Ramos F, Wiame J M
Eur J Biochem. 1982 Apr;123(3):571-6. doi: 10.1111/j.1432-1033.1982.tb06570.x.
Saccharomyces cerevisiae mutants lacking the anabolic L-threonine deaminase, the ilv1- mutants, have been found to exhibit a normal ability to grow, without auxotrophy towards isoleucine, on L-threonine of L-serine as only nitrogen nutrient. Starting from a strain carrying a ilv1- mutation, a new mutation affecting the ability to utilize L-threonine as nitrogen source was selected. This mutation, which also impairs the ability to utilize L-serine, has been denominated cha-, for 'catabolism of hydroxyamino acids' and was found to result in the lack of a catabolic L-serine (L-threonine) deaminase. This enzyme which, unlike the anabolic threonine deaminase, is more active towards serine than towards threonine, differs from the latter enzyme by a number of biochemical and regulatory properties. Whereas the anabolic enzyme is an allosteric enzyme sensitive to feedback inhibition by isoleucine, the catabolic enzyme exhibits Michaelian kinetics: no control of its activity has been detected. Its synthesis is induced by L-serine and L-threonine. These two enzymes, which thus can be easily differentiated by means of their regulations, display a limited ability to compensate for one another's absence and appear to play clearly distinct roles under normal physiological conditions.
缺乏合成型L-苏氨酸脱氨酶的酿酒酵母突变体，即ilv1-突变体，已被发现具有正常的生长能力，在仅以L-苏氨酸或L-丝氨酸作为氮源时，对异亮氨酸无营养缺陷。从携带ilv1-突变的菌株开始，筛选出一种影响利用L-苏氨酸作为氮源能力的新突变。这种突变也损害了利用L-丝氨酸的能力，已被命名为cha-，表示“羟基氨基酸的分解代谢”，并且发现它导致缺乏分解型L-丝氨酸（L-苏氨酸）脱氨酶。与合成型苏氨酸脱氨酶不同，这种酶对丝氨酸的活性比对苏氨酸的活性更高，在许多生化和调节特性上与后一种酶不同。合成型酶是一种对异亮氨酸反馈抑制敏感的别构酶，而分解型酶表现出米氏动力学：未检测到对其活性的控制。它的合成由L-丝氨酸和L-苏氨酸诱导。因此，这两种酶可以通过它们的调节很容易地区分开来，它们相互补偿缺失的能力有限，并且在正常生理条件下似乎发挥着明显不同的作用。
