D-3-HYDROXYBUTYRATE DEHYDROGENASE

Figure 22-5. Interrelationships of the ketone bodies. D(-)-3-hydroxybutyrate dehydrogenase is a mitochondrial enzyme.

Fig. 1. The metabolic cycle for the synthesis and degradation of poly(3HB). (1) 3-ketothiolase (2) NADPH-dependent acetoacetyl-CoA reductase (3) poly(3HB) synthase (4) NADH-dependent acetoacetyl-CoA reductase (5), (6) enolases (7) depolymerase (8) d-(-)-3-hydroxybutyrate dehydrogenase (9) acetoacetyl-CoA synthetase (10) succinyl-CoA transferase (11) citrate synthase (12) see Sect. 3...

Figure 22.19. Formation of Ketone Bodies. The Ketone bodies-acetoacetate, d-3-hydroxybutyrate, and acetone from acetyl CoA are formed primarily in the liver. Enzymes catalyzing these reactions are (1) 3-ketothiolase, (2) hydroxymethylglutaryl CoA synthase, (3) hydroxymethylglutaryl CoA cleavage enzyme, and (4) d-3-hydroxybutyrate dehydrogenase. Acetoacetate spontaneously decarboxylates to form acetone.

D-3-Hydroxybutyrate is formed by the reduction of acetoacetate in the mitochondrial matrix by D-3-hydroxybutyrate dehydrogenase. The ratio of hydroxybutyrate to acetoacetate depends on the NADH/NAD ratio inside mitochondria. 

In extrahepatic tissues, D-/3-hydroxybutyrate is oxidized to acetoacetate by D-/3-hydroxybutyrate dehydrogenase (Fig. 17-19). The acetoacetate is activated to its coenzyme A ester by transfer of CoA from suc-cinyl-CoA, an intermediate of the citric acid cycle (see Fig. 16-7), in a reaction catalyzed by /3-ketoacyl-CoA transferase. The acetoacetyl-CoA is then cleaved by thiolase to yield two acetyl-CoAs, which enter the citric acid cycle. Thus the ketone bodies are used as fuels. 

A complication arises if the p-dicarbonyl compound is substituted on the central carbon, since diastereomers could be formed. Nonetheless useful selectivity is often observed. The enzyme D-3-hydroxybutyrate dehydrogenase (HBDH) fromRhodopseudonwnas spheroides in the presence of NADH reduces only the (5)-enantiomer of 2-ketocyclohexanecarboxylic acid, whereas baker s yeast operates exclusively on the (/ )-antipode. 

Most of the acetyl-CoA formed by 3-oxidation in liver is converted to acetoacetate by the 3-hydroxy-3-methylglutaryl-CoA pathway (Guzman and Gelen, 1993). Acetoacetate is reversibly converted to D-3-hydroxybutyrate by D-3-hy-droxybutyrate dehydrogenase in the mitochondrial matrix in all tissues. 

It is also a precursor for synthesis of polyprenyl (isoprenoid) compounds, and it can give rise to free acetoacetate, an important constituent of blood. Acetoacetate is a (3-oxoacid that can undergo decarboxylation to acetone or can be reduced by an NADH-dependent dehydrogenase to D-3-hydroxybutyrate. Notice that the configuration of this compound is opposite to that of L-3-hydroxybutyryl-CoA which is... 

Fig. 3. (a) Biosynthesis of SCL PHA 1,3-ketothiolase 2, NADPH-dependent acetoacetyl-CoA reductase 3, SCL PHA polymerase 4, SCL PHA depolymerase 5, d(-)-3-hydroxybutyrate-dimer hydrolase, (b) Biosynthesis of P(HB-co-HV), 1, 3-ketothiolase 2a, NADPH-dependent acetoacetyl-CoA reductase 2b, NADH-dependent acetoacetyl-CoA reductase 3, SCL PHA polymerase 4, fatty acyl-CoA dehydrogenase 5, enoyl-CoA hydratase. 

Hydroxybutyrate dehydrogenase (3-D-(-) hydroxybutyrate NAD oxido reductase EC 1.1.1.30) is a good example of this group of phospholipid-requiring enz nnes. It is tightly bound to the inner... 

The / -keto esters are reduced to the respective chiral ft -hydroxy esters by at least two alternative enzymes one of which is D-directing the other one is L-directing (Fig. 3.4). A product mixture results that contains both enantiomeric forms, d and i.,b of the carbinol (/1-hydroxy ester) in varying degrees. In the case of ethyl acetoacetate (1) preferably the L-form of ethyl 3-hydroxybutyrate (l-4) is produced which is then secreted from the cell [55-58]. The L-directing enzyme is methyl butyralde-hyde reductase (MBAR EC 1.1.1.265), and the D-enantiomer is formed by the action of /J-ketoacyl reductase (KAR EC 1.1.1.100) which is a constituent of fatty acid anabolism (Fig. 3.4) [49, 59]. Alcohol dehydrogenase (ADH EC 1.1.1.1) L-directing activity is classically attributed to was shown to be inactive - moreover the enzyme is even inhibited by the substrate [2, 34, 37]. 

A /3-hydroxybutyric acid dehydrogenase that uses DPN has been known for many years. It is specific for i/-/3-hydroxybutyrate, and does not act with the CoA derivative. The original description of the CoA-requiring system included a specific requirement for D-hydroxy acids. Later a racemase that converts ii-/3-hydroxy CoA compounds to d-was reported, but it appears that the racemase is the resultant of an D-dehydrogenase together with the previously described D-enzyme. Racemization requires DPN. ... 

D-(-) HYDROXYBUTYRATE DEHYDROGENASE FROM RAT LIVER MITOCHONDRIA PURIFICATION AND INTERACTION WITH PHOSPHOLIPIDS... 

Figure 1. SCL-PHA production from non-related carbon sources. A. The tricarboxylic acid cycle. B. Poly-3-hydroxybutyrate [P(3HB)] monomer supply via the fatty acid biosynthesis enzymes, FabD, FabH, and FabG. C. P(3HB) monomer supply mediated by the beta-ketothiolase enzymes, PhaA or BktB, and the NADPH-reductase, PhaB. D. Poly-3-hydroxyvalerate (P3HV) monomer supply mediated by threonine deaminase (IlvA), pyruvate dehydrogenase, BktB, and PhaB. E. Poly-4-hydroxybutyrate monomer supply mediated by succinate dehydrogenase (SucD), 4-hydroxybutyrate dehydrogenase (4HbD), and acetyl transferase (Catl or Cat2).

Dichlorophenoxy)butyric acid is converted in the presence of ATP into dichlorophenoxybutyryl coenzyme A. This acyl-CoA is converted by the electron acceptor flavine adenine dinucleotide (FAD) into dichlorophenoxycrotonyl-CoA. One carbon atom of the unsaturated bond is hydroxilated and dichlorophenoxy- -hydroxybutyric acid-CoA is formed. In certain plants possessing specific /3-oxidase enzyme systems, -ketobutyric acid-CoA is formed from this intermediate compound by the mediation of NAD and NADH in a reaction catalysed by -hydroxyacyl-CoA dehydrogenase. This compound is decomposed by hydrolysis into 2,4-D and acetyl-CoA. 

Fig. 23.18. Synthesis of the ketone bodies acetoacetate, P-hydroxybulyrate, and acetone. The portion of HMG-Co A shown in blue is released as acetyl CoA, and the remainder of the molecule forms acetoacetate. Acetoacetate is reduced to P-hydroxybutyrate or decarboxy-lated to acetone. Note that the dehydrogenase that interconverts acetoacetate and P-hydroxybutyrate is specific for the D-isomer. Thus, it differs from the dehydrogenases of P-oxidation, which act on 3-hydroxy acyl CoA derivatives and is specific for the L-isomer.

In the solvent-producing clostridia, 3-hydroxybutyryl-CoA dehydrogenase (P-hydroxybutyr)d-CoA dehydrogenase enzyme 5) catalyzes the reduction of acetoacetyl-CoA to 3-hydroxybutyryl-CoA. The enzyme has been purified from C. beijerinckii NRRL B593, and it has subunit and native MWs of 30.8 and 213 kDa, respectively (Colby and Chen 1992). The enzyme can use either reduced nicotinamide adenine dinucleotide (NADH) or NADPH as a cosubstrate, but NADH gives a 60-fold higher catalytic efficiency and is likely the physiological cosubstrate. 

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