Biochemistry Carbohydrate Chemistry and Metabolism 2 — Questions and Answers
Question 1: Phosphofructokinase-1 (PFK-1) is the key regulatory enzyme of glycolysis. Which molecule allosterically activates it?
- ATP
- Citrate
- AMP (Correct answer)
- Acetyl-CoA
Correct answer: AMP
AMP (and ADP) allosterically activates PFK-1, signaling low energy status in the cell and stimulating glycolysis to increase ATP production.
Question 2: Pyruvate dehydrogenase complex (PDC) converts pyruvate to acetyl-CoA with release of:
- CO₂ and NADH (Correct answer)
- CO₂ and FADH₂
- H₂O and NADPH
- CO₂ and ATP
Correct answer: CO₂ and NADH
PDC catalyzes the oxidative decarboxylation of pyruvate: pyruvate + CoA + NAD⁺ → acetyl-CoA + CO₂ + NADH, using five cofactors including thiamine pyrophosphate.
Question 3: Under anaerobic conditions in muscle, pyruvate is converted to lactate to regenerate:
- ATP
- FADH₂
- NAD⁺ (Correct answer)
- Acetyl-CoA
Correct answer: NAD⁺
Lactate dehydrogenase reduces pyruvate to lactate, oxidizing NADH back to NAD⁺, which is required for glycolysis to continue producing ATP in the absence of oxygen.
Question 4: The pentose phosphate pathway (PPP) primarily generates which two products needed for biosynthesis?
- ATP and NADH
- NADPH and ribose-5-phosphate (Correct answer)
- Acetyl-CoA and oxaloacetate
- Pyruvate and lactate
Correct answer: NADPH and ribose-5-phosphate
The oxidative PPP produces NADPH for reductive biosynthesis and antioxidant defense, and ribose-5-phosphate for nucleotide synthesis.
Question 5: Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. Which of the following is NOT a gluconeogenic precursor?
- Lactate
- Glycerol
- Oxaloacetate
- Acetyl-CoA (Correct answer)
Correct answer: Acetyl-CoA
Acetyl-CoA cannot be converted to net glucose in mammals because the two carbons entering the TCA cycle as acetyl-CoA are lost as CO₂ before the cycle can regenerate oxaloacetate.
Question 6: Von Gierke disease (Type I glycogen storage disease) is caused by deficiency of:
- Glycogen phosphorylase
- Glucose-6-phosphatase (Correct answer)
- Debranching enzyme
- Glycogen synthase
Correct answer: Glucose-6-phosphatase
Glucose-6-phosphatase deficiency in Von Gierke disease prevents the liver from releasing free glucose from glycogen (or gluconeogenesis), causing severe fasting hypoglycemia.
Biochemistry Carbohydrate Chemistry and Metabolism 2
Phosphofructokinase-1 (PFK-1) is the key regulatory enzyme of glycolysis.
Which molecule allosterically activates it?