Selenium Biochemistry

硒酸磷酸（SeP）是合成许多硒酶所需的高能量硒化合物，由硒酸磷酸合成酶（SPS）从ATP和硒的无机形式形成。专门的硒传递蛋白参与向SPS提供硒，以维持细胞内硒水平低于毒性水平。某些类型的传递蛋白可以利用硒半胱氨酸作为底物，硒半胱氨酸裂解酶，包括来自大肠杆菌的三种NifS相关蛋白和来自vanielii甲烷球菌的一种厌氧生物，特别富含硒酶和硒蛋白生物合成所需的因子。牛罗丹斯，一种已知的硫转移酶，可以转化为稳定的过硒化物加合物，这可以作为硒传递蛋白的模型。Gerard Lacourciere博士正在研究可能在硒运输中起作用的其他蛋白质。人硫氧还蛋白还原酶是一种同二聚体酶，它含有一个必需的硒代半胱氨酸残基，位于每个亚基的c端-Cys， Secys， Gly三肽序列中。这种硒蛋白在大肠杆菌中的表达效率非常低，并且在UGA密码子处终止是经常发生的。为了促进全长活性酶物种的分离，Shoshana Bar-Noy博士对编码c端扩展的构建体进行了测试，这些构建体可以结合到特定的亲和基质上。由蛋白酶裂解位点分离的多组氨酸标签允许在镍亲和柱上选择性富集全长uga -read - through产物。随后的蛋白酶裂解去除了c末端标签。某些细菌酶，如嘌呤和黄嘌呤羟化酶，是硒依赖钼羟化酶家族的成员，缺乏硒半胱氨酸，而是含有硒的不稳定的辅因子形式。硒的直接来源及其插入这些酶的机制是威廉·赛尔夫博士的一项主要研究工作。硫辛酸的硒三硫化物加合物是这些羟化酶的潜在硒源，是一种更稳定的硒形式，也被视为一种潜在的硒递送抗氧化剂，用于治疗实验动物辐射引起的皮肤损伤。与杜克大学临床医生的合作研究正在进行中。劳拉·坎贝尔博士（Dr. Lara Campbell）的研究课题是，对早期观察到的hiv诱导的t细胞中硒酶的破坏，阴离子多硒化物化合物的积累，以及类似的低水平效应，仅用Tat蛋白进行模拟。有研究提出Tat蛋白可能通过与SECIS茎环RNA结构（类似于HIV mRNA茎环结构TAR）的竞争结合干扰硒蛋白的生物合成。

Selenophosphate (SeP), the energy-rich Se compound required for synthesis of many selenoenzymes is formed by selenophosphate synthetase (SPS) from ATP and an inorganic form of Se. The participation of specialized selenium delivery proteins to furnish Se specifically to SPS serves to maintain intracellular levels of selenium below toxic levels. Certain classes of delivery proteins that can utilize selenocysteine as substrate, selenocysteine lyases, include three NifS related proteins from E. coli and one from Methanococcus vannielii, an anaerobic organism that is particularly rich in selenoenzymes and factors required for selenoprotein biosynthesis. Bovine rhodanese, a known sulfur transferase, can be converted to a stable perselenide adduct and this serves as a Se delivery protein model. Additional proteins that may have roles in selenium transport are under study by Dr. Gerard Lacourciere. Human thioredoxin reductase, a homodimeric enzyme, contains an essential selenocysteine residue located in a tripeptide sequence, -Cys, Secys, Gly, at the C-terminus of each subunit. Expression of this selenoprotein in E. coli is very inefficient and termination at the UGA codon is frequent. To facilitate isolation of full-length active enzyme species, constructs encoding C-terminal extensions that would bind to specific affinity matrices were tested by Dr. Shoshana Bar-Noy. A polyhistidine tag separated by a protease cleavage site allowed selective enrichment of full-length UGA-readthrough products on a nickel affinity column. Subsequent protease cleavage removed the C-terminal tag. Certain bacterial enzymes, such as purine and xanthine hydroxylases that are members of the selenium-dependent molybdenum hydroxylase family, lack selenocysteine and instead contain a labile cofactor form of Se. The immediate source of selenium and its mechanism of insertion into these enzymes is a major research effort of Dr. William Self. The selenotrisulfide adduct of lipoic acid, a potential selenium source for these hydroxylases, is a more stable form of selenium that also is viewed as a potential Se-delivery antioxidant for treatment of radiation-induced skin damage in experimental animals. Collaborative studies with Duke University clinicians are in progress. Follow up of earlier observations of HIV-induced destruction of Se-enzymes in T-cells with accumulation of anionic polyselenide compounds and similar lower level effects mimicked with Tat protein alone is the subject of research by Dr. Lara Campbell. It has been proposed that Tat protein may interfere with selenoprotein biosynthesis by competitive binding of the SECIS stem loop RNA structure, which resembles the HIV mRNA stem loop structure, TAR, the normal target of Tat protein.