Page - (000043) - in Biomedical Chemistry: Current Trends and Developments

1.1.4.7.2 Claisen Condensation Claisen condensation is the base-catalysed condensation reaction of an ester with another carbonyl compound taking place through the mechanism depicted in Scheme 1.1.28. Scheme 1.1.28: General mechanism for a Claisen condensation reaction. Firstly, the enolate is generated from an ester by α-deprotonation with a base. The enolate performs an acyl substitution followed by elimination of the alcoxyl group, giving a β-keto ester (Scheme 1.1.28). 1.1.4.7.3 Aldolases ‒ Stabilization Strategies In order to better perform aldol additions, enzymes create stabilized intermediate forms that provide a lower energy reaction path (Scheme 1.1.29). In the case of class II fructose-biphosphate aldolases, a zinc(II) ion is used to further polarize the carbonyl C=O bond of dihydroxyacetone phosphate (DHAP). The latter deprotonates easily, as the enolate intermediate is stabilized by the zinc ion. The enolate then attacks the carbonyl carbon of the glyceraldehyde 3-phosphate (GA3P or GAP), producing the addition product fructose biphosphate. This reaction is completely stereospecific as with all enzyme catalysed reactions this statement in parenthesis should be in the description of Scheme 1.1.29. The reverse reaction (hydrolysis) is also catalysed by another enzyme.