Structure at 2.9-A resolution of aspartate carbamoyltransferase complexed with the bisubstrate analogue N-(phosphonacetyl)-L-aspartate.

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Abstract
In an X-ray diffraction study by the isomorphous replacement method, the structure of the complex of aspartate carbamoyltransferase (EC2.1.3.2) bound to the bisubstrate analog N-(phosphonacetyl)-L-aspartate was solved to 2.9-.ANG. resolution (r = 0.24). The large quaternary structural changes previously deduced by molecular replacement methods were confirmed: the 2 catalytic trimers (c3) move apart by 12 .ANG. and mutually reorient by 10.degree., and the regulatory dimers (r2) reorient each about its 2-fold fold axis by about 15.degree.. In this, the T-to-R transition, new polar interactions develop between equatorial domains of c chains and the Zn domain of r chains. Within the c chain the 2 domains, one binding the phosphonate moiety (polar) and the other binding the aspartate moiety (equatorial) of the inhibitor N-(phosphonacetyl)-L-aspartate, move closer together. The Lys-84 loop makes a large relocation so that this residue and Ser-80 bind to the inhibitor of an adjacent catalytic chain within c3. A very large change in tertiary structure brings the 230-245 loop nearer the active site, allowing Arg-229 and Gln-231 to bind to the inhibitor. His-134 is close to the carbonyl group of the inhibitor, and Ser-52 is adjacent to its phosphonate group. No evidence exists in the literature for phosphorylation of Ser in the mechanism. Residues studied by other methods, including Cys-47, Tyr-165, Lys-232 and Tyr-240, are too far from the inhibitor to have a direct interaction.