Biochemistry - MCQ Practice Questions

Trypsin is a serine protease that specifically recognizes and cleaves peptide bonds on the C-terminal side of positively charged amino acids (Lys and Arg).

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.

Enzyme specificity arises from the precise three-dimensional arrangement of amino acid residues in the active site, which determines substrate recognition and binding through complementary fit.

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.

Leucine is a nonpolar, hydrophobic amino acid that tends to cluster in the protein interior away from the aqueous environment, while polar and charged residues prefer the surface.

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.

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.

Amylase is a hydrolase enzyme that catalyzes the hydrolysis of alpha-1,4-glycosidic bonds in starch, converting it into sugars. Both salivary and pancreatic amylases perform this function.

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.

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

Km (Michaelis constant) is defined as the substrate concentration at which v = Vmax/2. It provides insight into enzyme-substrate affinity; lower Km indicates higher affinity.

Transferases catalyze the transfer of functional groups (e.g., methyl, phosphoryl, amino) from one substrate to another. Examples include kinases, methyltransferases, and transaminases.

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).

Negative cooperativity occurs when substrate binding to one subunit decreases the affinity of other subunits for substrate, resulting in a hyperbolic (rather than sigmoidal) binding curve.

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.

Turnover number (kcat) is the number of substrate molecules converted to product per enzyme molecule per unit time when the enzyme is fully saturated. It equals Vmax/[E]total.

Serine proteases have an extended substrate-binding site composed of multiple subsites (S1, S1', S2, etc.) that recognize and bind extended substrate peptides. The catalytic triad (Ser-His-Asp) performs the actual catalysis.

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.