Fatty acid synthesis and degradation are similar in that both processes

Although fatty acid biosynthesis and degradation are different processes, both are initiated by forming an acyl CoA intermediate. The synthetic process is said to sta in the cytosol when acetyl CoA is formed from citrate, which is cleaved by citrate lyase. The degrading process stas when the free fatty acid is transpoed into a cell and is conveed into the CoA derivative by fatty acid CoA synthetase (thiokinase). In degradation, the fatty acid is transpoed into the mitochondrion using the carnitine shuttle, where the CoA derivative is reconstituted and subjected to -oxidation, in which the various acyl moieties are always maintained as the CoA derivative. The reducing equivalents obtained by oxidation of the acyl chain are transferred onto NAD+ or FAD to form NADH or FADH2. These cofactors then pass their electrons to the electron transpo chain NADH and succinate dehydrogenase, respectively. Thus, most -oxidation takes place in the mitochondrial matrix in juxtaposition to the electron transpo system. In synthesis, which takes place entirely in the cytosol and uses NADPH as the source of reducing equivalents, the acetyl CoA is transformed to malonyl CoA by a biotin requiring carboxylation reaction, the rate-limiting and regulated reaction in the synthetic sequence. The malonyl moiety of malonyl CoA is transferred to another - SH group on an acyl carrier protein that is pa of the fatty acid synthase complex. The acyl chain then grows in two-carbon increments by the addition of an acetate moiety from acetyl CoA Ref : Biochemistry by U. Satyanarayana 3rd edition Pgno : 287