D. occurs in the cytosol.
E. requires the coenzyme biotin.
Correct answer = A. Lipoic acid is an intermediate acceptor of the acetyl group formed in the reaction. [Note: Lipoic acid linked to a lysine residue in E2 functions as a “swinging arm” that allows interaction with E1 and E3.] The PDHC catalyzes an irreversible reaction that is inhibited when the decarboxylase component (E1) is phosphorylated. The PDHC is located in the mitochondrial matrix. Biotin is utilized by carboxylases, not decarboxylases.
.2. Which one of the following conditions decreases the oxidation of acetyl coenzyme A by the citric acid cycle?
A. A high availability of calcium
B. A high acetyl CoA/CoA ratio
C. A low ATP/ADP ratio
D. A low NAD+/NADH ratio
Correct answer = D. A low NAD+/NADH (oxidized to reduced nicotinamide adenine dinucleotide) ratio limits the rates of the NAD+-requiring dehydrogenases. High availability of calcium and substrate (acetyl coenzyme
A) and a low ATP/ADP (adenosine tri-to diphosphate) ratio stimulate the cycle.
.3. The following is the sum of three steps in the citric acid cycle. A + B + FAD + H2O → C + FADH2 + NADH
Choose the lettered answer that corresponds to the missing “A,” “B,” and “C” in the equation.
Correct answer = B. Succinate + NAD+ + FAD + H2O → oxaloacetate + NADH + FADH2.
.4. A 1-month-old male shows neurologic problems and lactic acidosis. Enzyme assay for pyruvate dehydrogenase complex (PDHC) activity on extracts of cultured skin fibroblasts showed 5% of normal activity with a low concentration of thiamine pyrophosphate (TPP) but 80% of normal activity when the assay contained a thousand-fold higher concentration of TPP. Which one of the following statements concerning this patient is correct?
A. Administration of thiamine is expected to reduce his serum lactate level and improve his clinical symptoms.
B. A high-carbohydrate diet would be expected to be beneficial for this patient.
C. Citrate production from aerobic glycolysis is expected to be increased.
D. PDH kinase, a regulatory enzyme of the PDHC, is expected to be active.
Correct answer = A. The patient appears to have a thiamine-responsive PDHC deficiency. The pyruvate decarboxylase (E1) component of the PDHC fails to bind thiamine pyrophosphate at low concentration but shows significant activity at a high concentration of the coenzyme. This mutation, which affects the Km (Michaelis constant) of the enzyme for the coenzyme, is present in some, but not all, cases of PDHC deficiency. Because the PDHC is an integral part of carbohydrate metabolism, a diet low in carbohydrates would be expected to blunt the effects of the enzyme deficiency. Aerobic glycolysis generates pyruvate, the substrate of the PDHC. Decreased activity of the complex decreases production of acetyl coenzyme A, a substrate for citrate synthase. Because PDH kinase is allosterically inhibited by pyruvate, it is inactive.
.5. Which coenzyme–cosubstrate is used by dehydrogenases in both glycolysis and the tricarboxylic acid cycle?