Schroedl N A, Hartzell C R
Biochemistry. 1977 Nov 15;16(23):4966-71. doi: 10.1021/bi00642a004.
Oxidative titrations were performed on the electrostatic complex formed between cytochrome c and cytochrome aa3 at low ionic strength. Midpoint potentials of the redox centers in the proteins in 1:1 and 2:1 complexes were compared with those in mixtures of the cytochromes at high ionic strength. Computer simulations of all titrations yielded midpoint potentials for the components of cytochrome aa3 which were consistent with literature values for isolated cytochrome aa3 or mixture of cytochromes c and aa3. However, the unequal heme extinction coefficients observed previously (Schroedl, N.A., and Hartzell, C.R. (1977), Biochemistry 16, 1327) during oxidative titrations of cytochrome aa3 became equal in magnitude under these experimental conditions. The binding of cytochrome c to cytochrome aa3 changed the midpoint potentials of cytochrome aa3 by 15-20 mV, while the midpoint potentials for cytochrome c were altered by 50-60 mV. Careful analysis of these titrations including computer simulation revealed that cytochrome c was able to bind to cytochrome aa3 only after cytochrome aL2+ had become oxidized. When bound to cytochrome aa3, the midpoint potential of cytochrome c was 210 7V. Titrations performed under a carbon monoxide atmosphere revealed cytochrome aa3 midpoint potentials unchanged from reported values. Cytochrome c again exhibited a midpoint potential of 210 mV after binding to cytochrome aa3.
在低离子强度下，对细胞色素c与细胞色素aa3形成的静电复合物进行了氧化滴定。将1:1和2:1复合物中蛋白质氧化还原中心的中点电位与高离子强度下细胞色素混合物中的中点电位进行了比较。所有滴定的计算机模拟得出了细胞色素aa3各组分的中点电位，这些电位与分离的细胞色素aa3或细胞色素c和aa3混合物的文献值一致。然而，在这些实验条件下，先前在细胞色素aa3氧化滴定过程中观察到的不等血红素消光系数在大小上变得相等。细胞色素c与细胞色素aa3的结合使细胞色素aa3的中点电位改变了15 - 20 mV，而细胞色素c的中点电位改变了50 - 60 mV。对这些滴定进行仔细分析，包括计算机模拟，结果表明细胞色素c只有在细胞色素aL2+被氧化后才能与细胞色素aa3结合。当与细胞色素aa3结合时，细胞色素c的中点电位为210 7V。在一氧化碳气氛下进行的滴定显示，细胞色素aa3的中点电位与报道值没有变化。细胞色素c与细胞色素aa3结合后，中点电位再次显示为210 mV。
