Hsuanyu Y, Wedler F C
Department of Molecular & Cell Biology, P.M. Althouse Laboratory, Pennsylvania State University, University Park 16802.
Biochim Biophys Acta. 1988 Dec 2;957(3):455-8. doi: 10.1016/0167-4838(88)90236-1.
In contrast to holo-enzyme (c6r6), catalytic subunits (c3) of Escherichia coli aspartate transcarbamylase (carbamoyl-phosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) do not exhibit allosteric interactions or inhibition effects that complicate kinetic investigations of substrate binding order. Equilibrium isotope-exchange kinetic probes of c3 at pH 7.0 and 30 degrees C produced kinetic saturation patterns consistent with a strongly preferred order random kinetic mechanism, in which carbamoyl phosphate binds prior to aspartate and carbamoyl aspartate is released before Pi. Weak substrate inhibition effects observed with c6r6 did not occur with c3, possibly due to decreased affinity for ligands at the dianion inhibition site.
与全酶（c6r6）不同，大肠杆菌天冬氨酸转氨甲酰酶（氨甲酰磷酸：L-天冬氨酸氨甲酰转移酶，EC 2.1.3.2）的催化亚基（c3）不表现出变构相互作用或抑制作用，这些作用会使底物结合顺序的动力学研究变得复杂。在pH 7.0和30℃条件下对c3进行平衡同位素交换动力学探测，产生的动力学饱和模式与强烈偏好的有序随机动力学机制一致，其中氨甲酰磷酸在天冬氨酸之前结合，氨甲酰天冬氨酸在磷酸根离子（Pi）之前释放。c6r6观察到的弱底物抑制作用在c3中未出现，这可能是由于在二价阴离子抑制位点对配体的亲和力降低所致。
