Biosynthesis, Properties, and Functions of Selenoenzymes and Seleno-tRNAs

硒磷酸盐(SEP)是合成含硒半胱氨酸酶所需的一种富含能量的硒化合物，它可以由部分(但不是全部)SEP合成酶与三磷酸腺苷和游离硒结合而形成。通过一种硒传递蛋白将“硒烷硒”直接提供给SEP合成酶，避免了使用剧毒的游离硒。由于万尼氏甲烷球菌以甲酸盐为唯一碳源厌氧生长，并且必须合成大量代谢必需的硒酶，因此它被选为硒前体和辅助蛋白的来源。从细胞提取液中纯化了一种硒半胱氨酸裂解酶，纯化的蛋白可将硒从硒半胱氨酸直接输送到富含硒半胱氨酸的vannielii Sep合成酶和大肠杆菌Sep合成酶中。该裂解酶的氨基酸序列分析表明，该裂解酶与NIFS硫转运蛋白关系密切。从vannielii中纯化的另一种蛋白与同型半胱氨酸合成酶和胱硫酮裂解酶蛋白家族具有序列同源性，从而将硒蛋氨酸作为硒的来源。从vannielii分枝杆菌中提纯的一种未知的Se-75结合蛋白将被测试其运送无机形式的Se用于硒蛋白合成的能力。硫氧还蛋白和硫氧还蛋白还原酶是植物、动物和细菌中重要的氧化还原调节系统之一。最近，我们发现哺乳动物的硫氧还蛋白还原酶是一种含硒半胱氨酸的硒酶。正在进行的对硒半胱氨酸残基的性质和功能的研究证实了它在催化中的重要作用。相比之下，用半胱氨酸取代硒半胱氨酸的突变酶是一种非常差的催化剂。目前正在尝试优化在大肠杆菌以外的宿主中生产硒酶的方法。

Selenophosphate (SeP), the energy-rich selenium compound required for synthesis of specific selenocysteine-containing enzymes, can be formed by some but not all SeP synthetases from ATP and free selenide. Provision of "selenane selenium" directly to the SeP synthetase by a selenium delivery protein avoids use of highly toxic free selenide. Because Methanococcus vannielii grows anaerobically on formate as sole carbon source and must synthesize considerable amounts of several metabolically essential selenoenzymes, it was selected as a source of Se precursors and accessory proteins. A selenocysteine lyase was purified from cell extracts and the pure protein was shown to deliver Se from selenocysteine directly to the abundant M. vannielii SeP synthetase and also Escherichia coli SeP synthetase. The amino acid sequence of the lyase indicated a close relationship to NIFS sulfur delivery proteins. Another protein purified from M. vannielii exhibited sequence homology to the homocysteine synthase and cystathione lyase protein family, thus linking selenomethionine as the Se source. An unidentified Se-75 binding protein purified from M. vannielii will be tested for its ability to deliver inorganic forms of Se for selenoprotein synthesis. One of the important redox regulatory systems in plants, animals, and bacteria consists of thioredoxin and thioredoxin reductase. Recently, we discovered that the mammalian thioredoxin reductase is a selenoenzyme that contains selenocysteine. Ongoing investigation of the properties and function of the selenocysteine residue confirm its essential role in catalysis. By comparison a mutant enzyme in which selenocysteine is replaced with cysteine is a very poor catalyst. Attempts to optimize the production of the selenoenzyme in hosts other than Escherichia coli are currently in progress.