    Answer: A. Pepsin

    Explanation: Pepsin, secreted as pepsinogen and activated by gastric acid, hydrolyses proteins into peptides.

    Enzymes question 7 of 8

  8. Q8.The optimum pH for pepsin activity is approximately:

    Moderate
    • A2
    • B5
    • C7
    • D9
    +Show Answer & Explanation

    Answer: A. 2

    Explanation: Pepsin works in the highly acidic stomach, with peak activity near pH 2. Trypsin, by contrast, peaks near pH 8.

    Enzymes question 8 of 8

Enzymes — Frequently Asked Questions

What does the Michaelis constant Km tell you?+

It is the substrate concentration at which the reaction runs at half its maximum velocity. A low Km means the enzyme reaches half-maximal speed at low substrate levels — that is, it has a high affinity for the substrate.

How do competitive and non-competitive inhibitors differ?+

A competitive inhibitor binds the active site and can be outcompeted by extra substrate, so Km rises but Vmax is unchanged. A non-competitive inhibitor binds elsewhere and cannot be outcompeted, so Vmax falls while Km stays the same.

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