Glucose can be labeled at specific positions with a radiolabeled 14C carbon. Pre

Question

Glucose can be labeled at specific positions with a radiolabeled 14C carbon. Predict the distribution of the 1*C label in pyruvate if the following labeled glucose substrates are incubated with all of the enzymes of glycolysis. Offer a clear rationale for your prediction. (You can assume that the rate of the reaction catalyzed by triose phosphate isomerase is much faster than the rate of the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase) A. B. C. Glucose is labeled at C-1 with 14C Glucose is labeled at C-4 with 14C Glucose is labeled at C-6 with 14C

Explanation / Answer

Glycolysis is degradation of glucose to pyruvate by assitance from various enzymes

Tracing the labels in glycolysis is done to study the mechanism of the glycolysis reaction pathway

A. Glucose is labelled at C-1 with 14C

Glucose breaks down into dihydroxyacetone phosphase (DHAP) and D-glyceraldehyde-3-phosphate (GADP) both of which are interchangeble isomer. This gets transformed into phosphoenolpyruvate (PEP). PEP upon reaction with pyruvate kinase enzyme forms pyruvate. Here the C-1 has become the terminal methyl group. There fore when glucose is labelled at C-1 position by 14C, the label ends up in Methyl group of pyruvate product. Note, C-1 and C-6 are equivalent due to DHAP and GADP isomerization.

B. Glucose is labelled at C-4 with 14C

When glucose is labelled at C-4 position. It forms the carboxyl group of the carboxylic acid in phosphoenolpyruvate (PEP). This thus remains as such in the end product pyruvate. The label thus would be at carbonyl carbon of pyruvate.

C. Glucose is labelled at C-6 with 14C

As seen in A, glucose breaks down into GADP, which then converts to pyruvate. The 14C labelled carbon at C-6 now moves to become the terminal methyl group in pyruvate product. This is similar to the one we have seen in case whne glucose is labelled at the C-1 carbon.

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