Aldolase mechanism

An interesting case in the perspective of artificial enzymes for enantioselective synthesis is the recently described peptide dendrimer aldolases [36]. These dendrimers utilize the enamine type I aldolase mechanism, which is found in natural aldolases [37] and antibodies [21].These aldolase dendrimers, for example, L2Dl,have multiple N-terminal proline residues as found in catalytic aldolase peptides [38], and display catalytic activity in aqueous medium under conditions where the small molecule catalysts are inactive (Figure 3.8). As most enzyme models, these dendrimers remain very far from natural enzymes in terms ofboth activity and selectivity, and at present should only be considered in the perspective of fundamental studies. 

Antibody Catalysis. Recent advances in biocatalysis have led to the generation of catalytic antibodies exhibiting aldolase activity by Lemer and Barbas. The antibody-catalyzed aldol addition reactions display remarkable enantioselectivity and substrate scope [18]. The requisite antibodies were produced through the process of reactive immunization wherein antibodies were raised against a [Tdiketone hapten. During the selection process, the presence of a suitably oriented lysine leads to the condensation of the -amine with the hapten. The formation of enaminone at the active site results in a molecular imprint that leads to the production of antibodies that function as aldol catalysts via a lysine-dependent class I aldolase mechanism (Eq. 8B2.12). 

Scheme 5.1. The two types of aldolase mechanisms The type I Schiff-base forming aldolase is represented by rabbit muscle fructose disphosphate (FDP) aldolase (RAMA, top), and the type II zinc enolate aldolase is represented by E. coli fructose diphosphate (FDP) aldolase (bottom).

The development of the concept of reactive immunization yielded more effective antibody aldolases.119-120 In this new approach, rather than raise antibodies against an unreactive hapten designed to mimic the transition state, the antibodies were raised against a reactive moiety. Specifically, a p-diketone that serves as a chemical trap to imprint a lysine residue in the active site of the Ab (Scheme 5.65) was used.340 A reactive lysine is a requirement of the type I aldolase mechanism. By this method two aldolase catalytic antibodies, 38C2 and 33F12 were identified.119... 

Prior to the determination of the aldolase mechanism and the development of catalytic antibodies for the aldol reaction, Hajos and Parrish and independently Wiechert et al. discovered that (5)-proline catalyzes the intramolecular aldol reaction of cyclic triketones (Scheme 6.7). This is not only a catalytic effect the reaction proceeds with high yields and large enantiomeric excess. 

Figure 59 (a) Natural class 1 aldolase mechanism, (b) Selection of the covalently bound antibody intermediate through formation of a... 

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