The iron-sulfur clusters in Escherichia coli succinate dehydrogenase direct electron flow
The iron-sulfur clusters in Escherichia coli succinate dehydrogenase direct electron flow
复制标题
DOI：
10.1074/jbc.m604900200
复制
发表时间：
2006-09-15
影响因子：
4.8
通讯作者：
Weiner, Joel H.
中科院分区：
文献类型：
--
作者：
Cheng, Victor W. T.;Ma, Elysia;Weiner, Joel H.
琥珀酸脱氢酶是线粒体和某些原核生物中参与克雷布斯循环以及能量偶联的不可缺少的酶。在催化过程中，琥珀酸氧化通过电子转移中继与泛醌还原偶联，所述电子转移中继包括黄素腺嘌呤二核苷酸辅因子、三个铁-硫簇和可能的血红素B（556）。在电子传递链的中心是一个具有低中点电位的[4Fe-4S]簇，它作为电子传递的能量屏障。疏水残基周围的[4Fe-4S]簇突变，以确定其对簇的中点电位以及电子转移速率的影响。SdhB-I150 E和SdhB-I150 H突变体降低了该簇的中点电位;令人惊讶的是，His变体具有比Glu突变体更低的中点电位。SdhB-Leu-220突变为Ser并没有改变簇的氧化还原行为，而是降低了[3Fe-4S]簇的中点电位。为了将这些突变体的中点潜在变化与酶功能相关联，我们监测了琥珀酸盐基本培养基中的有氧生长、富马酸甘油基本培养基中的厌氧生长、非生理和生理酶活性以及血红素减少。发现[4Fe-4S]簇或[3Fe-4S]簇的中点电位的降低伴随着酶周转率的降低。我们假设，这是因为在天然酶的[Fe-S]簇的中点电位的平衡，使电子转移的方向从琥珀酸泛醌是有利的。
Succinate dehydrogenase is an indispensable enzyme involved in the Krebs cycle as well as energy coupling in the mitochondria and certain prokaryotes. During catalysis, succinate oxidation is coupled to ubiquinone reduction by an electron transfer relay comprising a flavin adenine dinucleotide cofactor, three iron-sulfur clusters, and possibly a heme b(556). At the heart of the electron transport chain is a [4Fe-4S] cluster with a low midpoint potential that acts as an energy barrier against electron transfer. Hydrophobic residues around the [4Fe-4S] cluster were mutated to determine their effects on the midpoint potential of the cluster as well as electron transfer rates. SdhB-I150E and SdhB-I150H mutants lowered the midpoint potential of this cluster; surprisingly, the His variant had a lower midpoint potential than the Glu mutant. Mutation of SdhB-Leu-220 to Ser did not alter the redox behavior of the cluster but instead lowered the midpoint potential of the [3Fe-4S] cluster. To correlate the midpoint potential changes in these mutants to enzyme function, we monitored aerobic growth in succinate minimal medium, anaerobic growth in glycerol-fumarate minimal medium, non-physiological and physiological enzyme activities, and heme reduction. It was discovered that a decrease in midpoint potential of either the [4Fe-4S] cluster or the [3Fe-4S] cluster is accompanied by a decrease in the rate of enzyme turnover. We hypothesize that this occurs because the midpoint potentials of the [Fe-S] clusters in the native enzyme are poised such that direction of electron transfer from succinate to ubiquinone is favored.