Enoyl long-chain

Enzymes 7,9, and 13 form a trifunctional protein associated with the inner face of the inner mitochondrial membrane. Very-long-chain acyl-CoA dehydrogenase is also associated with other inner mitochondrial membranes while the other enzymes are in the matrix and may be loosely associated with the inner face of the inner membrane. A medium-chain 2-enoyl-CoA hydratase may also be present in the mitochondrial matrix. 

Mitochondria contain three acyl CoA dehydrogenases which act on short-, medium- and long-chain acyl CoAs, respectively. In contrast, there is just one each of the enzymes enoyl CoA hydratase, hydroxyacyl CoA dehydrogenase and (3-ketothiolase which all have a broad specificity with respect to the length of the acyl chain. 

Very long chain fatty acids are initially oxidized in the peroxisome where the initial oxidation step is catalyzed by acyl-CoA oxidase and the subsequent steps in fS-oxidation are catalyzed by a multi-enzyme complex with hydratase, dehydo-genase, and thiolase activities. Unsaturated fatty acids require additional enzymatic activities, including enoyl-CoA isomerase and dienoyl-CoA reductase. Readers are directed to Vance and Vance (2) for additional details regarding fi-oxidation, including the details of the metabolic reactions. 

Isoniazid interferes with mycolic acid synthesis by inhibiting an enoyl reductase (InhA) which forms part of the fatty acid synthase system in mycobacteria. Mycolic acids are produced by a diversion of the normal fatty acid synthetic pathway in which short-chain (16 carbon) and long-chain (24 carbon) fatty acids are produced by addition of 7 or 11 malonate extension units from malonyl coenzyme A to acetyl coenzyme A. InhA inserts a double bond into the extending fatty acid chain at the 24 carbon stage. The long-chain fatty acids are further extended and condensed to produce the 60-90 carbon (3-hydroxymycolic acids which are important components of the mycobacterial cell wall. Isoniazid is converted inside the mycobacteria to a free radical species by a catalase peroxidase enzyme, KatG. The active free radicals then attack and inhibit the enoyl reductase, InhA, by covalent attachment to the active site. 

Yeasts are able to degrade long-chain alkanes. The initial hydroxylation is carried out in micrososmes by cytochrome P-450, while degradation of the alkanoate is carried out in peroxisomes that contain the P-oxidation enzymes alkanoate oxidase, enoyl-CoA hydratase, and 3-hydroxyacyl-CoA dehydrogenase. Further details are given in Chapter 4, Sections 4.4.1.2 and 4.4.4. 

Despite the fact that numerous enzymes have been characterized that catalyze the addition of water to unsaturated fatty acids that are coupled to CoA or ACP, such as methylglucatonyl-CoA hydratase (E.C. 4.2.1.18), lactoyl-CoA dehydratase (E.C. 4.2.1.54), 3-hydroxybutyryl-CoA dehydratase (E.C. 4.2.1.55), itaconyl-CoA dehydratase (E. C. 4.2.1.56), isohexenylglutaconyl-CoA hydratase (E. C. 4.2.1.57), farnesyl-CoA dehydratase (E.C. 4.2.1.57), long-chain enoyl-CoA hydratase (E.C. 4.2.1.74), 3-hydroxydecanoyl-ACP dehydratase (E.C. 4.2.1.60) and 3-hydroxypalmitoyl-ACP dehydratase (E.C. 4.2.1.61), these enzymes are seldomly applied in organic synthesis. 

Abbreviations FASN, fatty acid synthase ACC, acetyl-CoA-carboxylase ACL, ATP-citrate lyase NADPH, nicotinamide adenine dinucleotide phosphate MAT, malonyl acetyl transferases KS, ketoacyl synthase KR, p-ketoacyl reductase DH, p-hydroxyacyl dehydratase ER, enoyl reductase TE, thioesterase ACP, acyl carrier protein VLCFA, very long chain fatty acids ELOVL, elongation of very long chain fatty acids SCDl, stearoyl-CoA desaturase-1 AMPK, AMP-activated kinase ME, malic enzyme FASKOL, liver-specific deletion of FAS PPARa, Peroxisome Proliferator-Activating Receptor alpha HMG-CoA, 3-hydroxy-3-methyl-glutaryl-CoA SREBP, sterol response element binding protein SIP, site-one protease S2P, site-two... 

Fig. 8. P-Oxidation of fatty acids in E. coli. Long-chain fatty acids are transported into the cell by FadL and converted to their CoA thioesters by FadD (not shown). The acyl-CoAs are substrates for the (1) acyl-CoA dehydrogenase (YafH) to form a trans-2-enoyl-CoA. The double bond is reduced by (2) rrans-2-enoyl-hydratase (crotonase) activity of FadB. The P-hydroxyacyl-CoA is then a substrate for the NADP -dependent dehydrogenase activity of FadB (3). A thiolase, FadA (4), releases acetyl-CoA from the P-ketoacyl-CoA to form an acyl-CoA for subsequent cycles. (5) Polyunsaturated fatty acyl-CoAs are reduced by the 2,4-dienoyl-CoA reductase (FadH). (6) FadB also catalyzes the isomerization of c/s-unsaturated fatty acids to trans. (7) The epimerase activity of FadB converts O-P-hydroxy thioesters to their L-enantiomers via the /rans-2-enoyl-CoA.

Fig. 2. Model of the functional and physical organization of P-oxidation enzymes in mitochondria. (A) P-Oxidation system active with long-chain (LC) acyl-CoAs (B) P-oxidation system active with medium-chain (MC) and short-chain (SC) acyl-CoAs. Abbreviations T, camitineiacylcamitine translocase CPT 11, carnitine palmitoyltransferase 11 AD, acyl-CoA dehydrogenase EH, enoyl-CoA hydratase HD, t-3-hydroxyacyl-CoA dehydrogenase KT, 3-ketoacyl-CoA thiolase VLC, very-long-chain.

Fig. 1. Reactions in the two-carbon chain elongation of long-chain fatty acids in the ER. Elovl, elongation of long-chain fatty acids KAR, 3-ketoacyl-CoA reductase TER, rrons-2,3-enoyl-CoA reductase.

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