Structural requirements in 20-oxo-steroids for interaction with the catalytic site of 20.BETA.-hydroxysteroid dehydrogenase.
作者:TAKAO HAYAKAWA、TSUYOSHI TANIMOTO、JIRO KAWAMURA
DOI:10.1248/cpb.28.730
日期:——
Kinetic measurements were made to investigate the interaction of 20β-hydroxysteroid dehydrogenase and a series of steroids with C-17 and/or C-21 hydroxyl groups, and the role of the region around C-17 and C-21 on the interaction of the reacting 20-oxo group with the catalytic site of the enzyme was considered. Substitution of a hydroxyl group for hydrogen at C-21 of 17-deoxy-steroid derivatives caused a significant decrease in the apparent Vmax value (to about one-sixth to one-ninth), but a slight increase in the apparent Km value (about 1.1- to 1.8-fold). The same change at C-21 of 17-hydroxy-steroid derivatives caused little or no decrease in the apparent Vmax value, but an increase in the apparent Km (about 1.4- to 1.8-fold) occurred which was similar to that with 17-deoxy-steroid derivatives. In 21-deoxy-11-deoxy-steroid derivatives, a 17α-hydroxyl substituent had little effect on the apparent Km value (about 0.8- to 1.1-fold) and produced a slight decrease in the apparent Vmax value (to about three-quarters to two-fifths). Introduction of a 17α-hydroxyl group into 21-deoxy-11-oxo-steroid derivatives led to a significant decrease in the apparent Km value (to about one-seventh) and a moderate decrease in the apparent Vmax value (to about five-sixths to one-half). Introduction of a 17α-hydroxyl group into 21-hydroxy-steroid derivatives caused the apparent Vmax value to increase by about 1.8- to 11-fold, but caused little or no decrease in the apparent Km value. These results suggest that the hydroxyl group at the 21- or 17α-position directly restricted the conformation and the orientation of the 20-oxo group towards the catalytic site of the enzyme and influenced the hydrogen transfer stage in the catalytic process, and also that, of the substituents at the 21- and 17α-positions, the latter may preferentially affect the reaction efficiency of the 20-oxo group. The presence of an oxo group at C-11 had an indirect influence on the effects of the substituents at the 21- and 17α-positions. The optimum orientation of the 20-oxo group for the catalytic reaction may occur in 21-deoxy-17-deoxy-11-deoxy-steroid derivatives. In an ideal ternary complex, the 20-oxo group of the steroid may project towards the β-face of the steroid ring and the conformation between the 20-oxo group and α-hydrogen of C-17 is nearly staggered, while that between C-21 and the α-hydrogen of C-17, looking along the C-17 to C-20 axis, is in a skew form ; the 20-oxo group is orientated rather far from the methyl group at the β-position of C-13 and more towards the β-chain of C-16.
进行了动力学测量,以研究20β-羟类固醇脱氢酶与一系列具有C-17和/或C-21羟基的类固醇之间的相互作用,并考虑了C-17和C-21附近区域对反应的20-氧基与酶催化位点相互作用的影响。将C-21的氢替换为羟基的17-脱氧类固醇衍生物导致表观Vmax值显著下降(降至约六分之一到九分之一),但表观Km值略微增加(约1.1到1.8倍)。在17-羟基类固醇衍生物的C-21处进行相同的改变对表观Vmax值几乎没有或没有造成下降,但表观Km值却增加(约1.4到1.8倍),这与17-脱氧类固醇衍生物的变化相似。在21-脱氧-11-脱氧类固醇衍生物中,17α-羟基取代体对表观Km值影响不大(约0.8到1.1倍),并导致表观Vmax值轻微下降(降至约三分之四到五分之二)。在21-脱氧-11-氧基类固醇衍生物中引入17α-羟基使表观Km值显著降低(降至约七分之一),而表观Vmax值中度下降(降至约五分之六到一半)。在21-羟基类固醇衍生物中引入17α-羟基使表观Vmax值增加约1.8到11倍,但对表观Km值几乎没有影响。这些结果表明,位于21或17α位点的羟基直接限制了20-氧基与酶催化位点之间的构象和取向,并影响了催化过程中的氢转移阶段。此外,在21和17α位点的取代基中,后者可能更优先影响20-氧基的反应效率。C-11的氧基的存在对21和17α位点取代基的影响具有间接作用。20-氧基在催化反应中的最佳取向可能发生在21-脱氧-17-脱氧-11-脱氧类固醇衍生物中。在一个理想的三元复合物中,类固醇的20-氧基可能朝向类固醇环的β面突出,且20-氧基与C-17的α-氢之间的构象几乎呈交错状态;而C-21与C-17的α-氢之间的角度在C-17至C-20轴上是倾斜的;20-氧基在空间上较远离C-13的β位甲基,更靠近C-16的β链。