Aldolase catalyzed reactions

These compounds are close structural analogs of dihydroxyacetone phosphate, differing only in replacement of the hydroxyl with a halogen. The rationale for designing haloacetol phosphates was much the same as that which led Meloche to believe that bromopyruvate would be a likely affinity label for kdGtP aldolase. Reactions catalyzed by both... 

Figure 10.11 Aldol reactions catalyzed in vivo by the 2-keto-3-deoxy-6-phospho-o-gluconate and 2-keto-3-deoxy-6-phospho-o-galactonate aldolases.

Application of an aldolase to the synthesis of the tricyclic microbial elicitor (-)-syringolide (Figure 10.34) is another excellent example that enzyme-catalyzed aldolizations can be used to generate sufficient quantities of enantiopure material in multistep syntheses of complex natural and unnatural products [159]. Remarkably, the aldolase reaction established absolute and relative configuration of the only chiral centers that needed to be externally induced in the adduct (95) from achiral precursor (94) during the subsequent cyclization events, all others seemed to follow by kinetic preference. 

In contrast to transketolase and the DHAP-dependent aldolases, deoxyribose aldolase (DERA) catalyzes the aldol reaction with the simple aldehyde, acetaldehyde. In vivo it catalyzes the formation of 2-deoxyribose-5-phosphate, the building block of DNA, from acetaldehyde and D-glyceraldehyde-3-phosphate, but in vitro it can catalyze the aldol reaction of acetaldehyde with other non-phosphorylated aldehydes. The example shown in Scheme 6.28 involves a tandem aldol reaction... 

The stereochemical course of the reaction catalyzed by dihydroneopterin aldolase has been established <2002JBC28841>. By carrying out the reaction in deuterium oxide and using multinuclear NMR spectroscopy of folate derived from the reaction product, 6-hydroxymethylpterin, it was shown that the late-stage enol intermediate undergoes protonation to form 6-hydroxymethylpterin with deuterium predominantly in the A-configuration. 

Scheme 1 Reversible aldol addition reaction catalyzed by fructose diphosphate aldolase.

This chapter deals with the other group of aldolases that catalyzes the reversible aldol reaction of pyruvate as the nucleophilic donor and a sugar as the electrophilic acceptor. Table 1 lists the main aldolases using pyruvate that have been examined for synthetic... 

Enzymatic synthesis relying on the use of aldolases offers several advantages. As opposed to chemical aldolization, aldolases usually catalyze a stereoselective aldol reaction under mild conditions there is no need for protection of functional groups and no cofactors are required. Moreover, whereas high specificity is reported for the donor substrate, broad flexibility toward the acceptor is generally observed. Finally, aldolases herein discussed do not use phosphorylated substrates, contrary to phosphoenolpyruvate-dependent aldolases involved in vivo in the biosynthetic pathway, such as KDO synthetase or DAHP synthetase [18,19]. 

Cleavage of fructose-l,6-bisphosphate, an aldolase-catalyzed reaction. The aldolase reaction entails a reversal of the familiar aldol condensation. The first step involves abstraction of the hydrogen of the C-4 hydroxyl group, followed by elimination of an enolate anion. 

The transaldolase-catalyzed conversion of fructose-6-phosphate and erythrose-4-phosphate to glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate. This is a two-step conversion. The first step is similar to the aldolase reaction except that the dihydroxyacetone produced is held at the catalytic site while the aldose product diffuses away and is replaced by another aldose molecule. The second step involves an aldol condensation. 

Transaldolase catalyzes a two-step conversion. The first step, an aldol cleavage of the bond between C-3 and C-4 of a ketose, is essentially identical to the reaction catalyzed by aldolase. However, the dihydroxyacetone that is produced in the transaldolase reaction from carbons 1, 2, and 3 is not released. Rather, it is held at the catalytic site while the glyceraldehyde-3-phosphate produced diffuses away and is replaced by erythrose-4-phosphate. An aldol condensation then generates the second product of the reaction, a ketose that contains the first three carbon atoms of the original ketose attached to C-1 of the acceptor aldose (fig. 12.32). 

The transketolase-catalyzed conversion of xylulose-5-phosphate and ribose-5-phosphate to glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate. Although the aldolase and ketolase reactions superficially resemble each other, they proceed by very different mechanisms. This is because in the aldolase reaction the carbon adjacent to a carbonyl... 

Scheme 4.—The Reversible Reaction Catalyzed by the Glycolysis Aldolase.

The removal of the aldehyde, for instance as partner to an aldol reaction catalyzed by the same aldolase, displaces the equilibrium to the right. 

Scheme 6.—The D-threo Configuration of the Newly Built Vicinal Diol in the Reaction Catalyzed by the Glycolysis Aldolase.

The use of reactive immunization to generate catalytic antibodies (or abzymes) that catalyze aldolase reactions has been described, offering additional utility for this synthetically useful transformation.260 Two such abzymes, 38C2 and 84G3, are available commercially and their respective, diverse activities have been described.261-262... 

We routinely use these substrates for kinetic characterizations of new aldolase catalysts. However, they are not suitable for cell-based screenings because both substrate and product are readily cell permeable. The solution to this problem came when we discovered that our aldolase antibodies catalyze the -elimination (or r ro-Michael reactions) of (3-hetero substituted ketones 37 [Scheme 8 (1)]. 

Serine hydroxymethyltransferase is a PLP-dependent aldolase. It catalyzes interconversion between glycine and various P-hydroxy-a-amino acids, such as serine and threonine, via formation of a quinoid intermediate derived from PLP with the amino acid substrate (Scheme 2.9). This aldolase-type reaction is of interest as an asymmetric synthesis of a-amino acids via C-C bond formation. 

Energetics of the Aldolase Reaction Aldolase catalyzes the glycolytic reaction... 

Answer Problem 1 outlines the steps in glycolysis involving fructose 1,6-bisphosphate, glyceraldehyde 3-phosphate, and dihydroxyacetone phosphate. Keep in mind that the aldolase reaction is readily reversible and the triose phosphate isomerase reaction catalyzes extremely rapid interconversion of its substrates. Thus, the label at C-l of glyceraldehyde 3-phosphate would equilibrate with C-l of dihydroxyacetone phosphate (AG ° = 7.5 kJ/mol). Because the aldolase reaction has AG ° = -23.8 kJ/mol in the direction of hexose formation, fructose 1,6-bisphosphate would be readily formed, and labeled in C-3 and C-4 (see Fig. 14-6). 

Figure 7.6. The metabolic reactions involved in the conversion of glycerol to glucose, the required precursor in the formation of sophorose. Note Reaction 1 catalyzed by triose phosphate isomerase. Reaction 2 catalyzed by aldolase. Reaction 3 catalyzed by fructose 1,6-bisphosphatase. Reaction 4 catalyzed by phosphoglucose isomerase., Reaction 6 catalyzed by glucose 6-phosphatase.

Preparation of enantiometrically pure aldehyde substrates for DHAP-dependent aldolase reactions has been accomplished by a combination of enzymatic and chemical methods. The lipase-catalyzed resolution of racemic aldehyde precursors has been accomplished by enantioselective acetate hydrolysis, as exhibited in the preparation of enantiomerically pure R- and S -glycidaldehyde acetals (Scheme 5.10).31 Regioselective ring opening of the epoxides, followed by acetal hydrolysis, generated the aldehydes in enantiomerically pure form. 

Scheme 5.29. Aldol addition reaction catalyzed in vivo by N-acetylneuranumc acid (NeuAc) aldolase.

---