Biochemistry - MCQ Practice Questions

Enzyme specificity varies: absolute (one substrate only), group (substrates with similar functional groups), linkage (specific types of bonds), and stereochemical (stereoisomers). Specificity results from 3D active site structure and orientation of catalytic residues.

The Cori cycle (glucose-lactate cycle) allows muscles undergoing anaerobic glycolysis to produce lactate, which is transported to liver and converted back to glucose via gluconeogenesis, maintaining blood glucose homeostasis during intense exercise.

Sigmoidal (S-shaped) kinetics indicate positive cooperativity, typical of allosteric enzymes with multiple subunits (e.g., aspartate transcarbamoylase). Binding of substrate to one subunit increases affinity in others.

E3 ubiquitin ligases catalyze the attachment of ubiquitin chains (primarily through Lys48 linkages) to lysine residues on target proteins. The 19S proteasomal subunit recognizes these polyubiquitin chains and unfolds the protein for degradation.

Substituting a nonpolar residue with a charged, hydrophilic one in the protein core disrupts critical hydrophobic interactions that stabilize the tertiary structure, leading to misfolding, aggregation, or degradation.

Proline is an imino acid with its side chain bonded to the backbone nitrogen, eliminating the NH group needed for β-sheet hydrogen bonding between strands. High proline content disrupts β-sheet formation, commonly found in turns and loops instead.

In non-competitive inhibition, the inhibitor binds to an allosteric site on both E and ES, preventing product formation. This decreases Vmax (fewer active enzymes) while Km remains unchanged (substrate binding affinity unaffected).