Pyruvate aldolases

Pyruvate-dependent lyases serve catabolic functions in vivo in the degradation of sialic acids and KDO (2-keto-3-deoxy-manno-octosonate), and in that of 2-keto-3-deoxy aldonic acid intermediates from hexose or pentose catabolism. 

A few related enzymes have been identified that use phosphoenolpyruvate in place of pyruvate vhich, by release of inorganic phosphate upon C-C bond formation, renders aldol additions essentially irreversible [14, 25]. Although attractive for synthetic applications, such enzymes have not yet been intensively studied for preparative applications [64]. 

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Of the known classes of aldolase, DERA (statin side chain) and pyruvate aldolases (sialic acids) have been shown to be of particular value in API production as they use readily accessible substrates. Glycine-dependent aldolases are another valuable class that allow access to p-hydroxy amino acid derivatives. In contrast, dihydroxy acetone phosphate (DHAP) aldolases, which also access two stereogenic centres simultaneously,... 

Unlike other pyruvate aldolases, 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolase (EC 4.1.2.14), which catalyzes the reversible condensation... 

Whereas DHAP-aldolase catalyzed aldol reactions have been thoroughly investigated [2-12], the use of pyruvate-aldolases is still in its infancy [39,40]. As the first chemical synthetic equivalents of PEP, which are able to transfer the pyruvic acid d -synthon (boxed), we have synthesized chiral pyravate hydra-zones from simple chemicals and (S)-proline (scheme 11). 

M.l. Walters, E.l. Toone, Pyruvate aldolases in chiral carbon-carbon bond formation, Nat. Protoc. 2 (2007) 1825-1830. 

Classification of aldolases according to their donor selectivity (a) pyruvate aldolases, (b) dihydr-oxyacetone phosphate (DHAP)-dependent aldolases, (c) DHA-and other unphosphorylated analogues or DMA utilizing aldolases, (d) glycine/alanine aldolases, and (e) acetaldehyde-dependent aldolases. 

Pyruvate-dependent aldolases reversibly catalyze the aldol addition of p)u uvate or analogues to aldehydes yielding y-hydroxy-a-oxoacids (Figure 10.1). They exist as Class I aldolases (i.e., Schiff base/enamine formation) and Class 11 (i.e., metal cofactor and enolate formation) aldolases (Figure 10.2) [45, 46]. Class 11 pyruvate aldolases contain a Mg, Mn, or Co divalent metal cation in octahedral coordination, which stabilizes the nucleophile (i.e., p5U uvate anion) in the active site [45,47]. 

A pyruvate aldolase from Pseudomonas taetrolens catalyzes the aldol addition of pyru-vafe to indole-pyruvic acid (34), a ketone [85]. This is an interesting example of an aldolase fhaf cafalyzes an aldol addition to a ketone acceptor. A variant of this aldolase was used in fhe stereoselective synfhesis of a precursor (35) of monatin (36), whose 2R,4R stereoisomer is 2700-fold sweeter fhan sucrose (Scheme 10.3) [86]. 

Keto-3-deoxygluconate aldolase from hyperthermophile Sulfolobus solfataricus (KDGlUg is a Class I pyruvate aldolase with broad acceptor specificity, which includes... 

Wang, W., Mazurkewich, S., Kimber, M. S., and Seah, S. Y. K., Structural and kinetic characterization of 4-hydroxy-4-methyl-2-oxoglutarate/4-carboxy-4-hydroxy-2-oxoadipate aldolase, a protocatechuate degradation enzyme evolutionarily convergent with the Hpal and DmpG pyruvate aldolases. /. Biol. Chem. 2010,285 (47), 36608-36615. 

Wang, W. J., Baker, R, and Seah, S. Y. K., Comparison of two metal-dependent pyruvate aldolases related by convergent evolution Substrate specificity, kinetic mechanism, and substrate charmeling. Biochemistry 2010,49 (17), 3774-3782. 

While pyruvate aldolases form only a single stereogenic center, the aldolases specific for dihydroxyacetone phosphate (DHAP, 22) as a nucleophile create two new asymmetric centers at the termini of the new C—C bond. Particularly useful for synthetic applications is the fact that nature has evolved a full set of four stereochemically unique aldolases [27] for the retroaldol cleavage of ketose 1-phosphates 23-26 (Fig. 12). In the direction of synthesis this formally allows the deliberate preparation of any one of the possible four diastereomeric aldol adducts in a building block fashion [15,22,27] by simply choosing the complementary enzyme and starting materials for full control over constitution and absolute configuration of the desired product. 

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