THE PRESENCE OF A HYDROXYL GROUP AT THE C-1 ATOM OF THE TRANSKETOLASESUBSTRATE MOLECULE IS NECESSARY FOR THE ENZYME TO PERFORM THE TRANSFERASE REACTION

Transketolase catalyzes the transfer of an aldehyde residue from keto sugars to aldo sugars. The intermediate product is dihydroxyethylthiam ine pyrophosphate (DHETPP). In the absence of an acceptor substrate, t he reaction is stopped at this stage and DHETPP does not undergo subse quent transformations, Pyruvate decarboxylase catalyses pyruvate decar boxylation to yield free aldehyde. The intermediate product is hydroxy ethylthiamine pyrophosphate (HETPP). It differs from DHETPP only in th at it has no hydroxyl at the C-2 atom of the aldehyde residue. We have shown that transketolase can bind HETPP and split the aldehyde residu e from it, This fact suggests that the path of the reaction is determi ned by the absence (in HETPP) or presence (in DHETPP) of a hydroxyl gr oup. In the former case the reaction will yield free aldehyde, in the latter the aldehyde residue will be transferred onto an acceptor subst rate.