Chapter 18, Problem 18P

Distinguishing the Mechanisms of Class I and Class I Aldolases

Fructose bisphosphate aldolase in animal muscle is a class 1 aldolase, which forms a Schiff base intermediate between substrate (for example. fructose-1, 6-bisphosphate or dihydroxyacetone phosphate) and a lysine at the active site (see Figure I8.12). The chemical evidence for this intermediate conies from studies with aldolase and the reducing agent sodium borohydride, NaBH₄. Incubation of the enzyme with dihydroxyacetone phosphate and NaBH₄ inactivates the enzyme. Interestingly, no inactivation is observed if NabH₄ is added to the enzyme in the absence of substrate. Write a mechanism that explains these observations and provides evidence for the formation of a Schiff base intermediate in the aldolase reaction.