Other Pyruvate-Dependent Aldolases

Remarkably, HMG/CHA aldolase, as 4-hydroxy-2-keto-4-methyl glutarate aldolase (EC 4.1.3.17) [88, 89], utilizes a ketone (i.e., 1 or 38) as acceptor, an activity that has not been exploited synthetically. Insighfs info fhe active site and catalytic mechanism are reported by Wang et al, which will facilitate efforts for its development as a potential biocatalyst [87]. 

10 RECENT ADVANCES IN ENZYME-CATALYZED ALDOL ADDITION REACTIONS 

Y Gaipi X Glcpi Galal Z Galal Gaipi W Galal,2ManNGc 

NeuA catalyzed synthesis of disaccharides containing sialic acid at the reducing end (e.g., 26-31) by means of aldol addition of pyruvate to disaccharides bearing a mannose derivative at the reducing end. 

Example of the one-pot, three-enzyme synthesis of Siaa2-6GalbpNP (32) and Siaa2-3GalppNP (33). Abbreviations Pm NeuA aldolase from, Pasteurella multocida NmCSS CMP-synthetase CMP-sialic acid synthetase from Neisseria meningitidis PmSTI P. multocida sialyltransferase Pd2,6ST a2-6-sialyltransferase from Photobacterium damsela GaippNP p-nitrophenyl p-galactoside. 

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Pyruvate-dependent aldolases catalyze the breaking of a carbon-carbon bond in nature. This reaction can, however, be reversed if an excess of pyruvate is used, establishing one new stereocenter in the course of it. The natural function of phosphoenolpyruvate (PEP)-dependent aldolases on the other hand is to catalyze the synthesis of a-keto acids. Since PEP is a very reactive, unstable and difficult to prepare substrate, they are not commonly used in synthesis. 

Other pyruvate- and phosphoenolpyruvate-dependent aldolases have been isolated and purified, but have not yet been extensively investigated for synthetic use. Those showing promise for future applications include, 3-deoxy-D-arabino-2-heptulosonic acid 7-phosphate (DAHP) synthetase (EC 4.1.2.15), 2-keto-4-hydroxyglutarate (KHG) aldolase (EC 4.1.2.31), and 2-keto-3-deoxy-D-gluconate (KDG) aldolase (EC 4.1.2.20). DAHP synthetase has been used... 

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. 

The biochemistry of the lactic acid bacteria has received attention [4, 17-20]. Homofermentative strains such as the Pediococci use the glycolytic pathway for the dissimilation of carbohydrates, such as glucose, to yield pyruvic acid. Pyruvic acid acts as a hydrogen acceptor and is converted to lactic acid by means of an NADH-dependent lactic dehydrogenase. It is believed that the homofermentative strains use in addition the hexose monophosphate pathway and possibly a phosphoketolase pathway (Fig. 21.2) when pentoses are degraded. The heterofermentative strains on the other hand lack both aldolase and hexose isomerase, essential for the operation of the glycolytic pathway, while pyruvic acid will not readily function as a... 

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