Fatty acid elongation via elongase pathway of ER

There are two different types of fatty acid elongation takes place in different organisms. These elongation pathways use Coenzyme-A as acyl carrier rather than acyl carrier protein (ACP) as in fatty acid synthesis systems FAS I and FAS II. The first type, mitochondrial fatty acid elongation is the reversal of fatty acid oxidation. This utilises acetyl-CoA as a substrate and extends the chain length of fatty acids with two carbons. This process acts mainly on acyl-CoA shorter than C₁₆. The second process is the elongation pathway of endoplasmic reticulum, which is present in plants, mammals, yeast and other lower eukaryotes. This is a four-step reaction each catalysed by individual enzymes. These enzymes are beta-ketoacyl-CoA synthase, beta-ketoacyl-CoA reductase, beta-hydroxyacyl-CoA dehydratase and trans-2-enoyl-CoA reductase. This pathway mainly acts with acyl-CoA of chain length C₁₆ or larger and important in the generation very long chain fatty acids. This process utilises malonyl-CoA rather than acetyl-CoA for chain elongation. The first enzyme which leads to condensation of malonyl-CoA with acyl-CoA (beta-ketoacyl-CoA synthase) is also called as elongase. There is a single elongase gene in Cryptosporidia as opposed to three different elongase genes in yeast and Coccidia such as Toxoplasma gondii with different substrate specificities. In addition to fatty acid elongation, fatty acid salvage has also been included to this pathway. The fatty acids salvaged from host with the action of acyl-CoA binding proteins (ACBP) can be converted to triacylglycerides with the action of ER-localised enzymes diacylglycerol O-acyltransferase (DGAT) and can be stored in lipid bodies.

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Sources and fates of metabolites