Foote J, Ikeda D M, Kantrowitz E R
J Biol Chem. 1980 Jun 10;255(11):5154-8.
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.
在天冬氨酸转氨甲酰酶催化链的两个位置上用7-氮杂色氨酸取代色氨酸，会导致该酶的同促和异促相互作用发生变化，尽管该酶的比活性没有改变。通过氨基酸分析对氮杂色氨酸的掺入程度进行了定量，结果表明85%的色氨酸残基已被取代。与天然酶相比，被取代的酶被ATP激活且被CTP抑制的程度更大。两种酶在ATP存在下的天冬氨酸饱和曲线是相同的，但对于氮杂色氨酸取代的酶，在CTP存在且无效应物的情况下，曲线向更高的天冬氨酸浓度偏移。在低天冬氨酸浓度下，天然酶被底物类似物琥珀酸激活的程度更大。这些数据表明，这种取代使酶的低底物亲和力构象状态的催化效率降低。这一解释与在该酶三维结构中观察到的可能的侧链相互作用一致。
