Acetyl CoA carboxylase: the purified transcarboxylase component

Acetyl CoA carboxylase of Escherichia coli has been resolved into three functionally dissimilar proteins: (1) biotin-carboxyl carrier protein (BCCP); (2) a biotin carboxylase component that catalyzes the Mn-ATP-dependent carboxylation of BCCP to form CO(2) (-)-BCCP; and (3) a transcarboxylase component that catalyzes the transfer of the carboxyl group from CO(2) (-)-BCCP to acetyl CoA to form malonyl CoA.The transcarboxylase has been purified 1700-fold. Evidence that this protein catalyzes the transcarboxylase step includes the demonstration that it (a) catalyzes the carboxylation of BCCP, (b) catalyzes the BCCP-dependent exchange between [(14)C]acetyl CoA and malonyl CoA, (c) binds labeled acetyl CoA and malonyl CoA, and (d) catalyzes the decarboxylation of CO(2) (-)-BCCP. On the basis of this evidence, it is concluded that the transcarboxylase component contains sites for the acyl CoA group and for biotin, the covalently bound prosthetic group of BCCP.