Hydroxyacyl CoA dehydrogenase

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). 

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... 

Scheme 23.4 Production of methylketones from fatty acids by Penicillium roqueforti. 1 ATP-de-pendent acylcoenzyme A (acyl-CoA) synthase 2 flavin adenine dinucleotidedependent acyl-CoA dehydrogenase 3 enoyl-CoA hydratase 4 NAD-dependent 3-hydroxyacyl-CoA dehydrogenase 5 3-oxoacyl-CoA thiolase 6 3-oxoacyl-CoA thiolester hydrolase and 3-oxoacid decarboxylase. (Adapted from [46])...

Shen JJ, Matern D, Millington DS, et al (2000) Acylcarnitines in fibroblasts of patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency and other fatty acid oxidation disorders. J Inherit Metab Dis 23 27-44... 

In the third step, 1, -/3-hydroxyacyl-CoA is dehydrogenated to form /3-ketoacyl-CoA, by the action of /3-hydroxyacyl-CoA dehydrogenase NAD+ is the electron acceptor. This enzyme is absolutely specific for the l stereoisomer of hydroxyacyl-CoA The NADH formed in the reaction donates its electrons to NADH dehydrogenase, an electron carrier of the respiratory chain, and ATP is formed from ADP as the electrons pass to 02. The reaction catalyzed by /3-hydroxyacyl-CoA dehydrogenase is closely analogous to the malate dehydrogenase reaction of the citric acid cycle (p. XXX). 

The last three steps of this four-step sequence are catalyzed by either of two sets of enzymes, with the enzymes employed depending on the length of the fatty acyl chain. For fatty acyl chains of 12 or more carbons, the reactions are catalyzed by a multienzyme complex associated with the inner mitochondrial membrane, the trifunctional protein (TFP). TFP is a heterooctamer of 4/34 subunits. Each a subunit contains two activities, the enoyl-CoA hydratase and the /3-hydroxyacyl-CoA dehydrogenase the /3 subunits contain the thiolase activity. This tight association of three enzymes may allow efficient substrate channeling from one active site to the... 

Two of the enzymes of /3 oxidation are also regulated by metabolites that signal energy sufficiency. When the [NADH]/[NAD+] ratio is high, /3-hydroxyacyl-CoA dehydrogenase is inhibited in addition, high concentrations of acetyl-CoA inhibit thiolase (Fig. 17-12). 

More than 20 other human genetic defects in fatty acid transport or oxidation have been documented, most much less common than the defect in MCAD. One of the most severe disorders results from loss of the long-chain /3-hydroxyacyl-CoA dehydrogenase activity of the tri-functional protein, TFP. Other disorders include defects in the a or /3 subunits that affect all three activities of TFP and cause serious heart disease and abnormal skeletal muscle. ... 

As many as 1 in 10,000 persons may inherit such prob-lems.48 50a Tire proteins that may be defective include a plasma membrane carnitine transporter carnitine palmitoyltransferases camitine/acylcamitine trans-locase long-chain, medium-chain, and short-chain acyl-CoA dehydrogenases 2,4-dienoyl-CoA reductase (Eq. 17-1) and long-chain 3-hydroxyacyl-CoA dehydrogenase. Some of these are indicated in Fig. 17-2. 

The /3-hydroxyacyl-CoA dehydrogenase reaction is analogous to the malate dehydrogenase reaction both are NAD-requiring and act on /3-hydroxyacyl compounds ... 

Figure 19.6 indicates the oxidation of palmitoyl-CoA to myristoyl-CoA with the production of an acetyl-CoA molecule. The myristoyl-CoA molecule can undergo another oxidative cycle, and so on. Note that the /3-hydroxyacyl-CoA dehydrogenase is specific for the l isomer of /3-hydroxyacyl-CoA. Also note that at least three acetyl-CoA dehydrogenases exist, one favoring short-chain fatty acids, another intermediate-length fatty adds, and the third long-chain fatty adds. 

Acyl-CoA dehydrogenase. enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, thiolase, and A -cis-A -trans-cnoyl-CoA isomerase. 

The hydration of enoyl CoA is a prelude to the second oxidation reaction, which converts the hydroxyl group at C-3 into a keto group and generates NADH. This oxidation is catalyzed by l-3-hydroxyacyl CoA dehydrogenase, which is specific for the 1 isomer of the hydroxyacyl substrate. 

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