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首页> 外文期刊>Biochemistry >Subunit III of cytochrome c oxidase of Rhodobacter sphaeroides is required to maintain rapid proton uptake through the D pathway at physiologic pH
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Subunit III of cytochrome c oxidase of Rhodobacter sphaeroides is required to maintain rapid proton uptake through the D pathway at physiologic pH

机译：需要球形红球菌细胞色素c氧化酶的亚基III来维持在生理pH下通过D途径快速吸收质子

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摘要

The catalytic core of cytochrome c oxidase is composed of three subunits where subunits I and 11 contain all of the redox-active metal centers and subunit III is a seven transmembrane helix protein that binds to subunit I. The N-terminal region of subunit III is adjacent to D132 of subunit 1, the initial proton acceptor of the D pathway that transfers protons from the protein surface to the buried active site similar to30 Angstrom distant. The absence of subunit III only slightly alters the initial steady-state activity of the oxidase at pH 6.5, but activity declines sharply with increasing pH, yielding an apparent pK(a) of 7.2 for steady-state 02 reduction. When subunit III is present, cytochrome oxidase is more active at higher pH, and the apparent pK(a) of steady-state O-2 reduction is 8.5. Single-turnover experiments show that proton uptake through the D pathway at pH 8 slows from > 10000 s(-1) in the presence of subunit III to 350 s(-1) in its absence. At low pH (5.5) the D pathway of the oxidase lacking subunit III regains its capacity for rapid proton uptake. Analysis of the F --> O transition indicates that the apparent pK(a) of the D pathway in the absence of subunit III is 6.8, similar to that of steady-state O-2 reduction (7.2). The pK(a) of D 132 itself may decline in the absence of subunit III since its carboxylate group will be more exposed to solvent water. Alternatively, part of a proton antenna for the D pathway may be lost upon removal of subunit III. It is proposed that one role of subunit III in the normal oxidase is to maintain rapid proton uptake through the D pathway at physiologic pH. [References: 59]
机译：细胞色素c氧化酶的催化核心由三个亚基组成，其中亚基I和11包含所有氧化还原活性金属中心，而亚基III是与亚基I结合的七个跨膜螺旋蛋白。亚基III的N端区域是与亚基1的D132相邻，D路径的初始质子受体将质子从蛋白质表面转移到与30埃距离相似的掩埋活性位点。不存在亚基III只会稍微改变pH值为6.5时氧化酶的初始稳态活性，但活性会随着pH值的增加而急剧下降，从而使稳态02还原的表观pK（a）为7.2。当存在亚基III时，细胞色素氧化酶在较高的pH值下更具活性，稳态O-2还原的表观pK（a）为8.5。单周转实验表明，在存在亚基III的情况下，在pH 8下通过D途径吸收的质子从> 10000 s（-1）减慢到在不存在亚基时的350 s（-1）。在低pH（5.5）下，缺乏III亚基的氧化酶的D途径恢复了其快速吸收质子的能力。从F→O跃迁的分析表明，在没有亚基III的情况下D途径的表观pK（a）为6.8，类似于稳态O-2还原（7.2）。在不存在亚基III的情况下，D 132的pK（a）可能会下降，因为其羧酸酯基团将更暴露于溶剂水中。替代地，在去除亚基III时，用于D路径的质子天线的一部分可能丢失。有人提出，在正常的氧化酶中，亚基III的一个作用是在生理pH值下保持通过D途径快速吸收质子。 [参考：59]

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