Aldol dehydrogenases

Aldol dehydrogenases, from diverse species including yeast, horses and humans, catalyze the oxidation of ethanol or the reduction of acetaldehyde, using diphosphopyridine nucleotide (DPN) as a co-factor. Crystalline yeast alcohol dehydrogenase has a molecular weight of 150,000 and contains four Zn2+ ions and binds four DPN molecules per mole. Its structure and chemistry are not yet known in detail. 

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

As depicted in Figure 6.8 the stability screening was based on DERA activity assay, the retro-aldol reaction of 2-deoxy-D-ribose 5-phosphate to acetaldehyde and D-glyceraldehyde 3-phosphate. D-glyceraldehyde 3-phosphate is further converted by the auxiliary enzymes triose phosphate isomerase and glycerol phosphate dehydrogenase. As the latter reaction consumes NADH it can be measured spectro-pho to metrically by the decrease in absorbance at 340 nm. 

N-Acetyl-L-phenylalanylsarcosine amide, 348 O-Acetylserine, 148 N-0 Acyl migrations, 157 Acyl-enzyme, 342-350 Z-0-Acylisoamide, 295 1,4-Addition, 221-242 1,6-Addition, 231 L-Ala-L-Ala-pNA,350 f-Ala-T-Pro-pNA, 350 AicohoT dehydrogenase, 340-341 Aldehydes, 209-211 Aldol condensation, 304-306 2-Alkoxytetrahydrofuran, 86 2-Alkoxytetrahydropyran, 18, 85-90 Alkylation of enamine, 282 Alkylation of enolate, 280 C and 0-Alkylation, 240 O-Alkylbenzohydroximoyl chloride, 155... 

Adenosine 5 -(tr1hydrogen diphosphate), 5 5 ester with 3-(ami nocarbonyl)-alcohol dehydrogenase, 63, 17 Adogen 464 [50934-77-5], 60, 11 Alanine, L-, N-carboxybenzyl ester, 63, 182 ALANINE, PHENYL-, N-tert-BUTOXYCARBONYL-, L-, 63, 160 Alanine, N-[(phenylmethoxy)carbonylL- [1142-20-7], 63, 182 Alcohols, alkynyl-, synthesis of, 63, 61 Alcohols, amino-, by reduction of acids, 63, 138 Aldol condensation, intramolecular, asymmetric, 63, 26, 37 Aldol condensation, diastereoselectlve, 63, 94, 104 ALIPHATIC B-KETOESTERS, 61, 5 Alkene photodimerization, 62, 125... 

Screening kits/sets containing samples of the normal commercially available enzymes are also provided by other enzyme suppliers, such as Boehringer Mannheim/Roche (Chirazyme sets for lipases/esterases, aldol reaction kits), Altus Biologies (ChiroScreen Kits TE and EH (based on CLECs, see section 5) for the chiral resolution of alcohols, amines, and esters), Biocatalysts (kits with alcohol dehydrogenases), Enzymatix (lipase biotransformation research kit), and others. 

Scheme 19.28 One-pot synthesis of a 1,3-diol based on combination of organocatalytic aldol reaction under neat conditions and alcohol dehydrogenase-catalyzed reduction.

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