Tollin G, Hanson L K, Caffrey M, Meyer T E, Cusanovich M A
Proc Natl Acad Sci U S A. 1986 Jun;83(11):3693-7. doi: 10.1073/pnas.83.11.3693.
The relative reactivities toward reduction by free flavin semiquinones of cytochromes (c-type cytochromes, cytochrome b5, c'-type cytochromes) iron-sulfur proteins (high-redox-potential ferredoxins, rubredoxins, low-redox-potential ferredoxins), and blue copper proteins (plastocyanin, azurins) are shown to correlate with calculations of the solvent exposure of the various prosthetic groups. In the case of the c-type cytochromes, one of the major centers of exposure is the sulfur atom of the thioether bridge that covalently links heme ring C to the protein. Charge-iterative extended Hückel calculations on a heme c model indicate that both porphyrin pi and Fe(III)d pi orbitals can delocalize onto the bridging sulfur atom. Unpaired spin densities are comparable to those obtained for individual aromatic porphyrin ring carbon atoms. Thus, the exposed sulfur of ring C may act to facilitate electron transfer.
细胞色素（c型细胞色素、细胞色素b5、c'型细胞色素）、铁硫蛋白（高氧化还原电位铁氧化还原蛋白、红素氧还蛋白、低氧化还原电位铁氧化还原蛋白）和蓝铜蛋白（质体蓝素、天青蛋白）对游离黄素半醌还原的相对反应性与各种辅基的溶剂暴露计算结果相关。对于c型细胞色素，主要的暴露中心之一是将血红素环C与蛋白质共价连接的硫醚桥的硫原子。对血红素c模型进行的电荷迭代扩展休克尔计算表明，卟啉π轨道和Fe(III)dπ轨道都可以离域到桥连硫原子上。未成对自旋密度与单个芳香卟啉环碳原子的自旋密度相当。因此，环C暴露的硫可能有助于促进电子转移。
