Biochemistry - MCQ Practice Questions

Serine is an amino acid, not a cofactor. Cofactors include metal ions and coenzymes like NAD⁺, FAD, and CoA.

Km (Michaelis constant) represents the substrate concentration at which enzyme velocity is half of Vmax, indicating substrate affinity.

In competitive inhibition, the inhibitor competes with substrate for the active site and can be overcome by increasing substrate concentration.

Peptide bond formation is part of protein synthesis and structure, not denaturation. Denaturation involves disruption of secondary and tertiary structures.

Leucine, Isoleucine, and Valine are branched-chain hydrophobic amino acids typically buried in the interior of folded proteins.

Quaternary structure involves interactions between different polypeptide chains, not covalent peptide bonds. Only non-covalent interactions maintain quaternary structure.

Extreme pH causes ionization of amino acid side chains and disruption of protein structure, leading to denaturation and loss of enzymatic activity.

Lipase catalyzes the hydrolysis of ester bonds in lipids. Amylase acts on carbohydrates, protease on proteins, and nuclease on nucleic acids.

Allosteric enzymes have regulatory sites distinct from active sites and show cooperativity, resulting in sigmoidal kinetics rather than hyperbolic kinetics seen in simple enzymes.

In competitive inhibition, the inhibitor competes with substrate for the active site. The apparent Km increases (appears to require more substrate to reach Vmax), but true Vmax remains unchanged because the inhibitor can be outcompeted at high substrate concentrations.

An alpha-helix makes 3.6 residues per turn. Each C=O of residue n forms a hydrogen bond with the N-H of residue n+4, resulting in approximately 4 hydrogen bonds per turn.

Aldolase requires a Zn²⁺ cofactor as part of its active site, which is crucial for substrate binding and catalysis in aldol condensation reactions.

Molecular chaperones like Hsp70 and Hsp90 help nascent proteins fold into their correct three-dimensional structure and prevent inappropriate aggregation, essential for cellular proteostasis.

Digestive enzymes are synthesized as inactive zymogens (e.g., pepsinogen, trypsinogen) and are activated by proteolytic cleavage in the appropriate compartments (stomach, small intestine).

Heat or chemical denaturants disrupt hydrogen bonds and hydrophobic interactions, affecting secondary (alpha-helix, beta-sheet) and tertiary (3D fold) structures before affecting primary structure (peptide bonds).

Disulfide bonds (S-S) between cysteine residues form cross-links that stabilize the tertiary structure (within a protein) and quaternary structure (between subunits), particularly important in extracellular proteins.

Valine, with its branched nonpolar side chain, frequently appears in beta-sheets where it can form hydrophobic interactions and van der Waals contacts that stabilize the sheet structure.

Aconitase requires an iron-sulfur cluster [4Fe-4S] for catalyzing the isomerization of citrate to isocitrate. This is essential for its catalytic mechanism in the TCA cycle.

Competitive inhibitors compete with substrate for the active site, increasing apparent Km while Vmax remains unchanged. Non-competitive inhibitors bind to a site other than the active site, decreasing Vmax without changing Km.

Allosteric enzymes like phosphofructokinase show cooperative binding where substrate binding at one subunit increases affinity at others. This produces an S-shaped curve rather than the hyperbolic Michaelis-Menten curve, allowing for better metabolic control.