Aspartate carbamoyltransferase (also known as aspartate transcarbamoylase or ATCase) catalyzes the first step in the pyrimidine biosynthetic pathway (EC 2.1.3.2).
In E. coli, the enzyme is a multi-subunit protein complex composed of 12 subunits (300 kDa in total). The composition of the subunits is C6R6, forming 2 trimers of catalytic subunits (34 kDa) and 3 dimers of regulatory subunits (17 kDa). 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 low-activity, low-affinity "tight" or T and the high-activity, high-affinity "relaxed" or R states. The binding of substrate to the catalytic subunits results 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. Binding of ATP to the regulatory subunits results in an equilibrium shift towards the R state.
Read more about Aspartate Carbamoyltransferase: Reaction, Structure, Catalytic Center, Allosteric Site