Question

Cell in culture are fed glucose molecules labled with 14C at carbon 2. Trace the radioactive label through one pass through the pentose phosphate pathway?

Answer 1

This labeled atom 14C at carbon 2 will be part of all stages (all of the products) of the pentose phosphate pathway.

Oxidative phase: glucose-6-phosphate into 6-phosphoglucono-δ-lactone; 6-phosphoglucono-δ-lactone into  6-phosphogluconate;  6-phosphogluconate into ribulose 5-phosphate

Non-oxidative phase: ribulose 5-phosphate into ribose 5-phosphate; ribulose 5-phosphate into xylulose 5-phosphate; xylulose 5-phosphate +ribose 5-phosphate into glyceraldehyde 3-phosphate + sedoheptulose 7-phosphate; sedoheptulose 7-phosphate + glyceraldehyde 3-phosphate into erythrose 4-phosphate + fructose 6-phosphate; xylulose 5-phosphate + erythrose 4-phosphate into glyceraldehyde 3-phosphate + fructose 6-phosphate.

The scheme could be seen in the picture below.