Aspartate transcarbamylase (ATCase) is a multi-subunit regulatory enzyme of 310,000 mol wt. It catalyzes the first unique reaction in the pyrimidine biosynthetic pathway, the carbamylation of aspartic acid by carbamyl phosphate (Reichard and Hanshoff, 1956). Furthermore, it is inhibited by an end product of the pyrimidine pathway, cytidine triphosphate (CTP) and activated by the purine adenosine triphosphate (ATP). These results, observed both in vivo and in vitro (Yates and Pardee, 1956; Gerhart and Pardee, 1962) suggest a regulatory mechanism whereby pyrimidine biosynthesis is balanced against pyrimidine and purine availability for nucleic acid synthesis. The molecule also exhibits cooperative “sigmoidal” kinetics and is thus an ideal case for studying a feedback regulated “allosteric” complex.
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Wiley et al. (1972) studied this question.