Acetyl-CoA carboxylase catalyzes the first committed step in the biosynthesis of fatty acids. The Escherichia coli form of the enzyme consists of a biotin carboxylase protein, a biotin carboxyl carrier protein, and a carboxyltransferase protein. In this report the overexpression of the genes for the carboxyltransferase component is described. The steady-state kinetics of the recombinant carboxyltransferase are characterized in the reverse direction, in which malonyl-CoA reacts with biocytin to form acetyl-CoA and carboxybiocytin. The initial velocity patterns indicated that the kinetic mechanism is equilibrium-ordered with malonyl-CoA binding before biocytin and the binding of malonyl-CoA to carboxyltransferase at equilibrium. The biotin analogs, desthiobiotin and 2-imidazolidone, inhibited carboxyltransferase. Both analogs exhibited parabolic noncompetitive inhibition, which means that two molecules of inhibitor bind to the enzyme. The pH dependence for both the maximum velocity (V) and the (V/K)biocytin parameters decreased at low pH. A single ionizing group on the enzyme with a pK of 6.2 or lower in the (V/K)biocytin profile and 7. 5 in the V profile must be unprotonated for catalysis. Carboxyltransferase was inactivated by N-ethylmaleimide, whereas malonyl-CoA protected against inactivation. This suggests that a thiol in or near the active site is needed for catalysis. The rate of inactivation of carboxyltransferase by N-ethylmaleimide decreased with decreasing pH and indicated that the pK of the sulfhydryl group had a pK value of 7.3. It is proposed that the thiolate ion of a cysteine acts as a catalytic base to remove the N1' proton of biocytin.