15. Which of the following would increase V max in an enzyme-catalyzed reaction?
(A) Increasing substrate concentration.
(B) Decreasing product concentration.
(C) Increasing enzyme concentration.
(D) Decreasing Km.
(E) Decreasing Keat.

16. Which of the following statements best describes a competitive inhibitor? (A) It binds to the enzyme-substrate complex and increases Km without affecting Vmax. (B) It binds to the enzyme-substrate complex and decreases both Km and Vmax. (C) It binds to the active site and increases Km without affecting Vmax. (D) It binds to the active site and decreases both Km and Vmax. (E) It reacts with an active site residue and binds irreversibly to the enzyme.

17. An enzyme-catalyzed reaction has the following parameters:Km= 10 mMVmax = 200 µmol/minWhat is the initial rate (Vo) ofthis reaction when substrate concentration [S] = 10 mM(A) 50 µmol/min(B) 100 µmol/min(C) 133 µmol/min(D) 200 µmol/min(E) 10 µmol/min

18. An enzyme shows sigmoidal kinetics, meaning that the plot of initial velocity (Vo) versus substrate concentration ([S])displays an S-shaped curve. What does this suggest about the enzyme?(A) It binds a single substrate molecule and follows first-order kinetics.(B) It undergoes irreversible inactivation during catalysis.(C) It exhibits cooperative binding, often due to allosteric interactions between subunits.(D) It becomes saturated with substrate only at very high temperatures.(E) It is competitively inhibited at high substrate concentrations.

19.Which of the following amino acid substitutions is most likely to disrupt the formation of a 'heptad repeat' motif in a leucinezipper protein, assuming the substitution occurs at the 'a'or 'd' position of the (abcdefg)n sequence?(A) Leucine to Isoleucine.(B) Leucine to Valine.(C) Leucine to Aspartate.(D) Leucine to Methionine.(E) Leucine to Alanine.

20.A purified peptide was treated with Cyanogen Bromide (CNBr), resulting in two fragments. Treatment of the original peptide with Trypsin yielded three fragments. Chymotrypsin treatment resulted in no cleavage. Recall the specific cleavage sites for Trypsin (positively charged side chains) and Chymotrypsin (aromatic side chains). Which sequence is consistent with these findings? (A) Met-Gly-Phe-Lys-Ala-Gly (B) Ala-Met-Lys-Arg-Gly-Gly (C) Arg-Lys-Met-Tyr-Ser-Gly (D) Gly-Met-Arg-Lys-Trp-Gly (E) Lys-Met-Phe-Arg-Ala-Gly

21. In the context of Hemoglobin (Hb) allostery, the 'Perutz Mechanism' describes the transition from the T-state to the R-state.Which molecular event triggers the movement of the F-helix?(A) The formation of salt bridges between carbamylated N-termini.(B) The release of 2,3-BPG from the central cavity.(C)The dissociation of the alpha-beta dimers into monomers.(D) The transition of the heme iron from high-spin to low-spin state upon oxygen binding.(E) The expulsion ofwater molecules from the distal heme pocket.

22. G-protein coupled receptors (GPCRs) utilize different mechanisms for desensitization. What is the specific role of beta-arrestin in the termination of GPCR signaling? (A) It hydrolyzes cAMP to 5'-AMP to reduce second messenger levels. (B) It acts as a Guanine Nucleotide Exchange Factor (GEF) for the alpha-subunit. (C) It dephosphorylates the receptor to restore its basal state. (D) It sterically delays G-protein coupling and facilitates clathrin-mediated endocytosis. (E) It phosphorylates the Ga subunit to accelerate GTP hydrolysis.

23. Steroidogenesis involves the conversion of cholesterol into various hormones. Which intermediate serves as the universal precursor for all classes of steroid hormones? (A) Mevalonate (C6H1204). (B) Squalene (C30H50). (C) Pregnenolone (C21 H3202). (D) Lanosterol (C30H50О). (E) Progesterone (C21H3002).

24. Which of the following statements regarding purine nucleotide biosynthesis is INCORRECT?(A) Phosphoribosylation of hypoxanthine leads to the formation of IMP.(B) Phosphorylation of purine nucleotides, catalyzed by adenosine phosphoribosyltransferase,dAMP.(C) IMP can be converted into both AMP and GMP.(D) Salvage pathways require significantly less energy than de novo synthesis.(E) None of the above.

25. Which ofthe following statements about sphingomyelins is INCORRECТ?converts deoxyadenosine to(A) They are present in high concentrations in the myelin sheath surrounding nerve fibers.(B) Sphingomyelins contain glycerol and, upon hydrolysis, yield a fatty acid, phosphoric acid, choline, and sphingosine.(C) They are believed to play a role in apoptosis.(D) All of the above are correct.(E) None of the above.

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