Selenium Biochemistry

硒磷酸(SEP)是合成多种硒酶所需的富含能量的硒化合物，是由三磷酸腺苷(ATP)合成的硒合成酶(SPS)和无机形式的硒形成的。特殊的硒转运蛋白参与向SPS提供特异性的硒，从而将细胞内的硒水平维持在毒性水平以下。某些类可以利用硒半胱氨酸作为底物的递送蛋白，即硒半胱氨酸裂解酶，包括来自大肠杆菌的三种NIFS相关蛋白质和一种来自凡尼氏甲烷球菌的蛋白质，万氏甲烷球菌是一种厌氧生物，特别富含硒酶和硒蛋白质生物合成所需的因子。牛罗丹尼斯是一种已知的硫转移酶，可以转化为稳定的全硒加合物，这是一个硒输送蛋白模型。Gerard Lacourciere博士正在研究其他可能在硒转运中发挥作用的蛋白质。人硫氧还蛋白还原酶是一种同源二聚体酶，在每个亚基的C末端含有一个必需的硒半胱氨酸残基，位于-Cys，Secys，Gly三肽序列中。这种硒蛋白在大肠杆菌中的表达效率很低，并且经常在UGA密码子上终止。为了便于分离全长活性酶物种，Shoshana Bar-Noy博士测试了编码与特定亲和力基质结合的C末端延伸的构建物。多组氨酸标签由一个蛋白酶裂解位点隔开，允许在镍亲和层析柱上选择性地浓缩全长UGA通读产物。随后的蛋白酶切割去除了C-末端标签。某些细菌酶，如嘌呤和黄嘌呤羟基酶，属于依赖硒的钼羟基酶家族的成员，缺乏硒半胱氨酸，而含有一种不稳定的辅因子形式的硒。硒的直接来源及其插入这些酶的机制是威廉·赛尔夫博士的主要研究成果。硫辛酸的三硫化硒加合物是这些羟基酶的潜在硒来源，是一种更稳定的硒形式，也被认为是一种潜在的硒传递抗氧化剂，用于治疗实验动物的放射性皮肤损伤。与杜克大学临床医生的合作研究正在进行中。Lara Campbell博士的研究主题是对早先观察到的HIV导致T细胞中Se酶的破坏、阴离子多硒化合物的积累以及仅用Tat蛋白模拟的类似较低水平的影响的后续研究。已有研究表明，TAT蛋白可能通过与SECIS茎环RNA结构的竞争结合而干扰硒蛋白的生物合成，SECIS茎环结构类似于TAT蛋白的正常靶标TAR。

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.