Fatty acid synthase complex mammals

The answer is e. (Murray, pp 230-267. Scriver, pp 2297-2326. Sack, pp 121-138. Wilson, pp 287-320.) The fatty acid synthase complex of mammals is composed of two identical subunits. Each of the subunits is a multienzyme complex of seven enzymes and the acyl carrier protein component. All the components are covalently linked together thus, all the components are on a single polypeptide chain, which functions in the presence of another identical polypeptide chain. Each cycle of fatty acid synthesis employs the acyl carrier protein and six enzymes acetyl transferase, malonyl transferase, p-ketoacyl synthase, p-ketoacyl reductase, dehydratase, and enoyl reductase. When the final fatty acid length is reached (usually C16), thioesterase hydrolyzes the fatty acid off of the synthase complex. 

Which of the following answers completes the sentence correctly The major product of the fatty acid synthase complex in mammals is... 

In bacteria and plants, the individual enzymes of the fatty acid synthase system are separate, and the acyl radicals are found in combination with a protein called the acyl carrier protein (ACP). However, in yeast, mammals, and birds, the synthase system is a multienzyme polypeptide complex that incorporates ACP, which takes over the role of CoA. It contains the vitamin pantothenic acid in the form of 4 -phosphopan-tetheine (Figure 45-18). The use of one multienzyme functional unit has the advantages of achieving the effect of compartmentalization of the process within the cell without the erection of permeability barriers, and synthesis of all enzymes in the complex is coordinated since it is encoded by a single gene. 

Fatty acid synthase (FAS) carries out the chain elongation steps of fatty acid biosynthesis. FAS is a large multienzyme complex. In mammals, FAS contains two subunits, each containing multiple enzyme activities. In bacteria and plants, individual proteins, which associate into a large complex, catalyze the individual steps of the synthesis scheme. 

In mammals Fatty Acid Synthase (FAS) catalyzes fatty acid synthesis on a homodimeric enzyme, each monomer of which has seven catalytic activities, and eight sites (In bacteria such as E. colt there are seven separate enzymes plus an acyl-carrier protein. Plants also have individual proteins for the various activities which are associated in a quaternary complex. In eukaryotes other than plants the FAS are complexes of multifunctional proteins. The enzyme weighs approximately 500,000 Daltons. 

The reactions of de novo fatty acid biosynthesis are shown in Figure 18-10. They are carried out by two multienzyme systems functioning in sequence. The first is acetyl-CoA carboxylase, which converts acetyl-CoA to malonyl-CoA. The second is fatty acid synthase, which sequentially joins two-carbon units of malonyl-CoA, eventually producing palmitic acid. Both complexes consist of multifunctional subunits. The various catalytic functions can be readily separated in plant cells and prokaryotes, but in yeasts, birds, and mammals, attempts to subdivide catalytic functions lead to loss of activity. Important features of this system are as follows ... 

The subsequent series of reactions that lead eventually to the formation of palmitate in mammals are carried out by a multienzyme complex known as fatty acid synthase. The initial two carbons, which eventually form C-15 and C-16 of palmitate, are supplied in the form of acetyl-CoA which acts as a primer for the subsequent condensation of malonyl-CoA units which lose CO2 in the process. The hydrogens required for the reductive reactions are all supplied by NADPH and the overall process may be represented as follows ... 

In mammals and in the majority of bacteria, cobalamin regulates DNA synthesis indirectly through its effect on a step in folate metabolism, catalyzing the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate via two methyl transfer reactions. This cytoplasmic reaction is catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyl-transferase), which requires methyl cobalamin (MeCbl) (253), one of the two known coenzyme forms of the complex, as its cofactor. 5 -Deoxyadenosyl cobalamin (AdoCbl) (254), the other coenzyme form of cobalamin, occurs within mitochondria. This compound is a cofactor for the enzyme methylmalonyl-CoA mutase, which is responsible for the conversion of T-methylmalonyl CoA to succinyl CoA. This reaction is involved in the metabolism of odd chain fatty acids via propionic acid, as well as amino acids isoleucine, methionine, threonine, and valine. 

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