Oxidation of Fatty Acids:-The β Oxidation of Saturated Fatty Acids Has Four Basic Steps
Four enzyme-catalyzed reactions make up the first stage of fatty acid oxidation (Fig. 17–8a). First, dehydrogenation of fatty acyl–CoA produces a double bond between the and carbon atoms (C-2 and C-3), yielding a trans - Δ2- enoyl - CoA ( the symbol Δ2 designates the position of the double bond; you may want to re view fatty acid nomenclature, p. 343.) Note that the new double bond has the trans configuration, whereas the double bonds in naturally occurring unsaturated fatty acids are normally in the cis configuration. We consider the significance of this difference later. This first step is catalyzed by three isozymes of acyl-CoA dehydrogenase, each specific for a range of fatty-acyl chain lengths: very-long-chain acyl-CoA de hydrogenase (VLCAD), acting on fatty acids of 12 to 18 carbons; medium-chain (MCAD), acting on fatty acids of 4 to 14 carbons; and short-chain (SCAD), acting on fatty acids of 4 to 8 carbons. All three isozymes are flavoproteins with FAD (see Fig. 13–18) as a prosthetic group. The electrons removed from the fatty acyl–CoA are transferred to FAD, and the reduced form of the de hydrogenase immediately donates its electrons to an electron carrier of the mitochondrial respiratory chain, the electron-transferring flavoprotein (ETF) (see Fig. 19–8). The oxidation catalyzed by an acyl-CoA de hydrogenase is analogous to succinate dehydrogenation in the citric acid cycle (p. XXX); in both reactions the enzyme is bound to the inner membrane, a double bond is introduced into a carboxylic acid between the and β carbons, FAD is the electron acceptor, and electrons from the reaction ultimately enter the respiratory chain and pass to O2, with the concomitant synthesis of about 1.5 ATP molecules per electron pair.
In the second step of the β-oxidation cycle (Fig. 17–8a), water is added to the double bond of the trans- Δ2 -enoyl-CoA to form the L stereoisomer of β -hydroxyacyl -CoA(3-hydroxyacyl-CoA). This re action, catalyzed by enoyl-CoA hydratase, is formally analogous to the fumarase reaction in the citric acid cycle, in which H2O adds across an α–β double bond (p. XXX).
In the third step, L- β -hydroxyacyl-CoA is dehydrogenated to form -ketoacyl-CoA, by the action of β -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 O2. The reaction catalyzed by -hydroxyacyl-CoA de hydrogenase is closely analogous to the malate dehydrogenase reaction of the citric acid cycle (p. XXX).
The fourth and last step of the -oxidation cycle is catalyzed by acyl-CoA acetyltransferase, more commonly called thiolase, which promotes reaction of β-ketoacyl-CoA with a molecule of free coenzyme A to split off the carboxyl-terminal two-carbon fragment of the original fatty acid as acetyl-CoA. The other product is the coenzyme A thioester of the fatty acid, now shortened by two carbon atoms (Fig. 17–8a). This reaction is called thiolysis, by analogy with the process of hydrolysis, because the β- ketoacyl-CoA is cleaved by re action with the thiol group of coenzyme A. The last three steps of this four-step sequence are catalyzed by either of two sets of enzymes, with the en zymes 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 α4β4
subunits. Each subunit contains two activities, the enoyl-CoA hydratase and the β-hydroxyacyl-CoA dehydrogenase; the β subunits contain the thiolase activity. This tight association of three enzymes may allow efficient substrate channeling from one active site to the next, without diffusion of the intermediates away from the enzyme surface. When TFP has shortened the fatty acyl chain to 12 or fewer carbons, further oxidations are catalyzed by a set of four soluble enzymes in the matrix. As noted earlier, the single bond between methyl ene (-CH2-) groups in fatty acids is relatively stable. The β- oxidation sequence is an elegant mechanism for destabilizing and breaking these bonds. The first three reactions of β oxidation create a much less stable C-C bond, in which the α carbon (C-2) is bonded to two carbonyl carbons (the β- ketoacyl-CoA intermediate). The ketone function on the β carbon (C-3) makes it a good target for nucleophilic attack by the -SH of coenzyme A, catalyzed by thiolase. The acidity of the α hydrogen and the resonance stabilization of the carbanion generated by the departure of this hydrogen make the terminal -CH2-CO-S-CoA a good leaving group, facilitating breakage of the α –β bond.
