Aspartate Carbamoyltransferase - Catalytic Center

Catalytic Center

The catalytic site of ATCase is located at the interface between two neighboring catalytic chains in the same trimer and incorporates amino acid side-chains from both of these subunits. Insight into the mode of binding of substrates to the catalytic center of ATCase was first made possible by the binding of a bisubstrate analogue, N-(phosphonoacetyl)-L-aspartate (PALA). This compound is a strong inhibitor of ATCase and has a structure that is thought to be very close to that of the transition state of the substrates. Additionally, crystal structures of ATCase bound to carbamoylphosphate and succinate have been obtained. These studies, in addition to investigations using site-directed mutagenesis of specific amino acids, have identified several residues that are crucial for catalysis, such as Ser52, Thr53, Arg54, Thr55, Arg105, His134, Gln137, Arg167, Arg229, Glu231, and Ser80 and Lys84 from an adjacent catalytic chain. The active site is a highly positively charged pocket. One of the most critical side-chains is from Arg54, which interacts with a terminal oxygen and the anhydride oxygen of carbamoyl phosphate, stabilizing the negative charge of the leaving phosphate group. Arg105, His134, and Thr55 help to increase the electrophilicity of the carbonyl carbon by interacting with the carbonyl oxygen. In general, the rate enhancement of ATCase is achieved by orientation and stabilization of substrates, intermediates, and products rather than by direct involvement of amino acid residues in the catalytic mechanism.