Ariki M, Lanyi J K
J Biol Chem. 1984 Mar 25;259(6):3504-10.
Amino acid analysis of the halorhodopsin chromoprotein shows that this protein contains a cysteine residue. Such a residue is absent in bacteriorhodopsin. Low concentrations (micromolar) of HgCl2 inhibit light-dependent chloride transport by halorhodopsin in envelope vesicles prepared from Halobacterium halobium strain L-33 and increase the Km for chloride. The decay rate of the flash-induced absorption change of halorhodopsin, measured at 570 nm, is considerably slowed by HgCl2, and this effect is reversed at higher concentrations of chloride. In addition, the magnitude of the absorption changes is diminished by HgCl2. These effects of the mercurial are also seen with the purified, solubilized chromoprotein. Upon addition of HgCl2 to the chromoprotein at low chloride concentrations in the dark, a decrease of absorption at 580 nm and an increase at 380 nm occur, as well as a blue shift of the chromophore by about 20 nm. Sustained illumination of halorhodopsin results in a 410 nm photoproduct. The reconversion of this species to 580 nm in the dark is strongly inhibited by HgCl2. These results show that a thiol group is essential for the stability of the halorhodopsin chromophore and for its photochemical reactions and suggest that this group is in the vicinity of both the retinal Schiff's base and the chloride-binding site.
嗜盐菌视紫红质发色蛋白的氨基酸分析表明，该蛋白含有一个半胱氨酸残基。而在细菌视紫红质中不存在这样的残基。低浓度（微摩尔）的HgCl₂会抑制从嗜盐菌L-33菌株制备的包膜囊泡中嗜盐菌视紫红质介导的光依赖性氯离子转运，并增加氯离子的米氏常数。在570nm处测量，HgCl₂会显著减缓嗜盐菌视紫红质闪光诱导吸收变化的衰减速率，且在较高浓度的氯离子存在时这种效应会逆转。此外，HgCl₂会减小吸收变化的幅度。在纯化的、可溶解的发色蛋白中也观察到了汞化合物的这些效应。在黑暗中向低氯离子浓度的发色蛋白中加入HgCl₂后，会出现580nm处吸收下降、380nm处吸收增加以及发色团蓝移约20nm的现象。持续光照嗜盐菌视紫红质会产生410nm的光产物。在黑暗中该物种向580nm的再转化受到HgCl₂的强烈抑制。这些结果表明，一个巯基对于嗜盐菌视紫红质发色团的稳定性及其光化学反应至关重要，并表明该基团位于视网膜席夫碱和氯离子结合位点附近。
