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Tissues that synthesize fatty acids do not contain the enzymes that degrade fatty acids.

High NADPH levels inhibit β -oxidation.

In the presence of insulin, the key fatty acid – degrading enzyme is not induced.

Newly synthesized fatty acids cannot be converted to their CoA derivatives.

Transport of fatty acids into mitochondria is inhibited under conditions in which fatty acids are being synthesized.

In the first step, dehydrogenation of fatty acyl–CoA produces a double bond between the α and β carbon atoms (C-2 and C-3), yielding a trans -Δ 2 -enoyl-Co 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 dehydrogenase (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.

In the second step of the β-oxidation cycle, water is added to the double bond of the trans -Δ 2 -enoyl-CoA to form the L stereoisomer of β-hydroxyacyl-CoA (3-hydroxyacyl-CoA). This reaction, catalyzed by enoyl-CoA hydratase, is formally analogous to the fumarase reaction in the citric acid cycle, in which H 2 O adds across an α-β double bond.

In the third step, L β-hydroxyacyl-CoA is dehydrogenated to form β-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-Co 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 O 2 . The reaction catalyzed by β-hydroxyacyl-CoA dehydrogenase is closely analogous to the malate dehydrogenase reaction of the citric acid cycle.

The fourth and last step of the β-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-Co The other product is the coenzyme A thioester of the fatty acid, now shortened by two carbon atoms. This reaction is called thiolysis, by analogy with the process of hydrolysis, because the β-ketoacyl-CoA is cleaved by reaction with the thiol group of coenzyme  The thiolase reaction is a reverse Claisen condensation.

Palmitate, the principal product of the fatty acid synthase system in animal cells, is the precursor of other long-chain fatty acids. It may be lengthened to form stearate (18:0) or even longer saturated fatty acids by further additions of acetyl groups, through the action of fatty acid elongation systems present in the smooth endoplasmic reticulum and in mitochondria.

The more active elongation system of the ER extends the 16-carbon chain of palmitoyl-CoA by two carbons, forming stearoyl-Co Although different enzyme systems are used, and coenzyme A rather than ACP is the acyl carrier in the reaction, the mechanism of elongation in the ER is otherwise identical to that in palmitate synthesis: donation of two carbons by malonyl-CoA, followed by reduction, dehydration, and reduction to the saturated 18-carbon product, stearoyl-Co

Two key products of elongation pathways are linoleate, an ω-6 fatty acid, and α-linolenate, an ω-3 fatty acid. These are precursors for two extensive families of derivative unsaturated fatty acids, the ω-6 and ω-3 families. Humans cannot synthesize linoleate and α-linolenate and must obtain them in the diet.

The ratio of ω-6 to ω-3 fatty acids in the diet, if too high, can lead to cardiovascular disease. The importance of this ratio may reflect the multitude of signaling molecules in the ω-6 and ω-3 families, with their equally complex physiological effects.

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Long-chain odd-number fatty acids are oxidized in the same pathway as the even-number acids, beginning at the carboxyl end of the chain. However, the substrate for the last pass through the β-oxidation sequence is a fatty acyl-CoA with a five-carbon fatty acid. When this is oxidized and cleaved, the products are acetyl-CoA and propionyl-Co The acetyl-CoA can be oxidized in the citric acid cycle, of course, but propionyl-CoA enters a different pathway, having three enzymes.

Propionyl-CoA is first carboxylated to form the D stereoisomer of methylmalonyl-CoA by propionyl-CoA carboxylase, which contains the cofactor biotin. In this enzymatic reaction, as in the pyruvate carboxylase reaction, CO 2 (or its hydrated ion) is activated by attachment to biotin before its transfer to the substrate, in this case the propionate moiety.

The D-methylmalonyl-CoA thus formed is enzymatically epimerized to its L stereoisomer by methylmalonyl-CoA epimerase.

The L-methylmalonyl-CoA then undergoes an intramolecular rearrangement to form succinyl-CoA, which can enter the citric acid cycle. This rearrangement is catalyzed by methyl-malonyl-CoA mutase, which requires as its coenzyme 5′-deoxyadenosylcobalamin, or coenzyme B 12 , which is derived from vitamin B 12 (cobalamin).

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Palmitate and stearate serve as precursors of the two most common monounsaturated fatty acids of animal tissues: palmitoleate, 16:1(Δ 9 ), and oleate, 18:1(Δ 9 ); both of these fatty acids have a single cis double bond between C-9 and C-10.

The double bond is introduced into the fatty acid chain by an oxidative reaction catalyzed by fatty acyl–CoA desaturase, a mixed-function oxidase. Two different substrates, the fatty acid and NADPH, simultaneously undergo twoelectron oxidations.

The path of electron flow includes a cytochrome (cytochrome b 5 ) and a flavoprotein (cytochrome b 5 reductase), both of which, like fatty acyl–CoA desaturase, are in the smooth ER.

In plants, oleate is produced by a stearoyl-ACP desaturase (SCD) that uses reduced ferredoxin as the electron donor in the chloroplast stroma.