Monounsaturated fatty acids biosynthesis

The Organization of the Fatty Acid Synthase Is Different in E. coli and Animals Biosynthesis of Monounsaturated Fatty Acids Follows Distinct Routes in E. coli and Animal Cells... 

Biosynthesis of Monounsaturated Fatty Acids Follows Distinct Routes in E. coli and Animal Cells... 

Anaerobic pathway for biosynthesis of monounsaturated fatty acids in E. coli. Synthesis of monounsaturated fatty acids follows the pathway described previously for saturated fatty acids until the intermediate j8-hydroxydecanoyl-ACP is reached. At this point an apparent competition arises between the enzymes involved in saturated and unsaturated fatty acid synthesis. 

As the name anaerobic implies, the double bond of the fatty acid is inserted in the absence of oxygen. Biosynthesis of monounsaturated fatty acids follows the pathway described previously for saturated fatty acids until the intermediate /3-hydroxydecanoyl-ACP is reached (fig. 18.15). At this point, a new enzyme, /3-hydroxydecanoyl-ACP dehydrase, becomes involved. This dehydrase can form the a-j8 trans double bond, and saturated fatty acid synthesis can occur as previously discussed. In addition, this dehydrase is capable of isomerization of the double bond to a cis /3-y double bond as shown in figure 18.15. The /3-y unsaturated fatty acyl-ACP is subsequently elongated by the normal enzymes of fatty acid synthesis to yield pal-mitoleoyl-ACP (16 1A9). The conversion of this compound to the major unsaturated fatty acid of E. coli, cA-vacccnic acid (18 1A11), requires a condensing enzyme that we have not previously discussed, /3-ketoacyl-ACP synthase II, which shows a preference for palmitoleoyl-ACP as a substrate. The subsequent conversion to vaccenyl-ACP is cata-... 

Methyl-branched fatty acids are intermediates in branched alkane biosynthesis (Juarez et al., 1992). Thus, [l-14C]propionate labeled methyl-branched fatty acids of 16-20 carbons, but did not label straight chain-saturated and monounsaturated fatty acids (Chase et al., 1990). 

The released unesterified cholesterol can then be usedfor membrane biosynthesis. Alternatively, it can be reesterified for storage inside the cell. In fact, free cholesterol activates acyl CoA cholesterol acyltransferase (ACAT), the enzyme catalyzing this reaction. Reesterified cholesterol contains mainly oleate and palmitoleate, which are monounsaturated fatty acids, in contrast with the cholesterol esters in LDL, which are rich in linoleate, a polyunsaturated fatty acid (see Table 24.1). It is imperative that the cholesterol be reesterified. High concentrations of unesterified cholesterol disrupt the integrity of cell membranes. 

Murphy, D. J., Mukherjee, K. D. (1988) Biosynthesis of very long chain monounsaturated fatty acids by subcellular fractions of developing seeds. 

Holbrook LA, Magus JR, Taylor DC. Abscisic acid induction of elongase activity, biosynthesis and accumulation of very long chain monounsaturated fatty acids and oil body proteins in microspore-derived embryos of Brassica napus L cv Reston. Plant Sci 1992 84 99-115. 

SCHEME 3.2 Outline of the biosynthesis of the saturated fatty acid palmitic acid (1) and the monounsaturated fatty acid oleic acid (2) and... 

Initial investigations on the mechanism of unsaturated fatty acid biosynthesis in the eubacteria revealed that a variety of these organisms were incapable of forming long-chain monounsaturated acids by direct... 

The common fatty acids have a linear chain containing an even number of carbon atoms, which reflects that the fatty acid chain is built up two carbon atoms at a time during biosynthesis. The structures and common names for several common fatty acids are provided in table 18.1. Fatty acids such as palmitic and stearic acids contain only carbon-carbon single bonds and are termed saturated. Other fatty acids such as oleic acid contain a single carbon-carbon double bond and are termed monounsaturated. Note that the geometry around this bond is cis, not trans. Oleic acid is found in high concentration in olive oil, which is low in saturated fatty acids. In fact, about 83% of all fatty acids in olive oil is oleic acid. Another 7% is linoleic acid. The remainder, only 10%, is saturated fatty acids. Butter, in contrast, contains about 25% oleic acid and more than 35% saturated fatty acids. 

The reactions mentioned above account for many, but by no means all, pheromones whose biosynthesis is related to fatty acids. For example, the silkworm moth, Bombyx mori, has as its main pheromone component E, Z)-10,12-hexadecadien-l-ol (1). The corresponding diunsaturated fatty acid also is present in the gland (JjL), as is a large quantity of (Z) -11-hexadecenoic acid (15.). Recent studies have demonstrated that the monounsaturated acid is formed first, followed by what appears to be a 1,4-desaturation yielding the diunsaturated acid (1 ). 

Excess cholesterol can also be metabolized to CE. ACAT is the ER enzyme that catalyzes the esterification of cellular sterols with fatty acids. In vivo, ACAT plays an important physiological role in intestinal absorption of dietary cholesterol, in intestinal and hepatic lipoprotein assembly, in transformation of macrophages into CE laden foam cells, and in control of the cellular free cholesterol pool that serves as substrate for bile acid and steroid hormone formation. ACAT is an allosteric enzyme, thought to be regulated by an ER cholesterol pool that is in equilibrium with the pool that regulates cholesterol biosynthesis. ACAT is activated more effectively by oxysterols than by cholesterol itself, likely due to differences in their solubility. As the fatty acyl donor, ACAT prefers endogenously synthesized, monounsaturated fatty acyl-CoA. 

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