The condensation reaction of fatty acid biosynthesis. II. Requirement of the enzymes of the condensation reaction for fatty acid synthesis.

A yeast system differs only in that the major product is stearylCo.4 (5, 6) rather than the free fatty acid, palmitate, as in the other systems. The fatty acid synthetase reaction appears to be a multistage reaction catalyzed by a particulate enzyme complex estimated to have a molecular weight of 2,000,OOO in yeast (6) and to be readily sedimentable by ultracentrifugation in adipose tissue (4) and avian liver (7). Fractionation of the fatty acid synthetase complex in these systems has not been accomplished. A reaction catalyzed by the fatty acid synthetase complex that has been of particular interest is the condensation reaction responsible for chain elongation. The observation that a condensation reaction involving malonyl-CoA and fatty acyl-CoA derivatives, as measured by the malonyl-CoA-CO2 exchange reaction, was catalyzed by an enzyme system from extracts of Clostridium kluyveri (8, 9) led to extensive studies attempting to link this reaction with fatty acid synthesis. Evidence suggesting that this malonyl-CoA-CO2 exchange reaction represented the condensation reaction of fatty acid synthesis has been the finding that this reaction (4, 6, 9), or the analogous acyl-CoA-dependent decarboxylation of malonyl-CoA (9, lo), was catalyzed by a number of fatty acidsynthesizing systems. In addition the ratio of malonyl-CoACOz exchange activity to fatty acid-synthesizing activity remains constant through a 45-fold purification of the synthetase system in adipose tissue (4). Of great importance has been the demonstration by Lynen, in a purified yeast synthetase system, of a protein-bound P-keto acid which is the product of a condensation of malonyl-CoA with a fatty acyl-CoA (5, 6). The present paper describes the characteristics of fatty acid synthetase systems from two bacteria, C. kluyveri and Escherichia coli. These systems differ from those mentioned above in that the enzymes required for fatty acid synthesis are not found in a single complex. Fractionation of the bacterial systems shows that the two enzyme fractions that are required for the acylCoA-dependent malonyl-CoA-CO2 exchange reaction (11)’ are also required for fatty acid synthesis. The requirement of these enzymes both for the condensation reaction and for fatty acid synthesis in each bacterial system gives additional support for the condensation-decarboxylation mechanism in long chain fatty acid synthesis. Preliminary reports on each of these bacterial systems have appeared (12, 13).