Department of Bioscience, Aarhus University, DK-8000 Aarhus C, Denmark.
Department of Bioscience, Aarhus University, DK-8000 Aarhus C, Denmark.
J Inorg Biochem. 2018 May;182:133-140. doi: 10.1016/j.jinorgbio.2018.02.007. Epub 2018 Feb 10.
Ferric hemoglobin (metHb) and myoglobin (metMb), present at low levels in vivo, have been recently found to oxidize hydrogen sulfide (HS) in excess, thus potentially contributing to removal of toxic HS in blood and heart, respectively. Here, we present a kinetic and thermodynamic study of the reaction of metHb and metMb with HS under physiological conditions, i.e. at low HS concentrations and with protein in excess of HS. We show here that both proteins react with sub-stoichiometric HS:heme ratios following two processes: a fast reversible binding of HS to ferric heme that prevails at high HS and a slow heme reduction to the ferrous state that prevails at low HS. While these two processes are fast for metMb, HS-induced heme reduction is slow for metHb and the metHb-HS complex once formed is therefore relatively stable. We find that metHb binds HS reversibly and cooperatively with a pH-dependent ligand affinity that is within the physiological range of HS concentrations found in blood. Stopped-flow kinetics show identical association rate constants for HS at varying pH, demonstrating that HS and not HS enters the ferric heme pocket. Dissociation rates of the metHb-HS complex increase when decreasing pH, consistent with the pH-dependent affinity. Taken together, these data are consistent with a novel biological role of metHb as a HS carrier in the blood, in parallel with the oxygen carrier function of the much more abundant ferrous Hb. In contrast, metMb in the heart could participate to redox-signaling involving HS.
体内低水平存在的高铁血红蛋白（metHb）和肌红蛋白（metMb）最近被发现可以氧化过量的硫化氢（HS），因此分别有可能有助于清除血液和心脏中的有毒 HS。在这里，我们对 metHb 和 metMb 在生理条件下（即低 HS 浓度和蛋白过量）与 HS 的反应进行了动力学和热力学研究。我们在这里表明，两种蛋白都以亚化学计量的 HS:heme 比值与 HS 反应：一种是在高 HS 条件下占主导地位的 HS 与三价铁血红素的快速可逆结合，另一种是在低 HS 条件下占主导地位的血红素缓慢还原为二价状态。虽然这两个过程对于 metMb 来说很快，但对于 metHb 来说，HS 诱导的血红素还原很慢，因此形成的 metHb-HS 复合物相对稳定。我们发现 metHb 以 pH 依赖性配体亲和力可逆且协同地结合 HS，其亲和力处于血液中发现的 HS 浓度的生理范围内。在不同 pH 值下，HS 的停流动力学显示出相同的缔合速率常数，这表明是 HS 而不是 HS 进入了三价血红素口袋。当 pH 值降低时，metHb-HS 复合物的离解速率增加，这与 pH 依赖性亲和力一致。总的来说，这些数据与 metHb 作为血液中 HS 载体的新的生物学作用一致，与更丰富的二价 Hb 的氧载体功能平行。相比之下，心脏中的 metMb 可能参与涉及 HS 的氧化还原信号转导。
