Thioester Thiohemiacetal

Kozarich and Chari suggest that the enzymic reaction most likely involves a cis-enediol intermediate, on the basis of two observations. First, the enzyme has the surprising ability to catalyze the stereospecific conversion of the thiohemiacetal (11), formed from (glutathiomethyl)glyoxal (10) and j8-mercaptoethanoI, to the thioester 12, established to have the (8)-configuration at C-2 on the basis of chemical degradation to L-lactate (111) [Eq. (24)] ... 

Two mechanisms for the NAD -dependent oxidation of an alcohol to a carboxylate have been characterized in enzymatic reactions. In the first mechanism, an active-site cysteine plays a crucial role in the reaction. A hydride is transferred to NAD from the alcohol substrate to generate an aldehyde intermediate, then the cysteine thiolate attacks the aldehyde to form a thiohemiacetal intermediate. The thiohemiacetal is oxidized by the second NAD" " to form a thioester, which is hydrolyzed to generate the carboxylate product. The second mechanism is similar to the first, except that the aldehyde undergoes hydration instead of thiohemiacetal formation. The aldehyde hydrate is oxidized by NAD" " to form the observed product. This reaction proceeds... 

Glyceraldehyde phosphate dehydrogenase probably holds the distinction of being the classic thiol enzyme in the minds of most biochemists . The thiol is believed to be involved in the initial attachment of the aldehyde substrate as a thiohemiacetal. The em me-bound thiohemiacetal is then oxidized by NAD generating an enzyme-bound thioester. In more sophisticated proposals for this mechanism the nicotinamide cofactor interacts with the active centre thiol as a charge transfer type of complex. This facilitates the reaction of the thiol with the carbonyl of the substrate. The thiol addition and the electron transfer to nicotinamide occur... 

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