Cytochrome bd from Azotobacter vinelandii:: Evidence for high-affinity oxygen binding
Cytochrome bd from Azotobacter vinelandii:: Evidence for high-affinity oxygen binding
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DOI：
10.1021/bi700862u
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发表时间：
2007-10-02
期刊：
影响因子：
2.9
通讯作者：
Verkhovsky, Michael I.
中科院分区：
文献类型：
--
作者：
Belevich, Ilya;Borisov, Vitaliy B.;Verkhovsky, Michael I.
来自紫色固氮菌的细胞色素bd是一种呼吸性喹啉氧化酶，它能高效地降低细胞内氧浓度，从而在环境有氧条件下实现固氮。A. vinelandii酶以单电子还原形式与亚铁血红素d结合，得到K-d(O2)= 0.5 μ M，接近大肠杆菌细胞色素bd的值（约0.3 μ M）；因此，这两种酶对氧具有相似的高亲和力。a的反应vinelandii细胞色素bd one-electron-reduced和充分降低州02极其迅速接近diffusion-controlled限制在水里。在完全还原状态下，o2的结合速率与氧的浓度呈线性关系，这是一个简单的单步过程。相反，在单电子还原态中，氧的结合速率是双曲线的，这意味着一个更复杂的结合模式。考虑了饱和动力学的两种可能的解释：(A)氧在光谱上沉默地预结合到一个未识别的低亲和力饱和位点，然后氧转移到血红素d。(B)氧结合到血红素d需要酶的“激活”状态，在这种状态下，连接血红素d和主体的氧通道是开放的。该通道在完全还原酶中永久打开（因此没有饱和行为），但在单电子还原酶中在打开和关闭状态之间闪烁。
Cytochrome bd from Azotobacter vinelandii is a respiratory quinol oxidase that is highly efficient in reducing intracellular oxygen concentration, thus enabling nitrogen fixation under ambient aerobic conditions. Equilibrium measurements Of 02 binding to ferrous heme d in the one-electron-reduced form of the A. vinelandii enzyme give K-d(O2)= 0.5 mu M, close to the value for the Escherichia coli cytochrome bd (ca. 0.3 mu M); thus, both enzymes have similar, high affinity for oxygen. The reaction of the A. vinelandii cytochrome bd in the one-electron-reduced and fully reduced states with 02 is extremely fast approaching the diffusion-controlled limit in water. In the fully reduced state, the rate Of 02 binding depends linearly on the oxygen concentration consistently with a simple, single-step process. In contrast, in the one-electron-reduced state the rate of oxygen binding is hyperbolic, implying a more complex binding pattern. Two possible explanations for the saturation kinetics are considered: (A) There is a spectroscopically silent prebinding of oxygen to an unidentified low-affinity saturatable site followed by the oxygen transfer to heme d. (B) Oxygen binding to heme d requires an "activated" state of the enzyme in which an oxygen channel connecting heme d to the bulk is open. This channel is permanently open in the fully reduced enzyme (hence no saturation behavior) but flickers between the open and closed states in the one-electron-reduced enzyme.