Aldolase - gluconeogenesis direction

In aldol addition, the enolate serves as a nucleophile, with its α carbon attacking the electrophilic carbonyl carbon of the aldehyde or ketone form also present in solution. Aldol addition straddles both of the big categories of reactions of aldehydes and ketones. It involves both nucleophilic attack at the carbonyl carbon and electrophilic attack at the α-carbon.

The product of aldol addition is a β-hydroxy aldehyde or ketone, which readily undergoes dehydration to yield an αβ-unsaturated aldehyde or ketone. The complete mechanism, aldol addition with subsequent dehydration, is known as aldol condensation.

In biochemistry, many reactions of the aldol addition type actually involve a process which is directly analogous, imine-enamine tautomerism. Here, instead of a nucleophilic enolate anion attacking a carbonyl carbon, you have a nucleophilic imminium anion formed through imine-enamine tautomerism. In fact, everyone's favorite example of aldol addition in biochemistry, the aldolase mechanism from glycolysis and gluconeogenesis, actually involves nucleophlic attack by the iminium ion formed through enzyme-DHP Schiff base.