Anderson P M
Biochemistry. 1977 Feb 22;16(4):583-6. doi: 10.1021/bi00623a004.
Carbamyl-phosphate synthetase from Escherichia coli is an allosteric enzyme which undergoes reversible association reactions in phosphate buffer. The positive allosteric effectors, ornithine, inosine 5'-monophosphate (IMP), and ammonia, facilitate oligomer formation, whereas uridine 5'-monophosphate (UMP), a negative effector, prevents or decreases oligomer formation. When the enzyme is immobilized by reaction with activated Sepharose, under conditions where the enzyme exists only as a monomer, nearly full catalytic activity is retained and the effects of ornithine, IMP, and UMP on the catalytic activity as a function of MgATP concentration are not significantly altered. Gel-filtration chromatography on Sephadex G-200 of catalytic quantities of the enzyme in the presence of all substrates showed that the elution volume was the same as that measured for the enzyme under conditions where it is known to exist in the monomer form. The specific activity of the enzyme does not increase when the concentration of the enzyme is increased 100-fold from a concentration at which the enzyme exists as monomer to a level at which the enzyme exists predominantly as oligomer. These results indicate that the monomer form of the enzyme is the principle active species and that oligomer formation is not directly related to enzyme activity or enzyme regulation.
来自大肠杆菌的氨甲酰磷酸合成酶是一种别构酶，在磷酸盐缓冲液中会发生可逆的缔合反应。正别构效应剂鸟氨酸、5'-肌苷单磷酸（IMP）和氨促进寡聚体形成，而负效应剂5'-尿苷单磷酸（UMP）则阻止或减少寡聚体形成。当酶通过与活化的琼脂糖反应固定化时，在酶仅以单体形式存在的条件下，几乎保留了全部催化活性，并且鸟氨酸、IMP和UMP对催化活性随MgATP浓度变化的影响没有显著改变。在所有底物存在的情况下，对催化量的酶进行Sephadex G-200凝胶过滤色谱分析表明，洗脱体积与已知以单体形式存在的酶的洗脱体积相同。当酶的浓度从以单体形式存在的浓度增加100倍到主要以寡聚体形式存在的水平时，酶的比活性没有增加。这些结果表明，酶的单体形式是主要的活性物种，并且寡聚体形成与酶活性或酶调节没有直接关系。
