Question

You are carrying out an experiment in the lab to study the citric acid cycle. To do this, you add 14C-labeled pyruvate at carbon 3 (this is the methyl carbon) to metabolically active mitochondria. Based on your knowledge of the citric acid cycle, what is the location of the 14C-label in oxaloacetate after one turn of the cycle? How many turns of the cycle are required to release all of the14C-label as CO2? Be sure to justify your answers by drawing out the intermediates of the citric acid cycle.

Answer 1

1) Pyruvate (C14 carbon at methyl position) is the end product of glycolysis, and in order to enter the next pathway, pyruvate first get converted into Acetyl Co A and Acetyl CoA further converted to oxaloacetate, which starts the tri carboxylic acid (TCA) or Krebs cycle.

The conversation of Acetyl CoA to Pyruvate known as Pyruvate decarboxylation reaction and carried out by Pyruvate dehydrogenase complex (PDC) is a complex of three enzymes.

According to IUPAC nomenclature system, carbon of methyl group in pyruvate is 3^rd carbon in pyruvate molecule, and after converting to acetyl Co A it will still remain in the 3^rd position. But when OAA reacts with acetyl Co A and get converted into citrate then C14 carbon of pyruvate methyl group get on the position of 3nd carbon in the 6carbon compound citrate, after completion of one turn of TCA cycle, c14 methyl carbon of pyruvate, get converted into the keto group and it became the 2^nd Carbon, in 4carbon containing compound OAA.