Elimination of cooperativity in aspartate transcarbamylase by nitration of a single tyrosine residue.

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Abstract
Tetranitromethane can be employed to nitrate a limited number of tyrosine residues in aspartate transcarbamylase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2); such modification eliminates cooperativity, feedback inhibition and enzymatic activity, and reduces binding of the feedback inhibitor cytidine triphosphate. Cooperativity is lost more rapidly than other properties, and this loss correlates with the nitration of a single tyrosine residue. The saturation kinetics of hybrid species constructed from nitrated subunits of 1 type (either catalytic or regulatory) and native subunits of the other type are described. The modification responsible for loss of cooperativity is on the catalytic subunit. The tryptic peptide containing this modification was isolated and identified.