Acetaldehyde-dependent aldolases

To date, 2-deoxy-D-ribose 5-phosphate aldolase (DERA) is the only acetaldehyde-dependent aldolase being applied in organic synthesis. Thus the stereoselectivity of DERA is significant, all known enzymes from different organisms showing the same preferences, limiting the field of application to syntheses in which specifically the DERA-catalyzed enantiomer is needed. 

Acetaldehyde-dependent aldolases are the only aldolases that can catalyze the aldol formation between two aldehydes, i.e. with an aldehyde both as donor and acceptor [2-4, 40, 43]. More importantly, since they utilize, with high selectivity,... 

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).

A sequential aldol reaction leading to sialic acid derivatives has been also presented [59], Combination of acetaldehyde dependent aldolase (DERA) and Neu5Ac aldolase catalyzed reactions led to (R)-C-4 keto acids. In this case, however, one-pot reaction sequence was not possible due to the incompatibility of the reaction conditions for two enzymes. 

It is the only known member of the group of acetaldehyde-dependent aldolases. In vivo, DERA catalyzes the reversible aldol reaction of acetaldehyde and G3P. The donor substrate specificity of this enzyme is not as strict as with the other aldolases. 

Deo)y-D-ribose 5-phosphate aldolases (DERAs) belong to the class of acetaldehyde-dependent aldolases. In contrast to dihydroxyacetone phosphate (DHAP) aldolases, the donor substrate specificity is not as strict, allowing for chain elongation hy two or three carbon atoms. DERA aldolases have been successfully applied to the production of nucleosides. Horinouchi... 

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. 

Acetaldehyde-dependent Aldolases (DERA) The group of the acetaldehyde-dependent aldolase is actually constituted of only one enzyme, the DERA. The natural substrates of this enzyme are acetaldehyde 28 and o-glycer-aldehyde-3-phosphate (Ga3P) 35 for the synthesis of 2-deoxy- o-ribose 5-phosphate 36 (Scheme 28.18). 

Type I and Type II aldolases are also present in the group of pyruvate and acetaldehyde-dependent aldolases (DERA, type I). Only the glycine- and alanine-dependent aldolases work according to a different mechanism involving the PLP cofactor. 

In contrast to transketolase and the DHAP-dependent aldolases, deoxyribose aldolase (DERA) catalyzes the aldol reaction with the simple aldehyde, acetaldehyde. In vivo it catalyzes the formation of 2-deoxyribose-5-phosphate, the building block of DNA, from acetaldehyde and D-glyceraldehyde-3-phosphate, but in vitro it can catalyze the aldol reaction of acetaldehyde with other non-phosphorylated aldehydes. The example shown in Scheme 6.28 involves a tandem aldol reaction... 

The product of the PNP enzyme, FDRP 9 has been purified and characterised. The evidence suggests that FDRP 9 is then isomerised to 5-fluoro-5-deoxyribulose-1-phosphate 10, acted upon by an isomerase (Scheme 7). Such ribulose phosphates are well-known products of aldolases and a reverse aldol reaction will clearly generate fluoroacetaldehyde 11. Fluoroacetaldehyde 11 is then converted after oxidation to FAc 1. We have also shown that there is a pyridoxal phosphate (PLP)-dependent enzyme which converts fluoroacetaldehyde 11 and L-threonine 12 to 4-FT 2 and acetaldehyde in a transaldol reaction as shown in Scheme 8. Thus, all of the biosynthetic steps from fluoride ion to FAc 1 and 4-FT 2 can be rationalised as illustrated in Scheme 7. 

In nature, most aldolases are rooted in the sugar metabolic cycle and accept highly functionalized substrates for the aldol reaction. Nevertheless, the scope of enzymatic aldol reactions is limited, since aldolases strictly distinguish between the acceptor and the donor, yielding almost exclusively one product, and is furthermore restricted to only a few different possible natural donors. According to the donor molecules, aldolases are grouped in dihydroxyacetone phosphate-, phosphoenolpyruvate- or pyruvate-, acetaldehyde-, and glycine-dependent aldolases [41]. 

There is only a single literature example of a bi-directional skeleton elongation performed with an enzyme different from the DHAP dependent aldolases. The 2-deoxy-D-ribose 5-phosphate aldolase (RibA EC 4.1.2.4), a bacterial class I enzyme which requires acetaldehyde as the nucleophilic substrate [42], has been applied to the tandem aldolization of a thioether dialdehyde [95]. The substrate 14 was synthesized from optically homogenous (R)-glycidaldehyde and, because of its Q Synimetry, diastereoselective twofold addition of acetaldehyde led to a symmetrical 5,5 -sulfide-linked dipentofuranose 15. 

A second catabolic reaction of L-threonine (Eq. 24-37, step b) is cleavage to glycine and acetaldehyde. The reaction is catalyzed by serine hydroxymethyl-transferase (Eq. 14-30). Some bacteria have a very active D-threonine aldolase. A quantitatively more important route of catabolism in most organisms is dehydrogenation (Eq. 24-37, step to form 2-amino-3-oxobutyrate. This intermediate can be cleaved by another PLP-dependent enzyme to acetyl-CoA plus glycine (Eq. 24-38, step d). It can also be decarboxylat-ed (Eq. 24-38, step e) to aminoacetone, a urinary excretion product, or oxidized by amine oxidases to methylglyoxal (Eq. 24-37, The latter can... 

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]. 

Aldolases belong to the class of lyases and are found in the biosynthetic pathways of carbohydrates, ketoacids, and amino acids. They are generally divided into different types following their mechanism and groups according to their donor specificity. These four different groups are the pyruvate, dihydroxyacetone phosphate, acetaldehyde, and glyci-ne/alanine-dependent aldolases (Scheme 28.15). 

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