USMLE-Rx Step 1 Practice Q's

USMLE-Rx Step 1 Qmax Challenge #1529

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USMLE-Rx Step 1 Qmax Challenge #1529A metabolic process is shown in the image.

Which intermediate in this process inhibits the rate-limiting enzyme of glycolysis and activates the rate-limiting enzyme of fatty acid synthesis?

A. A
B. B
C. C
D. D
E. E


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2 replies »

  1. The correct answer is B. Citrate (B), formed from oxaloacetate and acetyl-CoA by the enzyme citrate synthase, inhibits phosphofructokinase and allosterically activates acetyl-CoA carboxylase. Phosphofructokinase is a key regulatory step in glycolysis, converting fructose-6-phosphate to fructose-1,6-bisphosphate. Inhibition of phosphofructokinase by citrate slows the production of acetyl-CoA, which is required for the production of citrate in the tricarboxylic acid cycle. Acetyl-CoA carboxylase is the key regulatory step in fatty acid synthesis; it converts acetyl-CoA to malonyl-CoA. Allosteric activation of acetyl-CoA carboxylase by citrate will increase fatty acid synthesis, because excess citrate indicates a high level of acetyl-CoA. Acetyl-CoA carboxylase is also inactivated by phosphorylation.

    A is not correct. α-Ketoglutarate (A) is not an important regulator of the tricarboxylic acid cycle, but it is an important intermediate in protein metabolism.

    C is not correct. Malate (C) is not an important regulator of the tricarboxylic acid cycle, but it is important in the malate shuttle.

    D is not correct. Oxaloacetate (D) is not an important regulator of the tricarboxylic acid cycle, but it is important in gluconeogenesis.

    E is not correct. Succinyl-CoA (E) downregulates its own synthesis by inhibiting α-ketoglutarate dehydrogenase, the enzyme responsible for dehydrogenation of α-ketoglutarate.

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