Shortly after adding

$$^{14}C-labeled$$

glyceraldehyde-3-phosphate to a yeast extract, fructose-1,6-bisphosphate labeled at C-3 and C-4 can be isolated. Where was the

$$^{14}C-label$$

in glyceraldehyde-3-phosphate?

Fructose-1,6-biphosphate in the presence of aldolase is dissociated into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. Dihydroxyacetone phosphate can be converted to glyceraldehyde-3-phosphate. The C-3 and C-4 are present on two different glyceraldehyde-3-phosphate molecules. The labeled carbons in fructose-1,6-bisphosphate were at C-3 and C-4 positions, so the $^{14}$C-label was present at the carboxyl group of the glyceraldehyde-3-phosphate molecule.