Pyruvate dependent aldolases

DHAP-dependent aldolases Pyruvate dependent aldolases Deoxyribose 5 -phosphate aldolase —h... 

The power of directed evolution has been demonstrated by the conversion of an aldolase into a new kind of aldolase. Wong and coworkers evolved a pyruvate-dependent sialic acid aldolase... 

A convenient chemoenzymatic access to sialic acid mimetics as important inhibitors of influenza sialidases has been established by Nelson et al. (Fig. 35b) [191, 192]. Application of a pyruvate-dependent sialic acid aldolase improved by directed evolution disclosed a new route to the core structure of important pharmaceuticals, such as zanamivir (Relenza ). 

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. 

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).

Pyruvate-dependent aldolases have catabolic activity in vivo, whereas their counterparts utilizing phosphoenolpyruvate as the donor substrate are involved in the biosynthesis of keto acids. Both classes of enzymes have been used in synthesis to prepare similar a-keto acids. The enzymes catalzye this type of reaction in vivo and their stereoselectivity are presented together in this section (Schemes 5.27, 5.28). 

Scheme 5.27. Pyruvate-dependent aldolases and the reactions they catalyze. P = PO32. ...

The aldolases which have been investigated for their synthetic utility can be classified on the basis of the donor substrate accepted by the enzyme. For the synthesis of 3-deoxy-2-ulosonic acids pyruvate- and phosphoenolpyruvate dependent aldolases are the most desirable enzymes as they are involved in the metabolism of sialic acids (or structurally related ones) in vivo. They use pyruvate or phosphoenolpyruvate as a donor to form 3-deoxy-2-keto acids (Table 1). Both of them add a three-carbons ketone fragment onto a carbonyl group of an aldehyde. The pyruvate dependent aldolases have a catabolic function in vivo, whereas the phosphoenolpyruvate dependent aldolases are involved in the biosynthesis of the keto acids. For synthetic purpose the equilibrium of the pyruvate dependent aldolases is shifted toward the condensation products through the use of an excess of pyruvate. 

W-acetyl-neuraminate lyase (NAL) family, pyruvate-dependent aldolases... 

For thermodynamic reasons, pyruvate-dependent aldolases have catabolic functions in vivo, whereas their counterparts employing (energy-rich) phosphoenol pyruvate as the donor are involved in the biosynthesis of keto-acids. However, both types of enzymes can be used to synthesize a-keto-p-hydroxy acids in vitro. 

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]. 

Hydroxy-4-methyl-2-oxoglufarafe/4-carboxy-4-hydroxy-2-oxoadipate (HMG/CHA) aldolase (EC 4.1.3.17) from Pseudomonas putida is a Class II pyruvate-dependent aldolase that catalyzes reversibly the homo-aldol addition of pyruvate (HMG aldolase) and the addition of pyruvate to oxaloacefate (38) (CHA aldolase), the last step of the bacterial protocatechuate 4,5-cleavage pathway (Scheme 10.4) [87]. 

Class II pyruvate-dependent 4-hydroxy-2-oxopentanoate aldolases (EC 4.1.2.-), BphI and Hpal, catalyze the retroaldol reactions of 4-hydroxy-2-oxopentanoate (43) to p5uiivate (1) and acetaldehyde and 4-hydroxy-2-oxo-l,7-heptanedioate (44) to 1 and succinic semialdehyde (45) (Scheme 10.7) [45,96]. BphI is selective for (S)-43 stereoisomer, whereas Hpal accepts both enantiomers [97]. 

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,... 

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