Acyl-CoA Desaturation System

There are three proteins required for the formation of unsaturated fatty acids from precursor saturated acids by the microsomal electron-transport chain. They are NADH-cytochrome reductase, cytochrome 5, and acyl-CoA desaturase. The reaction sequence (Fig. lA) requires molecular oxygen as a proton acceptor, and electrons from NADH. The electron-transport sequence commences with the reductase, passes through cytochrome b, and terminates with the cyanide-sensitive factor, acyl-CoA desaturase (Enoch et al., 1976). Both cytochrome b reductase and cytochrome b are amphipathic proteins in which the redox center is located at the hydrophilic end while the hydrophobic portion binds to lipid moieties (DePierre and Ernster, 1977). Several studies have demonstrated the ability of cytochrome b and cytochrome b reductase to undergo lateral diffusion both in microsomes and in liposomes during the course of electron transfer (Hackenbrock, 1976). 

Cytochrome b can be solubilized either by detergents or with trypsin in a form which retains biologic activity (DePierre and Emster, 1977). However, the proteins thus obtained differ in a number of properties. The protein extracted by trypsin treatment is soluble in aqueous solutions, does not bind phospholipids, and has a molecular weight of 11,000. The detergent-solubilized protein forms aggregates in aqueous 

The desaturase is more nonpolar than the other components of the microsomal acyl-CoA desaturation system, possessing 62% hydrophobic amino acid residues and one atom of nonheme iron and having a molecular weight of 53,000 (Strittmatter et al., 1974). 

The phospholipid requirement for this system has recently been studied by Enoch et al. (1976), who concluded that the hydrocarbon is necessary both as an initial attachment site for the stearoyl-CoA prior to binding to the enzyme and as an anchor for the hydrophobic regions of the desaturase system components. The study by Enoch et al. supports the view that the fluidity of the hydrocarbon chains in the membrane allows for lateral diffusion of the three enzymes of the system. In 

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