Brunori M, Giuffrè A, D'Itri E, Sarti P
Department of Biochemical Sciences A. Rossi-Fanelli, University of Rome La Sapienza, Italy.
Biofactors. 1998;8(3-4):191-3. doi: 10.1002/biof.5520080304.
Two alternative hypotheses have been proposed to account for the relatively slow (ms) internal eT observed in the oxidized cyt c oxidase. The thermodynamic control hypothesis states that eT between cyt a and a3 is very fast (microsecond), but the apparent reduction of cyt a3 is slow because thermodynamics favors reduced cyt a. Whereas the kinetic control hypothesis states that inter-heme eT is intrinsically slow (ms), for the oxidized binuclear center. Monitoring by stopped flow the anaerobic reduction of the oxidized enzyme by ruthenium hexamine in the absence and presence of CO or NO, used as "trapping" ligands for cyt a3(2+), we found that the rate of formation of the cyt a3(2+)-NO adduct (k' approximately 20-25 s-1) is independent of the concentration of ruthenium hexamine and NO. We conclude that in the oxidized enzyme the two hemes are not in very rapid redox equilibrium and internal eT is kinetically controlled.
为了解释在氧化型细胞色素c氧化酶中观察到的相对较慢（毫秒级）的内部电子转移（eT），人们提出了两种替代假说。热力学控制假说认为，细胞色素a和a3之间的电子转移非常快（微秒级），但细胞色素a3的表观还原速度较慢，因为热力学倾向于还原型细胞色素a。而动力学控制假说则认为，对于氧化型双核中心而言，血红素间的电子转移本质上是慢的（毫秒级）。通过停流监测在不存在和存在作为细胞色素a3(2+)“捕获”配体的CO或NO的情况下，六胺钌对氧化酶的厌氧还原，我们发现细胞色素a3(2+)-NO加合物的形成速率（k'约为20 - 25 s-1）与六胺钌和NO的浓度无关。我们得出结论，在氧化酶中，两个血红素并非处于非常快速的氧化还原平衡状态，内部电子转移受动力学控制。
