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

硒磷酸盐（SeP）是合成特定含硒半胱氨酸酶所需的高能硒化合物，可由部分但不是全部的SeP合成酶由ATP和游离硒形成。 通过硒递送蛋白直接向SeP合成酶提供“硒烷硒”避免使用高毒性游离硒化物。 由于甲烷球菌vannielii厌氧生长的甲酸作为唯一的碳源，必须合成相当数量的几个代谢必需的硒酶，它被选为硒前体和辅助蛋白质的来源。 从细胞提取物中纯化了硒代半胱氨酸裂解酶，并显示纯蛋白质将硒从硒代半胱氨酸直接递送到丰富的M。vannieli SeP合成酶以及大肠杆菌SeP合成酶。 裂解酶的氨基酸序列表明与NIFS硫递送蛋白的密切关系。 从M. vannielii表现出与同型半胱氨酸合酶和胱硫醚裂解酶蛋白家族的序列同源性，从而将硒代蛋氨酸作为Se源连接起来。 从M.将测试vannielii递送无机形式的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，我们发现哺乳动物硫氧还蛋白还原酶是一种含硒半胱氨酸的硒酶。 硒代半胱氨酸残基的性质和功能的持续研究证实了其在催化中的重要作用。 相比之下，其中硒代半胱氨酸被半胱氨酸取代的突变酶是非常差的催化剂。目前正在尝试在大肠杆菌以外的宿主中优化硒酶的生产。

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.