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Fatty acid synthesis starts with acetyl-CoA and builds up by the addition of two-carbon units. Fatty acid synthesis occurs in the cytoplasm of cells while oxidative degradation occurs in the mitochondria. Many of the enzymes for the fatty acid synthesis are organized into a multienzyme complex called fatty acid synthase. [5]
The fatty acids in phospho- and glycolipids usually contain an even number, typically between 14 and 24, of carbon atoms, with 16- and 18-carbon being the most common. FAs may be saturated or unsaturated, with the configuration of the double bonds nearly always cis .
Long chain fatty acids (more than 14 carbon) need to be converted to fatty acyl-CoA in order to pass across the mitochondria membrane. [6] Fatty acid catabolism begins in the cytoplasm of cells as acyl-CoA synthetase uses the energy from cleavage of an ATP to catalyze the addition of coenzyme A to the fatty acid. [6]
The free fatty acids and glycerol are then released into the blood. However more recent studies have shown that adipose triglyceride lipase has to first convert triacylglycerides to diacylglycerides, and that hormone-sensitive lipase converts the diacylglycerides to monoglycerides
These lipases cleave free fatty acids from their attachment to glycerol in the lipid droplet of the adipocyte. The free fatty acids and glycerol are then released into the blood. The activity of hormone sensitive lipase is regulated by the circulating hormones insulin, glucagon, norepinephrine, and epinephrine.
[2]: 634 Fatty acids are made by fatty acid synthases that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the acetyl group, reduce it to an alcohol, dehydrate it to an alkene group and then reduce it again to an alkane group.
The metabolism of fructose at this point yields intermediates in gluconeogenic pathway leading to glycogen synthesis, or can be oxidized to pyruvate and reduced to lactate, or be decarboxylated to acetyl CoA in the mitochondria and directed toward the synthesis of free fatty acid, resulting finally in triglyceride synthesis.
Fatty acid degradation is the process in which fatty acids are broken down into their metabolites, in the end generating acetyl-CoA, the entry molecule for the citric acid cycle, the main energy supply of living organisms, including bacteria and animals. [1] [2] It includes three major steps: Lipolysis of and release from adipose tissue