Kinetic Studies on the Function of All the Conserved Tryptophans Involved Inside and Outside the QW Motifs of Squalene-hopene Cyclase: Stabilizing Effect of the Protein Structure against Thermal Denaturation
作者:Tsutomu SATO、Tsutomu HOSHINO
DOI:10.1271/bbb.63.1171
日期:1999.1
Site-directed mutagenesis experiments were carried out to identify the responsibility of the eight QW motifs for the reaction catalyzed by squalene-hopene cyclase (SHC). Alterations of the conserved tryptophans, which are responsible for the stacking structure with glutamine, into aliphatic amino acids gave a significantly lower temperature for the catalytic optimum as for the mutageneses of QW motifs 4, 5a and 5b, which are specifically present in SHCs. However, there was no change in the optimal temperatures of the mutated SHCs targeted at the other five motifs 1, 2, 3, 5c and 6. Thus, reinforcement against heat denaturation can be proposed as a function of the three QW motifs 4, 5a and 5b, but no function could be identified for the QW motifs 1, 2, 3, 5c and 6, although they are commonly found in all the families of prokaryotic SHCs and eukaryotic oxidosqualene cyclases. On the other hand, the three conserved tryptophans of W169, W312 and W489, which are located inside the putative central cavity and outside the QW motifs, were identified as components of the active sites, but also had a function against thermal denaturation. The other two tryptophan residues of W142 and W558, which are located outside the QW motifs, were found not to be active sites, but also had a role for stabilizing the protein structure. It is noteworthy that the mutants replaced by phenylalanine had higher temperatures for the catalytic optimum than those replaced by aliphatic amino acids. The catalytic optimal pH values for all the mutants remained unchanged with an identical value of 6.0.
进行定点诱变实验以确定八个 QW 基序对角鲨烯-霍烯环化酶 (SHC) 催化反应的作用。将负责与谷氨酰胺堆叠结构的保守色氨酸改变为脂肪族氨基酸,对于特别存在于 SHC 中的 QW 基序 4、5a 和 5b 的诱变,提供了显着较低的催化最佳温度。然而,针对其他五个基序 1、2、3、5c 和 6 的突变 SHC 的最佳温度没有变化。因此,可以建议将抗热变性的强化作为三个 QW 基序 4、5a 的函数。和 5b,但无法鉴定 QW 基序 1、2、3、5c 和 6 的功能,尽管它们常见于原核 SHC 和真核氧化角鲨烯环化酶的所有家族中。另一方面,位于假定的中央腔内部和QW基序外部的三个保守色氨酸W169、W312和W489被鉴定为活性位点的组成部分,但也具有抗热变性的功能。位于QW基序之外的另外两个色氨酸残基W142和W558被发现不是活性位点,但也具有稳定蛋白质结构的作用。值得注意的是,苯丙氨酸取代的突变体比脂肪族氨基酸取代的突变体具有更高的催化最佳温度。所有突变体的催化最佳pH值保持不变,均为6.0。