Enzymes & Bioenergetics MCQs

Biochemistry · 22 free questions with answers & explanations.

1. A 4-year-old child presents with recurrent episodes of vomiting, lethargy, and hypoglycemia triggered by fasting. Enzyme assay shows that adding the substrate at increasing concentrations does not increase the reaction rate beyond a plateau, and a Lineweaver-Burk plot reveals that adding a fixed amount of an inhibitor shifts the y-intercept upward while leaving the x-intercept unchanged. Which type of inhibition is demonstrated?
2. Succinate dehydrogenase catalyzes the conversion of succinate to fumarate in the TCA cycle. Malonate, a structural analogue of succinate, strongly inhibits this enzyme. When the concentration of succinate is progressively increased in the presence of a fixed amount of malonate, the inhibition is overcome and maximum velocity is eventually restored. This pattern describes which of the following enzyme inhibition kinetics?
3. During a biochemistry practical, a student measures the rate of an enzyme-catalyzed reaction at several substrate concentrations and obtains a Km of 2 mM and a Vmax of 100 nmol/min. If the substrate concentration in the assay tube is exactly equal to the Km, what fraction of the enzyme's active sites are occupied by substrate at that moment?
4. A pharmaceutical company is developing an irreversible inhibitor targeting a serine protease involved in blood coagulation. The inhibitor forms a covalent bond with the serine residue in the active site. Enzymatic activity can only be restored by de novo synthesis of the enzyme protein. Which of the following best describes the mechanism by which this class of inhibitors acts?
5. Phosphofructokinase-1 (PFK-1), the key regulatory enzyme of glycolysis, displays sigmoidal kinetics with respect to its substrate fructose-6-phosphate. High concentrations of ATP allosterically inhibit PFK-1 by decreasing its affinity for the substrate. This regulatory behavior is best explained by which model of enzyme cooperativity?
6. An enzyme shows sigmoidal kinetics with Hill coefficient n=3. A homotropic allosteric activator shifts the curve leftward (lower S50). This activator most likely acts by:
7. Suicide (mechanism-based) inhibition occurs when an enzyme activates an inhibitor to form a reactive intermediate that covalently inactivates the enzyme. Which drug-enzyme pair exemplifies mechanism-based irreversible inhibition?
8. An enzyme shows a linear Eadie-Hofstee plot (V vs V/[S]) that passes through the origin with a slope of -Km and y-intercept of Vmax. In the presence of a competitive inhibitor, which change is observed in the Eadie-Hofstee plot?
9. Serum angiotensin-converting enzyme (ACE) is elevated in sarcoidosis. The biochemical basis for this is ACE being produced by which cell type that proliferates in sarcoid granulomas?
10. A competitive enzyme inhibitor reduces the apparent affinity of an enzyme for its substrate. In clinical pharmacology, which drug mechanism is an example of reversible competitive inhibition of a metabolic enzyme?
11. In Michaelis-Menten enzyme kinetics, an inhibitor is found to increase both Km and Vmax when present at increasing concentrations. This kinetic pattern is characteristic of:
12. Lactate dehydrogenase (LDH) consists of two subunit types: H (heart) and M (muscle). The LDH1 isoenzyme (H4) has higher affinity for lactate than LDH5 (M4). The CLINICAL implication of this difference is:
13. Ribosomal RNA (rRNA) contains self-splicing introns (ribozymes) that catalyse their own excision. The key feature that allows RNA to act as a catalyst is:
14. An enzyme inhibitor raises the Km (from 2 mM to 8 mM) without changing Vmax. Increasing substrate concentration overcomes the inhibition. This inhibitor is best characterised as:
15. A patient is found to have marked elevation of serum creatine kinase (CK) with CK-MB fraction 4% (reference <6%) but total CK of 12,000 U/L. Troponin I is normal. This CK pattern is most consistent with:
16. A graph of enzyme velocity (v) vs substrate concentration [S] for an allosteric enzyme shows a sigmoidal (S-shaped) curve rather than a hyperbolic curve. This sigmoidal kinetics indicates:
17. Covalent catalysis involves formation of a transient covalent intermediate between the enzyme and substrate. Serine proteases (chymotrypsin) use a catalytic triad. The role of the histidine residue in the catalytic triad (Asp-His-Ser) is:
18. An inhibitor increases the Km of an enzyme for its substrate without changing the Vmax. Increasing substrate concentration fully reverses the inhibition. This describes which type of inhibition?
19. Feedback inhibition is a key regulatory mechanism. Aspartate transcarbamoylase (ATCase) in bacteria is inhibited by CTP, the end product of pyrimidine biosynthesis, and is activated by ATP. ATCase is an example of which type of enzyme regulation?
20. In Michaelis-Menten kinetics, a competitive inhibitor increases the apparent Km without changing Vmax. The apparent Km is increased because:
21. Covalent modification by phosphorylation is a major regulatory mechanism for enzymes. Glycogen synthase is INACTIVATED by phosphorylation, whereas glycogen phosphorylase is ACTIVATED by phosphorylation. This reciprocal regulation ensures:
22. Succinylcholine (suxamethonium) causes prolonged apnoea in patients with pseudocholinesterase deficiency because: