A highly purified acyl-CoA carboxylase from the nametode Turbatrix aceti catalyzes the ATP, Mg2+, and HCO3- dependent alpha-carboxylation of acetyl-CoA, propionyl-CoA, and butyryl-CoA at the respective rates of 6.8, 39.7, and 9.1 mumol per min per mg. The enzyme is inhibited by avidin and sulfhydryl reagents. It is activated up to 30-fold by the monovalent cations K+, Rb+, Cs+, or NH4+, with the apparent activation constants of 11.0, 4.1, 10.0, and 6.7 mM, respectively. In the presence of K+, the apparent Km for ATP increases 5-fold, and the Km for HCO3- decreases 4-fold, whereas the Km for propionyl-CoA remains constant. Of the partial reactions, the ATP-32P exchange reaction and the carboxylation of free biotin have a nearly absolute requirement for K+. By contrast, the [14C]acetyl-CoA-malonyl-CoA exchange reaction proceeds without K+ at 80% of its maximum rate. The data indicate that K+ primarily stimulates the first half of the carboxylation reaction, i.e., the ATP-dependent carboxylation of the biotinyl residue.