PEP -dependent aldolases

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. 

The first group is the dihydroxyacetone phosphate (DHAP)-dependent aldolases, which use DHAP as the donor to produce 2-keto-l, 3, 4-trihydroxy motifs. The second group, the pyruvate- or phosphoenol pyruvate (PEP)-dependent aldolases, uses pyruvate to form 4-hydroxy-2-ketoacids. The third... 

Scheme 5.28. Phospoenolpyruvate (PEP) dependent aldolases and the reactions they catalyze.

Threonine aldolases (ThrA EC4.1.2.5) and serine hydroxymethyltransferases (SHMT EC 2.1.2.1) are pyridoxal-5 -phosphate (PEP) dependent aldolases that catalyze the aldol addition of glydne to aldehydes [155-158], Since two new stereogenic centers are formed, four possible stereoisomers can be formally obtained. However, contrary to the case of DHAP-dependent aldolases, the four set of stereocomplementary enzymes have not been found in nature yet (Scheme 10.7) [157,159,160]. 

Scheme 5.2. The four main groups of aldolase reactions classified by their donor substrate (1) Dihydroxyacetone phosphate (DHAP)- dependent aldolases, (2) phosphoenol pyruvate (PEP)-and pyruvate-dependent aldolases, (3) 2-deoxyribose-5-phosphate aldolase (DERA), a member of the acetaldehyde-dependent aldolases, and (4) glycine-dependent aldolases (GDA).

Figure 11.1 Proposed pathway for hex-ose metabolism of homofermentative LAB (1) and (2) phosphoenolpyruvate (PEP)-dependent sugar phosphotransferase system (PTS) (3) mannitol-specific PTS (4) phospho-glucose isomerase (5) mannitol-1-phosphate dehydrogenase (6) mannitol-1-phosphatase (7) 6-phosphofructokinase (8) fructose-diphosphatase (9) fructose-1,6-diphosphate aldolase (10) triosephosphate isomerase (11) glyceraldehyde-3-phosphate dehydrogenase...

The availability of both the cataboHc aldolase and the uniquely synthetic anabolic synthase made it possible to assemble a novel continuous assay for the determination of the metabolite N-acetylneuraminic acid [46]. A combination of both enzymes, in the presence of an excess of PEP, will start a cycle in which the determinant sialic acid will undergo a steady conversion of cleavage and re-syn-thesis as a futile cycle (Scheme 2.2.5.24). With each progression, however, 1 equiv of pyruvate is liberated simultaneously, which causes time-dependent signal amplification. Pyruvate is quantified spectrophotometrically by a corresponding NADH consumption when the system is coupled to the standard pyruvate dehy-... 

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