2. Which enzyme of the Tricarboxylic acid cycle (TCA cycle) is directly regulated by the cellular NADH/NAD⁺ ratio?
(A) Citrate synthase
(B) Fumarase
(C) Succinyl‑CoA synthetase
(D) Isocitrate dehydrogenase

3. Which statement best explains why alanine aminotransferase (ALT) is more specific than aspartate aminotransferase (AST) for hepatocellular injury? (A) ALT is located exclusively in hepatocyte mitochondria (B) AST has a longer plasma half‑life than ALT (C) ALT is predominantly cytosolic and has limited expression in extrahepatic tissues (D) ALT requires pyridoxal phosphate, whereas AST does not

4. A mutation that deletes the operator sequence of the lac operon would most likely result in: (A) No transcription under any conditions (B) Transcription only when lactose is present (C) Constitutive transcription regardless of lactose availability (D) Transcription only when glucose is absent

5. An enzyme shows strong substrate specificity, yet mutation of a single active‑site residue leads to a dramatic increase in catalytic rate but a decrease in substrate affinity. Which interpretation best fits this observation? (A)The mutation stabilizes the enzyme–substrate complex but destabilizes the transition state (B)The mutation decreases the activation energy but weakens binding interactions in the active site (C)The mutation increases both kcat and KM by stabilizing the transition state (D)The mutation eliminates a key catalytic group but improves substrate recognition

6. Which of the following intermediates can not serve as the material for the synthesis of glucose via gluconeogenesis? (A) acetyl-CoA (B) Glycerol (C) Oxaloacetate (D) Glycerol

7. Which of the following is unique to eukaryotic DNA replication? (A) Bidirectional replication forks (B) Requirement of primers (C) Use of ligase to join Okazaki fragments (D) Multiple origins of replication per chromosome

8. Which of the following is able to release an ammonium ion? (A) Conversion of hydroxymethylbilane to uroporphyrinogen III by uroporphyrinogen III synthase (B) Conversion of serine to glycine by serine hydroxymethyltransferase (C) Conversion of glutamate to glutamine by glutamine synthetase (D) Conversion of cystathionine to cysteine and α-ketobutyrate by cystathionine γ-lyase

9. A researcher finds that an enzyme follows the induced fit model rather than the lock and key model. Which experimental observation best supports this conclusion? (A) The enzyme binds multiple substrates with similar affinities (B) Substrate binding causes a detectable conformational shift in enzyme structure (C) The enzyme exhibits a very low turnover number (D) The enzyme undergoes irreversible inactivation after substrate binding

10. A loss‑of‑function mutation in the lacZ gene would most directly cause: (A) Inability to convert lactose into glucose and galactose (B) Inability to transport lactose into the cell (C) Failure of the repressor to bind lactose (D) Reduced binding of CAP to DNA

11. Which step of β‑oxidation directly generates FADH₂? (A) Thiolysis by β‑ketothiolase (B) Hydration by enoyl‑CoA hydratase (C) Oxidation by acyl‑CoA dehydrogenase (D) Oxidation by β‑hydroxyacyl‑CoA dehydrogenase

12. Which of the following statements correctly distinguishes the mode of gene silencing by microRNA versus RNAi? (A) microRNA almost always leads to slicing of mRNA, whereas RNAi does not (B) RNAi is typically associated with perfect complementarity, whereas microRNA often represses translation through imperfect pairing (C) RNAi uses Dicer but microRNA does not (D) RNAi functions only in the nucleus

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