4.In base-excision repair, the first enzyme to act is:
(A) AP endonuclease.
(B) DNA glycosylase.
(C) DNA polymerase.
(D) Dam methylase.
(E) DNA ligase.

5. In homologous recombination in E. coli, the protein that moves along a double-stranded DNA, unwinding the strands ahead of it and degrading them, is: (A) RuvC protein (resolvase). (B) RecBCD enzyme. (C) RecA protein. (D) DNA ligase. (E) chi.

6. Which enzyme in the urea cycle catalyzes the reaction that provides a direct metabolic link to the TCA (Citric Acid) cycle through the production of fumarate? (A) Ornithine transcarbamoylase (OТС). (B) Argininosuccinate synthase (ASS). (C) Argininosuccinate lyase (ASL). (D) Carbamoyl phosphate synthetase I (CPS1). (E) Arginase-1 (ARG1).

7. An enzyme requires an organic non-protein component to facilitate its catalytic mechanism. Ifthis organic molecule is very tightly or even covalently bound to the enzyme's structure and remains associated with it throughout the reaction cycle, it is most specifically classified as a: (A) Apoenzyme. (B) Prosthetic group. (C) Coenzyme. (D) Holoenzyme. (E) Metalloenzyme.

8. In the context of cellular physiology, which of the following transport mechanisms is specifically characterized by the use ofa pre-existing electrochemical gradient (such as a sodium gradient) to drive the movement of a solute against its own concentration gradient? (A) Primary active transport. (B) Simple diffusion. (C) Osmosis. (D) Facilitated diffusion. (E) Secondary active transport.

9. Phospholipids are the primary structural components of the cell membrane bilayer. Which of the following descriptions regarding the function of phospholipids in the membrane bilayer is the most accurate? (A) Phospholipids, through their amphipathic nature, spontaneously form a barrier that prevents the free movement of water-soluble substances in and out of the cell. (B) The primary function of phospholipids is to act as active transport pumps, directly consuming ATP to move ions across the membrane. (C) Phospholipids remain in a fixed, static state within the membrane to provide mechanical rigidity and replace the need for a cytoskeleton. (D) The main function of phospholipids is to act as enzymes that catalyze the translation and modification of membrane (E) proteins. Phospholipids use their hydrophilic tails to create channels in the membrane core to facilitate the simple diffusion of large polar molecules.

10. Pyridoxal phosphate (PLP) is one of the most versatile cofactors in human metabolism. Which of the following catalytic processes is PLP-dependent and involves the formation of a Schiff base intermediate with a lysine residue in the enzyme's active site? (A) The carboxylation of pyruvate to form oxaloacetate. (B) The oxidative deamination of glutamate to release free ammonia. (C) The transamination of amino acids to generate a-keto acids. (D) The activation of fatty acids for ẞ-oxidation. (E) The conversion of methyl malonyl-CoA to succinyl-CoA.

11. In biochemistry, amino acids are classified as glucogenic or ketogenic based on their degradation products. Which of the following pairs of amino acids are classified as "purely ketogenic," meaning their carbon skeletons can only be converted into acetyl-CoA or acetoacetyl-CoA and cannot be used for gluconeogenesis? (A) Alanine and Glycine. (B) Phenylalanine and Tyrosine. (C) Isoleucine and Threonine. (D) Leucine and Lysine. (E) Valine and Histidine.

12.Which of the following best describes the hydrogen bonding pattern in an a-helix?(A) Between adjacent side chains.(B) Between carbonyl oxygen of residue i and amide hydrogen of residue i+4.(C) Between carbonyl oxygen of residue i and amide hydrogen of residue i+1.(D) Between backbone atoms of adjacent ẞ-strands.(E) Between backbone atoms and water molecules.

13. In the folding process of globular proteins, hydrophobic side chains tend to:(A) Expose themselves to water for hydrogen bonding.(B) Cluster on the protein surface.(C) Remain randomly distributed.(D) Be buried in the protein core.(E) Be post-translationally modified.

14. Which of the following amino acid substitutions is most likely to disrupt protein function by destabilizing a protein's hydrophobic core? (A) Leucine to Isoleucine. (B) Isoleucine to Leucine. (C) Leucine to Methionine. (D) Leucine to Valine. (E) Leucine to Arginine.

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