DECARBOXYLATION OF MALONYL-(ACYL CARRIER PROTEIN) BY 3-OXOACYL-(ACYL CARRIER PROTEIN) SYNTHASES IN PLANT FATTY-ACID BIOSYNTHESIS

In order to identify regulatory steps in fatty acid biosynthesis, the influence of intermediate 3-oxoacyl-(acyl carrier proteins) (3-oxoacyl -ACPs) and end-product acyl-ACPs of the fatty acid synthase reaction o n the condensation reaction was investigated in vitro, using total fat ty acid synthase preparations and purified 3-oxoacyl-ACP synthases (KA Ss; EC 2.3.1.41) from Cuphea lanceolata seeds. KAS I and II in the fat ty acid synthase preparations were assayed for the elongation of octan oyl- and hexadecanoyl-ACP respectively, and the accumulation of the co rresponding condensation product 3-oxoacyl-ACP was studied by modulati ng the content of the reducing equivalents NADH and NADPH. Complete om ission of reducing equivalents resulted with either KAS in the abnorma l synthesis of acetyl-ACP from malonyl-ACP by a decarboxylation reacti on. Supplementation with NADPH or NADH, separately or in combination w ith recombinant 3-oxoacyl-ACP reductase (EC 1.1.1.100), led to a decre ase in the amount of acetyl-ACP and a simultaneous increase in elongat ion products. This demonstrates that the accumulation of 3-oxoacyl-ACP inhibits the condensation reaction on the one hand, and induces the d ecarboxylation of malonyl-ACP on the other. By carrying out similar ex periments with purified enzymes, this decarboxylation was attributed t o the action of KAS. Our data point to a regulatory mechanism for the degradation of malonyl-ACP in plants which is activated by the accumul ation of the fatty acid synthase intermediate 3-oxoacyl-ACP.