Biochemistry: The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Types of Reactions Catalyzed by Enzymes
ß-Cleavage and Condensation

In the preceding sections of this chapter, we examined substitution and addition reactions that typically involve nucleophiles containing O, N, or S. Bonds between carbon and these heteroatoms are generally readily cleaved under acid- or base-catalyzed conditions. The Cleavage and formation of C—C bonds, however, proceed with much greater difficulty, and the carbon skeletons of organic molecules are normally held together by strong bonds. Nevertheless, a vast number of complex, branched carbon compounds are continuously formed and broken down within living Cells.

The fundamental challenge in cleaving or forming carbon—carbon bonds lies in generating a nucleophilic center on a carbon atom. In most cases, such reactions are facilitated by the activating effect of a carbonyl group, which enables The formation of a Resonance-stabilized enolate anion.

To the same extent that the presence of a carbonyl group facilitates the cleavage of an adjacent C—H bond, this group can also promote the cleavage of a C—C bond:

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The most extensively studied reactions of this type are aldol cleavage (aldol retro-reaction) and the decarboxylation of ß-keto acids. The latter is known as ß-decarboxylation, and its reverse reaction as ß-carboxylation. We will use this term in a broader sense to include other reactions in which bonds between the α- and ß-carbon atoms of a carbonyl compound are broken or formed. Such reactions are referred to as ß-cleavage or ß-Condensation.

ß-Condensation reactions include substitution or addition processes in which an enzyme-bound enolate anion acts as a nucleophile. We can classify these condensation reactions into three categories, designated in Table 7-1 as types 5.A, 5.B, and 5.C.