Biochemistry — MCQ Practice Questions & Answers

Question 1

A protein with multiple subunits shows cooperative binding of substrate. This phenomenon is best explained by:

Accepted Answer

Correct answer: B. Allosteric regulation — Cooperative binding (positive cooperativity) occurs when binding of one substrate molecule increases affinity for subsequent molecules, a classic example of allosteric regulation as seen in hemoglobin.

Answer

A. Competitive inhibition

Answer

C. Non-competitive inhibition

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D. Covalent modification

Question 2

During protein synthesis, a missense mutation changes codon GAA to GUA. Which amino acid substitution occurs?

Accepted Answer

Correct answer: A. Glutamic acid to Valine — GAA codes for Glutamic acid (Glu), GUA codes for Valine (Val). This is a non-conservative substitution of a charged to hydrophobic residue.

Answer

B. Aspartate to Leucine

Answer

C. Glutamine to Isoleucine

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D. Glutamic acid to Alanine

Question 3

In a double displacement enzyme reaction mechanism, the enzyme forms a covalent intermediate. Which protease follows this mechanism?

Accepted Answer

Correct answer: B. Serine proteases — Serine proteases (like trypsin, chymotrypsin) form an acyl-enzyme intermediate through a nucleophilic attack by the active site serine residue.

Answer

A. Pepsin

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C. Metalloproteases

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D. Cysteine proteases

Question 4

A competitive inhibitor with Ki = 0.5 mM and Km = 2 mM is added to an enzyme reaction. What is the apparent Km in presence of this inhibitor at [I] = 1 mM?

Accepted Answer

Correct answer: C. 6 mM — Apparent Km = Km(1 + [I]/Ki) = 2(1 + 1/0.5) = 2(1 + 2) = 6 mM. Competitive inhibition increases apparent Km without changing Vmax.

Answer

A. 2 mM

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B. 4 mM

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D. 8 mM

Question 5

Which statement about the relationship between protein structure and function is INCORRECT?

Accepted Answer

Correct answer: B. Tertiary structure can exist without secondary structure — Tertiary structure requires secondary structure elements. Secondary structures (α-helix, β-sheet) fold into tertiary structure; one cannot exist without the other.

Answer

A. Primary structure determines all higher-order structures

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C. Quaternary structure requires multiple polypeptide chains

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D. Disulfide bonds stabilize tertiary structure in extracellular proteins

Question 6

A protease enzyme shows reduced activity when Ca²⁺ is removed from the reaction mixture. This indicates that Ca²⁺ acts as a:

Accepted Answer

Correct answer: C. Cofactor — Ca²⁺ is required for enzymatic activity and is not consumed in the reaction, making it a cofactor. Many proteases require metal ions as structural or catalytic cofactors.

Answer

A. Substrate

Answer

B. Allosteric regulator

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D. Competitive inhibitor

Question 7

In non-competitive inhibition, what is the graphical representation on a Lineweaver-Burk plot?

Accepted Answer

Correct answer: A. Lines intersect on the y-axis — In non-competitive inhibition, both Km and Vmax are affected proportionally. On a Lineweaver-Burk plot (1/v vs 1/[S]), this results in lines with different slopes that intersect on the y-axis.

Answer

B. Lines intersect on the x-axis

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C. Lines intersect to the left of the y-axis

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D. Parallel lines

Question 8

In the Michaelis-Menten equation, what does Km represent when Km >> [S]?

Accepted Answer

Correct answer: C. The reaction is first-order with respect to substrate concentration — When Km >> [S], the Michaelis-Menten equation simplifies to v = (Vmax/Km)[S], making the reaction essentially first-order. The enzyme has low affinity for substrate under these conditions.

Answer

A. The enzyme is saturated

Answer

B. The reaction rate is zero-order with respect to substrate

Answer

D. The enzyme has very high affinity for substrate

Biochemistry - MCQ Practice Questions