Aspartate carbamoyltransferase

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

The enzyme is a multi-subunit protein complex composed of 12 subunits (300 kDa in total). The stoichiometry of the subunits is C6R6; forming 2 trimers of catalytic subunits (34 kDa) and 3 dimers of regulatory subunits (17 kDa). The overall structure is roughly triangular in shape. The particular arrangement of catalytic and regulatory subunits in this enzyme afford the complex with strongly allosteric behaviour with respect to its substrates. This protein is an archetypal example of allosteric modulation of fine control of metabolic enzyme reactions. It therefore does not follow Michaelis-Menten kinetics, but lies between the R and 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. Binding of ATP to the regulatory subunits results in an equilibrium shift towards the R state.