Aspartate carbamoyltransferase (also known as aspartate transcarbamoylase or ATCase) catalyzes the first step in the pyrimidine biosynthetic pathway (EC2.1.3.2).

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.