l ‐Ornithine Carbamoyltransferase from Saccharomyces cerevisiae: Steady‐State Kinetic Analysis

Ornithine carbamoyltransferase of Saccharomyces cerevisiae is subjected to an enzymatic regulation of its anabolic activity when it is bound to the inducible catabolic arginase as described earlier. This regulatory ornithine carbamoyltransferase essentially catalyzes the synthesis of citrulline, but the reverse reaction could be demonstrated using arsenate instead of phosphate. Steady‐state initial velocity studies of the reverse reaction indicate that the mechanism is consistent with a rapid‐equilibrium random model (in which all steps are in equilibrium, except that concerned with the interconversion of the central ternary complexes) involving the formation of enzyme · ornithine · arsenate and enzyme · citrulline · phosphate dead‐end complexes. In the forward direction, although the mechanism also appears to be random, the results are in better agreement with a preferred ordered binding of substrates, with carbamoylphosphate adding first. This degenerate form of the random mechanism is discussed.