Fatty 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 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. 

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.)... 

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... 

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])...

Table 1.4.14 Data from a patient affected with a long-chain 3-hydroxyacyl enzyme A dehydrogenase defect. UEFA Unesterified fatty acids...

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... 

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... 

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. ... 

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. 

Treem, W.R., Rinaldo, P., Hale, D.E., Stanley, C.A., Millington, D.S., Hyams, J.S., Jackson, S., Turnbull, D.M. Acute fatty liver of pregnancy and long-chain 3-hydroxyacyl coenzyme A dehydrogenase deficiency Hepatology 1994 19 339-345... 

LeFlir M, Dubach U. Peroxisomal and mitochondrial b-oxidation in the rat kidney. Distribution of fatty acyl coenzyme A oxidase and 3-hydroxyacyl-coenzyme A dehydrogenase activities along the nephron. J Flistochem Cytochem 1982 30 441-444. 

D.W. Johnson, M.-U. Trinh, Analysis of isomeric long-chain hydroxy fatty acids by MS-MS application to the diagnosis of long-chain 3-hydroxyacyl CoA dehydrogenase deficiency. Rapid Commun. Mass Spectrom., 17 (2003) 171. 

As noted above, there have been reports that link some cases of APLP with a defect in fatty acid metabolism in the fetus. These include fetal deficiencies of long chain 3-hydroxyacyl-coenzyme A dehydrogenase (LCHAD), carnitine-palmitoyl transferase 1 (CPT 1), and medium chain acyl-coenzyme A dehydrogenase (MCAD). The mechanism by which defective fetal fatty acid oxidation causes maternal illness is not known. However, since the fetus uses primarily glucose metabolism for its energy needs, it is likely that toxic products from the placenta, which does use fatty acid oxidation, cause the maternal liver failure. 

The rate of fatty acid oxidation is most likely regulated by the carnitine-dependent transport of acyl residues across the mitochondrial inner membrane, the supply of fatty acids to the cell, and the concentration of cofactors such as CoA and carnitine. Moreover, feedback inhibition is exerted by NADH on 3-hydroxyacyl-CoA dehydrogenase and by acetyl-CoA on 3-ketothiolase. Interestingly, malonyl-CoA, an intermediate in the fatty acid biosynthetic pathway, is a strong inhibitor ofCAT-I. 

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