Urea carboxylase from Saccharomyces cerevisiae. Evidence for a minimal two-step reaction sequence.

Urea carboxylase, an avidin-sensitive enzyme catalyzing the ATP-dependent carboxylation of urea, has been shown to catalyze this reaction in a sequence of at least two steps. In the presence of all of the required carboxylation cofactors except urea, an enzyme-CO2 complex is formed. This complex was isolated by Sephadex chromatography and was capable, in the absence of added cofactors, of transferring the bound COe to urea. In addition to the carboxylation of urea this protein was capable of carboxylating free d-biotin, as well as the urea analogues formamide, formylurea, and acetamide. These data are consistent with the suggestion that urea carboxylase follows a reaction mechanism similar to those of the more thoroughly studied propionyl-CoA, acetyl-CoA, and pyruvate carboxylases.