Enzymes aldolase, general reaction

The stereochemistry of the aldol reaction is highly predictable since it is generally controlled by the enzyme and does not depend on the structure or stereochemistry of the substrates. Aldolases generally show a very strict specificity for the donor substrate (the ketone), but tolerate a broad range of acceptor substrates (the aldehyde). Thus, they can be functionally classified on the base of the donor substrate accepted by the enzyme. 

Trost et al. inspired the Zn(ll)-mediated direct catalytic reaction by aldolase reaction (Scheme 8.44). Two types of aldolases are known to promote aldol reaction in a biological system. A type I system includes enamine reaction by participation of lysine residue. A type II system involves the aldol reaction mediated with Zn(ll) cofactor in the active site of the enzyme. In general, type 1 aldolases are primarily found in animals and higher plants, whereas type II aldolases are found in bacteria and fungi. 

As we look at some of the reactions of intermediary metabolism, we shall rationahze them in terms of the chemistry that is taking place. In general, we shall not consider here the involvement of the enzyme itself, the binding of substrates to the enzyme, or the role played by the enzyme s amino acid side-chains. In Chapter 13 we looked at specific examples where we know just how an enzyme is able to catalyse a reaction. Examples such as aldolase and those phosphate isomerase, enzymes of the glycolytic pathway, and citrate synthase from the Krebs cycle were considered in some detail. It may... 

An advantage of these enzymes is that they are stereocomplementary, in that they can synthesize the four possible diastereoisomers of vicinal diols from achiral aldehyde acceptors and DHAP (Scheme 4.2). Although this statement is generally used and accepted, it is not completely true since tagatose-l,6-bisphosphate aldolase (TBPA) from Escherichia coli-the only TBPA that has been investigated in terms of its use in synthesis-does not seems to control the stereochemistry of the aldol reaction when aldehydes different from the natural substrate were used as acceptors [7]. However, this situation could be modified soon since it has been demonstrated that the stereochemical course of TBPA-catalyzed C—C bond formation may be modified by enzyme-directed evolution [8]. 

The enzymes which catalyze this reaction, the aldolases, are members of the general group called lyases (see Table I). They have been isolated from many living cells, and vary in specificity. The reader will find, in Methods of... 

Enzymes turned out to be very helpful in the de novo synthesis of certain monosaccharides. Generally, two chiral carbonyl compounds are combined in an aldol-type reaction. In carbohydrate metabolism, aldolases catalyze the condensation of dihydroxyacetone phosphate (DHAP) and aldehydes to higher sugar components. To date, about thirty aldolases have been classified, but only... 

In reactions catalyzed by DHAP-aldolases, hydroxylated aldehydes are generally superior to unsubstituted aldehydes presumably because of their higher reactivity (electrophilicity), higher affinity to the enzyme active site (lower values), and the fact that the products are stabilized by the formation of cyclic isomers [42].Accordingly,substrates with dual 2- or 3-hydroxyaldehyde termini seemed to be a logical choice for potential tandem aldolizations. 

Bromopyruvic acid inhibits S-deoxy-D-erj t ro-hexulosonate 6-phosphate aldolase " and 3-deoxy-D-ar-a6mo-heptulosonate 7-phosphate synthetase " in a similar way. The enzymes are protected from inhibition by pyruvate and phospho-enol pyruvate respectively. Since aldolases must possess a nucleophilic group to initiate the reaction, this type of inhibition should be a general property of aldolases. 

FDP aldolase catalyzes the reversible aldol addition reaction of DHAP and d-glyceraldehyde 3-phosphate (D-Gly 3-P) to form d-FDP (Fig. 14.1-1). The equilibrium constant for this reaction has a value of -104 m-1 in favor of FDP formation. The enzyme has been isolated from a variety of eukaryotic and prokaryotic sources, both in type I and type II forms[7 21). Generally, the type I FDP aldolases exist as tetramers (M.W. 160 KDa), while the type II enzymes are dimers (M. W. 80 KDa). For the... 

An exhaustive review of all of the types of reactions that are catalyzed by metal-requiring enzymes and the specific functions of these metals, as currently understood, is beyond the scope of this chapter. To complicate this general area of investigation, even within a single group of enzymes, the metal ions may play different roles in the catalytic processes for reaction-related enzymes. Not all enzymes of a specific class necessarily require a cation for activity. In some cases, the roles of the cation may be substituted by specific amino acid residues in the protein. A classic example of such a case is the muscle and yeast fmctose-bisphosphate aldolases. The muscle enzyme catalyzes the aldol condensation using a Schiff base intermediate to activate the substrate, whereas the yeast enzyme is a Zn +-metalloenzyme (1). The cation appears to serve as the electrophile in the activation of the substrate for the same reaction. 

Enzymes catalyze the formation of carbon-carbon bonds between allylic and homoallylic pyrophosphate species by mechanisms that are very different from those for carbonyl compounds. Here, carbonium ions, stabilized as ion pairs and generated from allylic pyrophosphates, are likely to be the intermediates that add to the TT-electron density of carbon-carbon double bonds to form new carbon-carbon single bonds. Reaction patterns are consistent with model systems and the mechanisms are based on analogies with the models, stereochemical information (which is subject to interpretation), and the structural requirements for inhibitors. Detailed kinetic studies, including isotope effects, which provide probes in the aldolase and Claisen enzymes discussed in Section II, have not yet been performed in these systems. The possibility for surprising discoveries remains and further work is needed to confirm the proposed mechanisms and to generalize them. 

The general class of enzymes catalyzing aldol or retroaldol condensation reactions are the aldolases (Table VI) (156-161). In principle, there are four possible stereochemical routes to product depending on whether C-C bond formation involves retention or inversion of configuration at the methyl or methylene carbon atom a to the ketonic carbonyl (Eq. (30)] ... 

The observation of net retention of configuration by the muscle enzyme, as well as the other aldolases, suggests that a single active site base could alternatively function in a general base and general acid capacity in the exchange and condensation reactions, respectively (Fig. 7) (76). However, the stereochemistry does not absolutely require a single-base mechanism. For example, Hupe and co-workers propose a two-base mechanism in which the phosphate at C-1 of the... 

Schiirmann M, Schiirmann M, Sprenger GA (2002) Fructose 6-phosphate aldolase and 1-deoxy-D-xylulose 5-phosphate synthase from Escherichia coli as tools in enzymatic synthesis of 1-deoxysugars. J Mol Catal B Enzym 19 247-252 Shelton CM, Toone EJ (1995) Differential dye-ligand chromatography as a general purification protocol for 2-keto-3-deoxy-6-phosphogluconate aldolases. Tetrahed Asymm 6 207-211 Silvestri MG, Desantis G, Mitchell M et al. (2003) Asymmetric aldol reactions using aldolases. Top Stereochem 23 267-342... 

The aldolases are a diverse class of enzymes that catalyse the coupling of a carbonyl-containing compound (nucleophile), containing one, two or three carbons, with an aldehyde (electrophile). In most cases the nucleophile is either pyruvic acid or dihydroxyacetone phosphate, whereas the electrophilic aldehyde is much more variable in structure. In many cases the reaction generates two new stereogenic centres in the product. In general, only one isomer is obtained from the four possible stereoisomeric products (Scheme 5.1). 

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