Microsomal fatty acid elongation

Keyes, S.R. and D.L. Cinti (1980). Biochemical properties of cytochrome b5-dependent microsomal fatty acid elongation and identification of products. J Biol. Chem. 255, 11357-11364. 

This pathway (the microsomal system ) elongates saturated and unsaturated fatty acyl-CoAs (from Cjg upward) by two carbons, using malonyl-CoA as acetyl donor and NADPH as reductant, and is catalyzed by the microsomal fatty acid elongase system of enzymes (Figure 21-5). Elongation of stearyl-CoA in brain increases rapidly during myehnation in order to provide C22 and C24 fatty acids for sphingoEpids. 

One of the sex pheromone components of the housefly, Musca domestica, is Z9-21 H that is found on the cuticular surface of the fly. This compound is formed by the elongation of Z9-18 CoA using malonyl-CoA and NADPH to Z15-24 CoA which is decarboxylated to form Z9-21 Hc (Fig. 3) [78-80]. Other pheromone components include an epoxide and ketone that are produced from Z9-21 Hc by a cytochrome P450 [81,82] and methyl-branched alkanes that are produced by the substitution of methylmalonyl-CoA in place of malonyl-CoA at specific points during chain elongation [83,84]. A novel microsomal fatty acid synthase is involved in production of methyl-branched alkanes in most insects [85-87]. This fatty acid synthase is different from the ubiquitous soluble fatty acid synthase that produces saturated straight chain fatty acids in that it is found in the microsomes and prefers methylmalonyl-CoA. The amino acids valine and isoleucine can provide the carbon skeletons for methylmalonyl-CoA as well as propionate [83]. 

St. John, L.C., Lunt, D.K., and Smith, S.B. 1991. Fatty acid elongation and desaturation enzyme activities of bovine liver and subcutaneous adipose tissue microsomes. J. Anim. Sci. 69,... 

Figure 7. Enzymatic steps in long-chain fatty acid elongation. Enzymatic steps of microsomal fatty acyl chain elongation. ELOVL, elongation of very-long-chain fatty acids KAR, 3-ketoacyl-CoA reductase HADC, 3-hydroxyacyl-CoA dehydratase TER, /rowi-2,3-enoyl-CoA reductase [108].

In order to examine the apparently selective inhibition of very long chain fatty acid synthesis in vivo, experiments have been carried out in our laboratories using microsomal fractions these contain endoplasmic reticulum, which is the site of fatty acid elongation (Section 3.2). 

Figure 21-S. Microsomal elongase system for fatty acid chain elongation. NADH is also used by the reductases, but NADPH is preferred.

Figure 23-3. Biosynthesis of the co9, co6,and co3 families of polyunsaturated fatty acids. Each step is catalyzed by the microsomal chain elongation or desaturase system 1,elongase 2,A desaturase 3,A desaturase 4,A desaturase. ( .Inhibition.)...

The FAS multi-enzyme complex synthesizes saturated C16 fatty acids, but cells and tissues need unsaturated and longer chain fatty acids. The palmitoyl-CoA can be modified by either chain elongation and/or oxidation in order to produce different fatty acid molecules. Both elongation and desaturation occur within the smooth endoplasmic reticulum (SER, microsomal fraction) of the cell. 

In liver mitochondria, palmitic acid, as its CoA ester, is lengthened by successive additions of acetyl CoA. There is also a liver microsomal enzyme capable of elongating saturated and unsaturated fatty acids by addition of acetyl CoA or malonyl CoA. 

The biosynthesis of hydrocarbons occurs by the microsomal elongation of straight chain, methyl-branched and unsaturated fatty acids to produce very long-chain fatty acyl-CoAs (Figure 11.1). The very long chain fatty acids are then reduced to aldehydes and converted to hydrocarbon by loss of the carboxyl carbon. The mechanism of hydrocarbon formation has been controversial. Kolattukudy and coworkers have reported that for a plant, an algae, a vertebrate and an insect, the aliphatic aldehyde is decarbonylated to the hydrocarbon and carbon monoxide, and that this process does not require cofactors (Cheesbrough and Kolattukudy, 1984 1988 Dennis and Kolattukudy, 1991,1992 Yoder et al., 1992). In contrast, the Blomquist laboratory has presented evidence that the aldehyde is converted to hydrocarbon and carbon dioxide in a process that... 

Elongation microsomal process (ER), requires malonylCoA and two NADPH, process is similar to that occurring with fatty acid synthase. 

One molecule of oxygen accepts two pairs of electrons, one from palmitoyl-CoA and the other from NADPH or NADH. The electrons NAD(P)H are transported via cytochrome-bs reductase to cytochrome bs (microsomal electron transport Chapter 14). An enzyme-bound superoxide radical is responsible for the oxidation of acyl-CoA. Four desaturases specific for introducing cis double bonds at C9, Ca, C5, and C4, respectively, are known. If the substrate is saturated, the first double bond introduced is C9. With an unsaturated substrate, other double bonds are introduced between the carboxyl group and the double bond nearest the carboxyl group. Desaturation yields a divinylmethane arrangement of double bonds (—CH=CH—CH2—CH=CH—). Usually desaturation alternates with chain elongation. Desaturation is inhibited by fasting and diabetes. The oxidation of unsaturated fatty acids occurs in mitochondria. 

The metabolic pathways for synthesis of n-6 and n-3 families of polyunsaturated fatty acids from the essential fatty acids, linoleic acid (LA) (18 2 [n-6]) and a-linolenic acid (18 3 [n-3]), respectively, are showninFig. 2. Conversion of LA to arachidonic acid (AA) occurs via A6 desaturation to yield y-linolenic acid (GLA), then an elongation step to produce dihomo-y-linolenic acid (DHGL A) and A5 desaturation, to form AA. The A6 and A5 microsomal desaturases have been reported to utilize both NADH and NADPH as cofactors in vitro (Brenner 1977). Whether there is a more stringent pyridine nucleotide requirement in vivo is not known with certainty. Desaturase activities are especially abundant in the liver. 

There are other cases of inhibition of lipid enzymatic pathways by trans fatty acid isomers. The above reported mono-14 -trans isomer of arachidonic acid is inhibitor of the synthesis of thromboxane B2 and, therefore, can prevent rat platelet aggregation [52]. The transformation of mono-trans isomers of linoleic acid by rat liver microsomes showed that the 9-cis,12-trans isomer is better desaturated, whereas the 9-trans, 2-cis isomer (Scheme 6.1) is better elongated [53]. 

In animals, fatty acids are synthesized in the cytoplasm from acetyl-CoA and malonyl-CoA. Microsomal enzymes elongate and desaturate newly synthesized fatty acids as well as those obtained in the diet. 

The desaturation of fatty acids is usually assayed by incubating radioactive fatty acids with microsomes in the presence of appropriate cofactors. This general protocol has been used by many investigators to assay A9, A6, and A5 desaturase activities. It was thus assumed that 7,10,13,16-22 4 and 7,10,13,16,19-22 5 were desaturated by a microsomal A4 desaturase. In 1991, we showed that when rat liver microsomes were incubated with [1- C]7,10,13,16,19-22 5, it was not desaturated to 4,7,10,13,16,19-22 6. However, when malonyl-CoA was included in the incubation, the substrate was chain elongated to 9,12,15,18,21-24 5, which was then desaturated, at position-6, to yield 6,9,12,15,18,21-24 6. When [1- C]7,10,13,16,19-22 5 and the two [3- C]-labeled 24-carbon acids were incubated with hepatocytes, all three acids were metabolized to esterified [1- C]4,7,10, 13,16,19-22 6 (19). The findings implied that 4,7,10,13,16, 19-22 6 was made from 9,12,15-18 3 as follows ... 

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