CBSE BIOCHEM Exam With
Complete Questions And Answers |
2026/2027 | Graded A+
1. What is the most common cause of hyperhomocysteinemia?
Correct Answer: Cystathionine beta-synthase deficiency, leading to
homocystinuria. It presents with intellectual disability, ectopia lentis (downward
lens dislocation), and thromboembolic events.
2. What is the function of the enzyme phenylalanine hydroxylase, and what is
the consequence of its deficiency?
Correct Answer: It converts phenylalanine to tyrosine. Its deficiency causes
phenylketonuria (PKU), leading to intellectual disability, fair skin, eczema, and a
musty body odor.
3. What is the treatment for classic PKU?
Correct Answer: Dietary restriction of phenylalanine and supplementation with
tyrosine.
4. What cofactor is required for the conversion of phenylalanine to tyrosine?
Correct Answer: Tetrahydrobiopterin (BH4).
5. What is alkaptonuria, and what enzyme is deficient?
Correct Answer: A deficiency of homogentisate 1,2-dioxygenase in the tyrosine
degradation pathway, leading to accumulation of homogentisic acid.
Manifestations include ochronosis (bluish-black connective tissue) and arthritis.
6. What are the four major classes of enzyme cofactors derived from vitamin B6
(pyridoxine)?
,Correct Answer: Pyridoxal phosphate (PLP) is a cofactor for transaminases (ALT,
AST), decarboxylases, glycogen phosphorylase, and cystathionine synthase.
7. What is the function of glycogen phosphorylase, and what is its activator?
Correct Answer: It cleaves glucose-1-phosphate from glycogen. It is activated by
AMP and phosphorylation.
8. What is the function of the pyruvate dehydrogenase complex?
Correct Answer: It converts pyruvate to acetyl-CoA in the mitochondria, linking
glycolysis to the TCA cycle.
9. What cofactors are required for the pyruvate dehydrogenase complex?
Correct Answer: Thiamine pyrophosphate (TPP), lipoic acid, FAD, NAD+, and CoA.
(Remember: "The PDH complex Tenderly Loves Flavin and NAD+ CoA").
10. What is the cause and presentation of thiamine (B1) deficiency?
Correct Answer: Causes beriberi (dry: neuropathy; wet: high-output heart failure)
and Wernicke-Korsakoff syndrome (confusion, ataxia, nystagmus, and memory
loss).
11. What is the function of alpha-ketoglutarate dehydrogenase?
Correct Answer: It converts alpha-ketoglutarate to succinyl-CoA in the TCA cycle.
It uses the same cofactors as the pyruvate dehydrogenase complex.
12. What is the rate-limiting enzyme of the TCA cycle?
Correct Answer: Isocitrate dehydrogenase.
13. What are the irreversible steps of the TCA cycle?
Correct Answer: The three irreversible steps are catalyzed by: (1) Citrate
synthase, (2) Isocitrate dehydrogenase, and (3) Alpha-ketoglutarate
dehydrogenase.
14. What is the function of malate-aspartate shuttle?
Correct Answer: It transfers reducing equivalents from NADH (from glycolysis)
into the mitochondrial matrix in the heart and liver, yielding 2.5 ATP per NADH.
, 15. What is the function of the glycerol-3-phosphate shuttle?
Correct Answer: It transfers reducing equivalents from NADH into the
mitochondria in skeletal muscle and brain, yielding 1.5 ATP per NADH via FADH2.
16. What is the net ATP yield from one molecule of glucose under aerobic
conditions?
Correct Answer: Approximately 30-32 ATP (2 from glycolysis, 2 from TCA, and 26-
28 from oxidative phosphorylation).
17. What is the net ATP yield from one molecule of glucose under anaerobic
conditions?
Correct Answer: 2 ATP (from glycolysis).
18. What is the function of the Cori cycle?
Correct Answer: Lactate produced by anaerobic glycolysis in muscles is
transported to the liver, where it is converted back to glucose via
gluconeogenesis.
19. What are the key enzymes of gluconeogenesis?
Correct Answer: Pyruvate carboxylase, PEP carboxykinase (PEPCK), fructose-1,6-
bisphosphatase, and glucose-6-phosphatase.
20. Where is glucose-6-phosphatase found?
Correct Answer: In the liver and kidneys, but not in muscles or the brain. This
allows the liver to release free glucose into the blood.
21. What is the function of hexokinase vs. glucokinase?
Correct Answer: Hexokinase is present in most tissues, has a high affinity (low
Km) for glucose, and is inhibited by G6P. Glucokinase is present in the liver and
pancreas, has a low affinity (high Km) for glucose, and is not inhibited by G6P,
allowing it to sense blood glucose levels.
22. What is the rate-limiting enzyme of glycolysis?
Correct Answer: Phosphofructokinase-1 (PFK-1). It is activated by AMP and
fructose-2,6-bisphosphate, and inhibited by ATP and citrate.
Complete Questions And Answers |
2026/2027 | Graded A+
1. What is the most common cause of hyperhomocysteinemia?
Correct Answer: Cystathionine beta-synthase deficiency, leading to
homocystinuria. It presents with intellectual disability, ectopia lentis (downward
lens dislocation), and thromboembolic events.
2. What is the function of the enzyme phenylalanine hydroxylase, and what is
the consequence of its deficiency?
Correct Answer: It converts phenylalanine to tyrosine. Its deficiency causes
phenylketonuria (PKU), leading to intellectual disability, fair skin, eczema, and a
musty body odor.
3. What is the treatment for classic PKU?
Correct Answer: Dietary restriction of phenylalanine and supplementation with
tyrosine.
4. What cofactor is required for the conversion of phenylalanine to tyrosine?
Correct Answer: Tetrahydrobiopterin (BH4).
5. What is alkaptonuria, and what enzyme is deficient?
Correct Answer: A deficiency of homogentisate 1,2-dioxygenase in the tyrosine
degradation pathway, leading to accumulation of homogentisic acid.
Manifestations include ochronosis (bluish-black connective tissue) and arthritis.
6. What are the four major classes of enzyme cofactors derived from vitamin B6
(pyridoxine)?
,Correct Answer: Pyridoxal phosphate (PLP) is a cofactor for transaminases (ALT,
AST), decarboxylases, glycogen phosphorylase, and cystathionine synthase.
7. What is the function of glycogen phosphorylase, and what is its activator?
Correct Answer: It cleaves glucose-1-phosphate from glycogen. It is activated by
AMP and phosphorylation.
8. What is the function of the pyruvate dehydrogenase complex?
Correct Answer: It converts pyruvate to acetyl-CoA in the mitochondria, linking
glycolysis to the TCA cycle.
9. What cofactors are required for the pyruvate dehydrogenase complex?
Correct Answer: Thiamine pyrophosphate (TPP), lipoic acid, FAD, NAD+, and CoA.
(Remember: "The PDH complex Tenderly Loves Flavin and NAD+ CoA").
10. What is the cause and presentation of thiamine (B1) deficiency?
Correct Answer: Causes beriberi (dry: neuropathy; wet: high-output heart failure)
and Wernicke-Korsakoff syndrome (confusion, ataxia, nystagmus, and memory
loss).
11. What is the function of alpha-ketoglutarate dehydrogenase?
Correct Answer: It converts alpha-ketoglutarate to succinyl-CoA in the TCA cycle.
It uses the same cofactors as the pyruvate dehydrogenase complex.
12. What is the rate-limiting enzyme of the TCA cycle?
Correct Answer: Isocitrate dehydrogenase.
13. What are the irreversible steps of the TCA cycle?
Correct Answer: The three irreversible steps are catalyzed by: (1) Citrate
synthase, (2) Isocitrate dehydrogenase, and (3) Alpha-ketoglutarate
dehydrogenase.
14. What is the function of malate-aspartate shuttle?
Correct Answer: It transfers reducing equivalents from NADH (from glycolysis)
into the mitochondrial matrix in the heart and liver, yielding 2.5 ATP per NADH.
, 15. What is the function of the glycerol-3-phosphate shuttle?
Correct Answer: It transfers reducing equivalents from NADH into the
mitochondria in skeletal muscle and brain, yielding 1.5 ATP per NADH via FADH2.
16. What is the net ATP yield from one molecule of glucose under aerobic
conditions?
Correct Answer: Approximately 30-32 ATP (2 from glycolysis, 2 from TCA, and 26-
28 from oxidative phosphorylation).
17. What is the net ATP yield from one molecule of glucose under anaerobic
conditions?
Correct Answer: 2 ATP (from glycolysis).
18. What is the function of the Cori cycle?
Correct Answer: Lactate produced by anaerobic glycolysis in muscles is
transported to the liver, where it is converted back to glucose via
gluconeogenesis.
19. What are the key enzymes of gluconeogenesis?
Correct Answer: Pyruvate carboxylase, PEP carboxykinase (PEPCK), fructose-1,6-
bisphosphatase, and glucose-6-phosphatase.
20. Where is glucose-6-phosphatase found?
Correct Answer: In the liver and kidneys, but not in muscles or the brain. This
allows the liver to release free glucose into the blood.
21. What is the function of hexokinase vs. glucokinase?
Correct Answer: Hexokinase is present in most tissues, has a high affinity (low
Km) for glucose, and is inhibited by G6P. Glucokinase is present in the liver and
pancreas, has a low affinity (high Km) for glucose, and is not inhibited by G6P,
allowing it to sense blood glucose levels.
22. What is the rate-limiting enzyme of glycolysis?
Correct Answer: Phosphofructokinase-1 (PFK-1). It is activated by AMP and
fructose-2,6-bisphosphate, and inhibited by ATP and citrate.
