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 "tense" 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.

ATCase is a highly regulated enzyme that catalyses the first committed step in pyrimidine biosynthesis, the condensation of l-aspartate and carbamoyl phosphate to form N-carbamyl-L-aspartate and inorganic phosphate. ATCase controls the rate of pyrimidine biosynthesis by altering its catalytic velocity in response to cellular levels of both pyrimidines and purines. The end-product of the pyrimidine pathway, CTP, decreases catalytic velocity, whereas ATP, the end-product of the parallel purine pathway, increases catalytic velocity.

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