家蚕酪氨酸羟化酶His^(394)突变对酶结构和活性的影响

Impact of His^(394) Mutation on Structure and Activity of Tyrosine Hydroxylase in the Silkworm,Bombyx mori

酪氨酸羟化酶作为儿茶酚胺类合成的限速酶,是人类、哺乳动物和昆虫新陈代谢不可或缺的酶类。以家蚕(Bombyxmori)酪氨酸羟化酶(BmTH)为研究对象,分析其氨基酸位点突变对酶结构与活性的改变,为研究酪氨酸羟化酶的功能位点以及表达调控提供依据。利用重叠延伸PCR技术将BmTH蛋白质序列的第394位氨基酸残基组氨酸(His394)突变为谷氨酰胺(Gln),成功获得家蚕酪氨酸羟化酶突变型BmTH-H394Q。BmTH-H394Q的活性仅为野生型BmTH活性的38%,且其热稳定性降低,易于聚沉;荧光光谱分析His394突变为Gln使酪氨酸羟化酶的色氨酸和疏水基团暴露增多;圆二色光谱分析显示BmTH-H394Q的α-螺旋结构含量减少了9.6个百分点,而β-折叠结构含量上升了8.0个百分点。据此可以推测,BmTH的氨基酸位点His394对酶的活性和结构稳定性起着重要作用,是维持其结构和功能所必需的基团。

As a rate-limiting enzyme in biosynthesis of catecholamines,tyrosine hydroxylase is an indispensable enzyme in metabolism of insects,human and other mammals.In this study,Bombyx mori tyrosine hydroxylase（BmTH） was selected as research subject to analyze structure and activity variation of the enzyme caused by amino acid site mutation so as to provide guidance for studying the functional sites and expressional regulation of tyrosine hydroxylase.BmTH-H394Q,a mutant of which the 394th amino acid histidine（His394） of Bombyx mori tyrosine hydroxylase was replaced by glutamine（Gln）,was successfully obtained by overlap PCR.Activity of BmTH-H394Q was only 38% of the wild type BmTH.Meanwhile,the thermal stability of BmTH-H394Q decreased and was easy to coagulate.Fluorescent spectral analysis showed that mutation of His394 into Gln led to increased exposure of tryptophan and hydrophobic groups.Circular dichroism analysis indicated that α-helix content in BmTH-H394Q was reduced by 9.6 percentage points,while β-sheet content was increased by 8.0 percentage points.It is inferred that His394 of BmTH plays an important role in enzymatic activity and structural stability,being an essential group for maintaining structure and functions of Bombyx mori tyrosine hydroxylase.