Question

What would be the products of the reaction, if aldolase used glucose-6-phosphate instead of fructose-1,6-bisphosphate? Based on your answer, can you suggest a possible rationale for why nature chose to include two extra steps prior to the aldolase step?

Answer 1

Based on the chemical requirements of an aldol cleavage, you must cleave such that the“carbanion” ends up added to the C=O, which would now be the aldehyde instead of the ketone.Therefore, the products would be asymmetrical (two different products; one with two carbons, one with four carbons) if isomerization to and phosphorylation of F-6-P did not occur.You might then need twice as many enzymes to break down the two different “halves” instead of having one common pathway.Also, the 2-carbon unit would not have a phosphate attached, and therefore be prone to diffusion out of the cell.