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首页> 外文期刊>Biochemistry >Redox potentials for yeast, Escherichia coli and human glutathione reductase relative to the NAD(+)/NADH redox couple: Enzyme forms active in catalysis
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Redox potentials for yeast, Escherichia coli and human glutathione reductase relative to the NAD(+)/NADH redox couple: Enzyme forms active in catalysis

机译：相对于NAD（+）/ NADH氧化还原对，酵母，大肠杆菌和人谷胱甘肽还原酶的氧化还原电位：酶形式在催化中具有活性

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The flavoenzyme glutathione reductase catalyzes the NADPH-dependent reduction of glutathione disulfide, yielding two molecules of glutathione. The oxidation-reduction potentials, E-ox/EH2 (two-electron reduced enzyme), for yeast, Escherichia coli, and human glutathione reductase have been determined between pH 6.0 and 9.8 relative to the nonphysiological substrate couple NAD(+)/NADH and were found to be -237, -243, and -227 mV (+/-5 mV) at pH 7.0 and 20 degrees C, respectively. The potential as a function of pH demonstrated slopes of -51, -45, and -42 mV/pH unit, respectively, at low pH and -37, -31, and -34 mV/pH unit, respectively, at high pH. The change in slope indicated pK(a) values of 7.4, 8.5, and 7.6, respectively. The slopes indicate that two protons are associated with the two-electron reduction of E-ox at low pH and that only one proton is involved with the two-electron reduction of E-ox at high pH, provided that the effects of nearby titratable residues are considered in the data analysis. The influence of four such groups, Cys(50), Cys(45), His(456)', and either Tyr(107) or the flavin-(N3), has been included (residue numbering refers to the yeast sequence). The enzyme loses activity upon deprotonation of the acid-base catalyst at high pH. Since the pK(a) ascribed to the EH2-to-EH- ionization is lower than the pK(a) of the acid-base catalyst, both the EH2 and EH- forms of glutathione reductase must be catalytically active, in contrast to the closely related enzyme lipoamide dehydrogenase, for which only EH2 is active. [References: 34]
机译：黄酮酶谷胱甘肽还原酶催化NADPH依赖性的二硫代谷胱甘肽还原，产生两个分子的谷胱甘肽。相对于非生理性底物对NAD（+）/ NADH和NAD（+）而言，已确定了酵母，大肠杆菌和人谷胱甘肽还原酶的氧化还原电位E-ox / EH2（双电子还原酶）。分别在pH 7.0和20摄氏度时分别为-237，-243和-227 mV（+/- 5 mV）。电位随pH的变化在低pH时分别为-51，-45和-42 mV / pH单位，在高pH时分别为-37，-31和-34 mV / pH单位。斜率的变化分别表明pK（a）值为7.4、8.5和7.6。斜率表明，在低pH下，两个质子与E-ox的双电子还原有关，而在高pH下，只有一个质子与E-ox的双电子还原有关。在数据分析中被考虑。 Cys（50），Cys（45），His（456）'和Tyr（107）或黄素-（N3）这四个基团的影响已包括在内（残基编号是指酵母序列）。在高pH下酸碱催化剂去质子化时，酶失去活性。由于归因于EH2-到EH-电离的pK（a）低于酸碱催化剂的pK（a），因此与之相比，谷胱甘肽还原酶的EH2和EH-形式都必须具有催化活性。紧密相关的脂酰胺脱氢酶，仅EH2具有活性。 [参考：34]

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