Fatty acid synthetase from chloroplasts of soybean cotyledons: ACP activation and CoA inhibition

Fatty acid synthetase from chloroplasts of soybean cotyledons was activated by preincubation with acyl carrier protein and dithiothreitol. The synthetase reaction had a 3–10 min lag which was not eliminated by the preincubation. Acetyl-CoA and malonyl-CoA had no effect on the activation. Fatty acid synthetase from spinach chloroplasts was neither activated by preincubation nor had a lag. The variability of the activity of the soybean enzyme with preincubation suggested that the fatty acid synthetase was present in two forms and that the acyl carrier protein caused conversion to the active form. This fatty acid synthetase and the same synthetase from spinach chloroplasts were inhibited by CoA. The type of inhibition by CoA in soybean was competitive with respect to malonyl-CoA and the Ki was 80μM.