Yang Y R, Syvanen J M, Nagel G M, Schachman H K
Proc Natl Acad Sci U S A. 1974 Mar;71(3):918-22. doi: 10.1073/pnas.71.3.918.
Reconstitution of aspartate transcarbamoylase (EC 2.1.3.2) from dilute solutions of the isolated regulatory and catalytic subunits, with the latter in large excess, led to the formation of appreciable amounts of a second, stable component in addition to the reconstituted enzyme. The purified component, designated r(4)c(6), was found to have a molecular weight about 3 x 10(4) less than that of the native enzyme, and it combined with isolated regulatory subunit to form aspartate transcarbamoylase. It also combined with one succinylated regulatory subunit to form a hybrid species that was identified electrophoretically. These findings indicate that r(4)c(6) differs from the native enzyme in that only two (rather than three) regulatory subunits participate in "crosslinking" the two catalytic trimers. The "incomplete" enzyme, r(4)c(6), exhibits the characteristic sigmoidal saturation behavior and CTP inhibition of aspartate transcarbamoylase; however these allosteric effects are reduced in extent by about one-third in comparison to the native enzyme and free catalytic subunits. The complex, which may be an intermediate in the assembly and dissociation of the native enzyme, is useful in assessing the role of the various bonding domains responsible for the stability and regulatory properties of the native enzyme.
从分离出的调节亚基和催化亚基的稀溶液中重构天冬氨酸转氨甲酰酶（EC 2.1.3.2），其中催化亚基大量过量，结果除了重构酶之外，还形成了相当数量的第二种稳定成分。纯化后的该成分，命名为r(4)c(6)，其分子量比天然酶小约3×10⁴，并且它与分离出的调节亚基结合形成天冬氨酸转氨甲酰酶。它还与一个琥珀酰化调节亚基结合形成一种通过电泳鉴定的杂交物种。这些发现表明，r(4)c(6)与天然酶的不同之处在于，只有两个（而非三个）调节亚基参与“交联”两个催化三聚体。“不完全”酶r(4)c(6)表现出天冬氨酸转氨甲酰酶典型的S形饱和行为和CTP抑制作用；然而，与天然酶和游离催化亚基相比，这些别构效应的程度降低了约三分之一。该复合物可能是天然酶组装和解离过程中的一个中间体，有助于评估负责天然酶稳定性和调节特性的各种结合结构域的作用。
