T-State active site of aspartate transcarbamylase: Crystal structure of the carbamyl phosphate and L-alanosine ligated enzyme

摘要

An X-ray diffraction study to 2.0 angstrom resolution shows that this enzyme, ATCase, is in the T-state (the c(3) to c(3) distance is 45.2 angstrom) when ATCase is bound to carbamyl phosphate (CP) and to L-alanosine (an analogue of aspartate). This result strongly supports the kinetic results that alanosine did not inhibit the carbamylation of aspartate in the normal reaction of native ATCase plus CP and aspartate [Baillon, J., Tauc, P., and Herve, G. (1985) Biochemistry 24, 7182-7187]. The structure further reveals that the phosphate of CP is 4 A away from its known position in the R-state and is in the T-state position of P-i in a recent study of ATCase complexed with products, phosphate (P-i) and N-carbamyl-L-aspartate [Huang, J., and Lipscomb, W. N. (2004) Biochemistry 43, 6422-6426]. Moreover, the alanosine position in this T-state is somewhat displaced from that expected for its analogue, aspartate, from the R-state position. The relations of these structural aspects to the kinetics are presented.