d-Methylmalonyl CoA synthesis: First, propionyl CoA is carboxylated, forming D-methylmalonyl CoA. The enzyme propionyl CoA carboxylase has an absolute requirement for the coenzymes biotin and ATP, as do ACC and most other carboxylases.
b.
l-Methylmalonyl CoA formation: Next, the D-isomer is converted to the L-form by the enzyme methylmalonyl CoA racemase.
c.
Succinyl CoA synthesis: Finally, the carbons of L-methylmalonyl CoA are rearranged, forming succinyl CoA, which can enter the TCA cycle (see p. 113). [Note: This is the only example of a glucogenic precursor generated from fatty acid oxidation.] The enzyme methylmalonyl CoA mutase requires a coenzyme form of vitamin B12 (deoxyadenosylcobalamin). The mutase reaction is one of only two reactions in the body that require vitamin B12 (see p. 379). [Note: In patients with vitamin B12 deficiency, both propionate and methylmalonate are excreted in the urine. Two types of heritable methylmalonic acidemia and aciduria have been described: one in which the mutase is missing or deficient (or has reduced affinity for the coenzyme) and one in which the patient is unable to convert vitamin B12 into its coenzyme form. Either type results in metabolic acidosis and neurologic manifestations.] 7.
Unsaturated fatty acid β-oxidation: The oxidation of unsaturated fatty acids generates intermediates that cannot serve as substrates for 2,3-enoyl CoA hydratase (see Fig. 16.17). Consequently, additional enzymes are required. Oxidation of a double bond at an odd-numbered carbon, such as 18:1(9) (oleic acid), requires one additional enzyme, 3,2-enoyl CoA isomerase, which converts the 3-cis derivative obtained after three rounds of β-oxidation to the 2-trans derivative required by the hydratase. Oxidation of a double bond at an even-numbered carbon, such as 18:2(9,12) (linoleic acid), requires an NADPH-dependent 2,4-dienoyl CoA reductase in addition to the isomerase. [Note: Because unsaturated fatty acids are less reduced than saturated fatty acids, fewer reducing equivalents are produced by their oxidation.] 8.
Peroxisomal β-oxidation: VLCFA ≥22 carbons in length undergo a preliminary β-oxidation in peroxisomes, because peroxisomes and not mitochondria are the primary site of the synthetase that activates fatty acids of this length. The shortened fatty acid (linked to carnitine) diffuses to a mitochondrion for further oxidation. In contrast to mitochondrial βoxidation, the initial dehydrogenation in peroxisomes is catalyzed by a FAD-containing acyl CoA oxidase. The FADH2 produced is oxidized by O2, which is reduced to hydrogen peroxide (H2O2). Therefore, no ATP is generated from this step. The H2O2 is reduced to H2O by catalase (see p. 148). [Note: Genetic defects in the ability either to target matrix proteins to peroxisomes (resulting in Zellweger syndrome, a peroxisomal biogenesis disorder) or to transport VLCFA across the peroxisomal membrane (resulting in X-linked adrenoleukodystrophy) lead to accumulation of VLCFA in the blood and tissues.]
C. Peroxisomal α-oxidation