Fatty acid synthetase. A steady state kinetic analysis of the reaction catalyzed by the enzyme from pigeon liver.

Abstract

The kinetic mechanism of pigeon liver fatty acid synthetase action has been studied using steady state kinetic analysis. Initial velocity studies are consistent with an earlier suggestion that the enzyme catalyzes this reaction by a seven-site ping-pong mechanism. Although the range of substrate concentrations that could be used was limited by several factors, the initial velocity patterns showing the relationship between the substrates acetyl coenzyme CoA, malonyl-CoA, and NADPH appear to be a series of parallel lines, regardless of which substrate is varied at fixed levels of a second substrate. However, two of the substrates, acetyl-CoA and malonly-CoA, apparently exhibit a competitive substrate inhibition with respect to each other, but NADPH shows no inhibition of any kind. Product inhibition patterns suggest that free CoA is competitive versus acetyl-CoA and malonyl-CoA and is uncompetitive versus NADPH, and that NADP+ is competitive versus NADPH and uncompetitive versus acetyl-CoA or malonyl-CoA. These results are consistent with a seven-site ping-pong mechanism with intermediates covalently bound to 4'-phosphopantetheine (part of acyl carrier protein). Double competitive substrate inhibition by acetyl-CoA and malonyl-CoA is consistent with the rate equation derived for the over-all mechanism. The kinetic mechanism developed from these results is capable of explaining the formation of fatty acids from malonyl-CoA and NADPH alone (Katiyar, S. S., Briedis, A. V., and Porter, J. W. (1974) Arch. Biochem. Biophys. 162, 412-420) and also the formation of triacetic acid lactone from either malonyl-CoA alone or acetyl-CoA plus malonyl-CoA.