Multienzyme complexes fatty acid synthetases

Flg. 13. The chloroplast envelope plays a predominant role in the assembly of the three parts of the galactolipid molecule (galactose, glycerol, fatty acids). Saturated and monounsat-urated fatty acids are synthesized in the stroma by a multienzyme complex (fatty acid synthetase). Then, the different steps occur on the envelope, probably at the level of the inner membrane. Under these conditions, massive transport of galactolipids should occur very rapidly between the inner layer of the inner envelope membrane and the thylakoids (reproduced from Douce and Joyaid, 1979a, by permission). 

In the organism tissues, fatty acids are continually renewed in order to provide not only for the energy requirements, but also for the synthesis of multicomponent lipids (triacylglycerides, phospholipids, etc.). In the organism cells, fatty acids are resynthetized from simpler compounds through the aid of a supramolecular multienzyme complex referred to as fatty acid synthetase. At the Lynen laboratory, this synthetase was first isolated from yeast and then from the liver of birds and mammals. Since in mammals palmitic acid in this process is a major product, this multienzyme complex is also called palmitate synthetase. 

Lin, C. Y. and Smith, S. 1978. Properties of the thioesterase component obtained by limited trypsinization of fatty acid synthetase multienzyme complex. J. Biol. Chem. 253, 1954-1962. 

Although fatty acid 8-oxidation is catalyzed by a series of intramitochon-drial enzymes, and the fatty acyl chain is carried by CoA, fatty acid synthesis is catalyzed by a cytosolic-multienzyme complex in which the growing fatty acyl chain is bound by thioester linkage to an enzyme-bound 4 -phosphopantetheine residue. This component of the fatty acid synthetase complex is ACP. 

Fatty acid synthesis occurs by way of a rather complex series of reactions catalyzed by a multienzyme complex called the fatty acid synthetase system. This system is made up of six enzymes and an additional protein, acyl carrier protein (ACP), to which all intermediates are attached. 

A soluble extract was obtained by Lynen and Tada (1961) fromP. patulum that required acetyl-CoA and malonyl-CoA plus NADPH for activity synthesis of 6-methylsalicylic acid, therefore, involves a reductive step to remove an oxygen function. These substrates are identical to those encountered in fatty acid synthesis, and this has led to the proposal that a multienzyme complex similar to fatty acid synthetase might be involved (Lynen and Tada, 1961 Lynen, 1961). Active thiol sites associated with this enzyme were also implicated in binding the intermediates by thioester linkage. The general mechanism applicable to the two synthetases, therefore, appears to be very similar, and the importance of their relationship to each other may be stressed. 

The metabolic routes leading to phenol synthesis, with which this chapter is concerned, utilize the same thioester substrates and depend upon the presence of a multienzyme complex with many properties similar to those of fatty acid synthetase (Dimroth et al., 1970, 1976). They therefore appear to follow genetically determined pathways. These reactions, and those of fatty... 

Fatty acid biosynthesis utilizes acetyl CoA. Radioactive acetate is the common experimental substitute, but in the developing seed sucrose from the mother plant is the initial source of substrate. Biosynthesis is a multi-step process (Fig. 3.14) which firstly involves the formation of malonyl CoA by carboxylation of acetyl CoA with carbon dioxide. This malonyl CoA is then accepted by acyl carrier protein (ACP) which is part of a multienzyme complex called the ACP fatty acid synthetase complex. The malonyl CoA is then condensed with... 

Fig. 67. Model of fatty acid synthetase. In the center of the multienzyme complex is the acyl carrier protein with its pantetheine arm which is shown again in detail below. The arm wheels around from one enzyme of the complex to the next in the direction of the arrow. In the process the individual reactions of chain elongation take place, reactions which are marked with the same letters as in Figure 66 (modified from Lynen 1969).

F. Lynen, H. Engeser, J. Friedrich, W. Schindlbeck, R. Seyffert, and F. Wieland, Fatty acid synthetase of yeast and 6-methylsalicylate synthetase of Penicillium patulum - two multienzyme complexes, iji "Microenvironments and Metabolic Conpartmentation," P.A. Srere and R.W. Estabrook, eds.. Academic Press, New York (1978). 

Attention has been directed also at fatty acid synthetase as a control site for the regulation of fatty acid synthesis. This multienzyme complex mediates the synthesis of saturated fatty acids from malonyl-CoA in a reaction requiring acetyl-CoA and NADPH. Details of the enzyme complex and reaction sequence have been reviewed recently (Volpe and Vagelos, 1976 Katiyar and Porter, 1977 Block and Vance, 1977). All available data suggest that while modulation of acetyl-CoA carboxylase activity represents the essential site of short-term control of fatty acid synthesis, long-term control of fatty acid synthesis may rest with the rate of de novo synthesis of the fatty acid synthetase complex. 

Fatty acids are made initially by a combination of the action of acetyl-Co A carboxylase and fatty acid synthetase. The acetyl-Co A carboxylase enzymes from different organisms vary in their structure with multifunctional proteins being found in mammals and higher plants while E, coli uses a multienzyme complex. The mechanism of acetyl-Co A carboxylase is, however, broadly similar and takes place in two main stages. First biotin is carboxylated by an ATP-dependent reaction. Second, the carboxyl group is transferred to acetyl-CoA thus producing malonyl-CoA. 

Type II fatty acid synthetases are found in bacteria such as E. coli, cyanobacteria and the chloroplasts of algae, higher plants and Euglena. They are multienzyme complexes which comprise a number of dissociable proteins which are responsible for acyl transfer, condensation, dehydration and reduction reactions. ACP can also be isolated as a small molecular mass acidic protein. In E. coli the fatty acid synthetase has the additional feature that it produces monounsaturated fatty acids (mainly cw-vaccenate) in addition to long chain saturated products. 

The two well-studied lipopeptide bioemulsifiers produced by Bacilli—surfactin (Peypoux et al. 1999) and lichenysin (Yakimov et al. 1995)—are structurally similar. As already mentioned, both are composed of a cyclic heptapeptide linked to a fatty acid and differ in the last amino acid of the peptide—leucine and isoleucine, respectively. The peptide moiety, like many small peptides in microorganisms, is synthesized non-ribosomally by a multienzyme peptide synthetase complex (Marahiel 1997). The srfA operon of B. subtilis was defined by a transposon... 

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