The role of tryptophan in aspartate transcarbamylase.

Replacement of 7-azatryptophan for tryptophan in two positions on the catalytic chain of aspartate transcarbamylase results in changes in the enzyme's homotropic and heterotropic interactions although there is no change in the enzyme's specific activity. The extent of azatryptophan incorporation was quantitated by amino acid analysis which showed that 85% of the tryptophan residues had been replaced. The substituted enzyme is activated by ATP and inhibited by CTP to a greater extent than is the native enzyme. The aspartate saturation curve in the presence of ATP is identical for the two enzymes, but the curve in the presence of CTP and without effectors is shifted toward higher aspartate concentrations for the azatryptophan-substituted enzyme. At low aspartate concentrations, the native enzyme is activated to a greater extent by the substrate analog succinate. These data suggest that the substitution renders the low substrate affinity conformational state of the enzyme less catalytically efficient. This interpretation is in agreement with possible side chain interactions observed in the three-dimensional structure of the enzyme.