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Biosynthesis of Fatty Acids and Eicosanoids:- The Fatty Acid Synthase of Some Organisms Consists of Multifunctional Proteins

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  794

2026-06-29

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Biosynthesis of Fatty Acids and Eicosanoids:- The Fatty Acid Synthase of Some Organisms Consists of Multifunctional Proteins

In E. coli and some plants, the seven active sites for fatty acid synthesis (six enzymes and ACP) reside in seven separate polypeptides (Fig. 21–7, top). In these complexes, each enzyme is positioned with its active site near that of the preceding and succeeding enzymes of the sequence. The flexible pantetheine arm of ACP can reach all the active sites, and it carries the growing fatty acyl chain from one site to the next; intermediates are not released from the enzyme complex until it has formed the finished product. As we have seen in earlier chapters, this channeling of intermediates from one active site to the next increases the efficiency of the overall process. The fatty acid synthases of yeast and of vertebrates are also multienzyme complexes, and their integration is even more complete than in E. coli and plants. In yeast, the seven distinct active sites reside in two large, multifunctional polypeptides, with three activities on the α subunit and four on the β subunit. In vertebrates, a single large polypeptide (Mr 240,000) contains all seven enzymatic activities as well as a hydrolytic activity that cleaves the finished fatty acid from the ACP-like part of the enzyme complex. The vertebrate enzyme functions as a dimer (Mr 480,000) in which the two identical subunits lie head-to-tail. The subunits appear to function independently. When all the active sites in one subunit are inactivated by mutation, palmitate synthesis is only modestly reduced.

FIGURE 21–7 Structure of fatty acid synthases. The fatty acid synthase of bacteria and plants is a complex of at least seven different polypeptides. In yeast, all seven activities reside in only two polypep tides; the vertebrate enzyme is a single large polypeptide.