Hydroxyacyl dehydrogenase

The mitochondrial system uses acetyl-CoA, not malonyl-CoA, by a slightly modified reversal of j6-oxidation. The substrates are saturated and unsaturated Ci2, Ci4, and Ci6 fatty acids, and the products are Cig, C20, C22, and C24 fatty acids. The first reduction step utilizes NADH, and the enzyme is /5-hydroxyacyl dehydrogenase (a -oxidation enzyme) the second reduction step utilizes NADPH, and the enzyme is enoyl reductase. 

This enzyme catalyses the second step in the Oxidation of fatty acids. It has been called il-hydroxyacyl dehydrogenase and ketoreductase. The pH optimum for oxidation in vitro is 9.6 to lO.O. At pH 6.0 to 7.0 in vitro it catalyses the reverse reaction. It can be assayed by u.v. spectroscopy [436] or radiochemically using the oxidation of C labelled oleic acid and evolution of C02 [437]. 

The enzyme hydroxyacyl dehydrogenase described above is specific for the L-isomer. Apparently, some mammalian tissue can also oxidize the D-isomer, but it is not clear what enzymic mechanism is responsible for that reaction. Although the presence of an enzyme that specifically catalyzes the oxidation of the D-hy-droxyacyl ester to yield the keto acid has been proposed by some, others believe that the D-hydroxyacyl is transformed to the L-hydroxy acid by enzymes with racemase activity—namely, crotonase and another racemase. The equilibrium of the enzyme reaction is modified by the presence of magnesium in the medium. The modification of the equilibrium probably results from the complexion of magnesium with the keto acid. Eliminating the product favors the formation of the hydroxyacyl. Hydroxybutyrate can also be oxidized by an enzyme found in the mitochondria of many tissues, such as brain, kidney, heart, and liver. Hydroxybutyrate dehydrogenase has been isolated, solubilized, purified from beef heart, and demonstrated to require lecithin for activity. 

The degradation reactions of methylbutyryl CoA are again catalyzed by the enzymes that are involved in the oxidation of the straight-chain fatty acids acyl dehydrogenase, crotonase, hydroxyacyl dehydrogenase. Only the last step of the sequence of reactions requires a specialized enzyme, ketoacyl thiolase. The products of that reaction are propionic acid and acetyl-CoA. 

Acyl CoA synthetase synthesises, in the presence of HS-CoA, acyl-CoA from a saturated fatty acid. Acyl CoA is transformed by acyl-CoA dehydrogenase into 2,3-dehydroacyl-CoA, and isomerism leads to trhydroxyacyl dehydrogenase the latter compound is hydrolysed by 3-ketoacyl-CoA thiolase to HS-CoA and 3-oxoacid. This acid eliminates carbon dioxide and yields methylketone, with catalysis of decarboxylase. Reductase can reduce methylketone to a secondary alcohol. 

The third reaction of this cycle is the oxidation of the hydroxyl group at the /3-position to produce a /3-ketoacyl-CoA derivative. This second oxidation reaction is catalyzed by L-hydroxyacyl-CoA dehydrogenase, an enzyme that requires NAD as a coenzyme. NADH produced in this reaction represents metabolic energy. Each NADH produced in mitochondria by this reaction drives the synthesis of 2.5 molecules of ATP in the electron transport pathway. L-Hydroxyacyl-... 

CoA dehydrogenase shows absolute specificity for the L-hydroxyacyl isomer of the substrate (Figure 24.16). (o-Hydroxyacyl isomers, which arise mainly from oxidation of unsaturated fatty acids, are handled differently.)... 

Step 3 of Figure 29.3 Alcohol Oxidation The /3-hydroxyacyl CoA from step 2 is oxidized to a /3-ketoacyl CoA in a reaction catalyzed by one of a family of L-3-hydroxyacyl-CoA dehydrogenases, which differ in substrate specificity according to the chain length of the acyl group. As in the oxidation of sn-glycerol 3-phosphate to dihydroxyacetone phosphate mentioned at the end of Section 29.2, this alcohol oxidation requires NAD+ as a coenzyme and yields reduced NADH/H+ as by-product. Deprotonation of the hydroxyl group is carried out by a histidine residue at the active site. 

Inherited defects in the enzymes of (3-oxidation and ketogenesis also lead to nonketotic hypoglycemia, coma, and fatty hver. Defects are known in long- and short-chain 3-hydroxyacyl-CoA dehydrogenase (deficiency of the long-chain enzyme may be a cause of acute fetty liver of pr nancy). 3-Ketoacyl-CoA thiolase and HMG-CoA lyase deficiency also affect the degradation of leucine, a ketogenic amino acid (Chapter 30). 

Treem WR et al Acute fatty liver of pregnancy and long-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency. Hepatology 1994 19 339. 

The oxidation of fatty acids within the Knoop-Lynen cycle occurs in the matrix. The Knoop-Lynen cycle includes four enzymes that act successively on acetyl-CoA. These are acyl-CoA dehydrogenase (FAD-dependent enzyme), enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase (NAD-dependent enzyme), and acetyl-CoA acyltrans-ferase. Each turn, or revolution, of the fatty acid spiral produces... 

Short-chain 3-hydroxyacyl-CoA dehydrogenase deficiency has been described in three patients. It is associated with additional defects of (3-oxidation, which have been associated with limb weakness and attacks of myoglobinuria, and it is potentially fatal. 

LCHAD long chain 3-hydroxyacyl-CoA dehydrogenase mtDNA mitochondrial DNA... 

PLP proteolipid protein SCHAD short chain 3-hydroxyacyl-CoA dehydrogenase... 

Yang and Schulz also formulated a treatment of coupled enzyme reaction kinetics that does not assume an irreversible first reaction. The validity of their theory is confirmed by a model system consisting of enoyl-CoA hydratase (EC 4.2.1.17) and 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) with 2,4-decadienoyl coenzyme A as a substrate. Unlike the conventional theory, their approach was found to be indispensible for coupled enzyme systems characterized by a first reaction with a small equilibrium constant and/or wherein the coupling enzyme concentration is higher than that of the intermediate. Equations based on their theory can allow one to calculate steady-state velocities of coupled enzyme reactions and to predict the time course of coupled enzyme reactions during the pre-steady state. 

ACID-BASE RELATIONSHIPS OXYGEN, OXIDES 0X0 ANIONS d-2-HYDROXY-ACID DEHYDROGENASE (S)-2-HYDROXY-ACID OXIDASE 3-HYDROXYACYL-CoA DEHYDROGENASE /3-HYDROXYACYL THIOESTER (or, ACP) DE-HYDRASE... 

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