Aspartate Carbamoyltransferase (ATCase)

Aspartate carbamoyltransferase catalyzes the first step in the pyrimidine biosynthetic pathway.The composition of the subunits is C₆R₆, forming 2 trimers of catalytic subunitsand 3 dimers of regulatory subunits. The particular arrangement of catalytic and regulatory subunits in this enzyme affords the complex with strongly allosteric behaviour with respect to its substrates. The enzyme is an archetypal example of allosteric modulation of fine control of metabolic enzyme reactions.

ATCase does not follow Michaelis-Menten kinetics, but lies between the high-activity, high-affinity "relaxed" or R and the low-activity, low-affinity "tense" or T states. The binding of substrate to the catalytic subunits result in an equilibrium shift towards the R state, whereas binding of CTP to the regulatory subunits results in an equilibrium shift towards the T state.