Mao Junjun, Hauser Karin, Gunner M R
Physics Department J-419, City College of New York, 138th Street and Convent Avenue, New York, New York 10031, USA.
Biochemistry. 2003 Aug 26;42(33):9829-40. doi: 10.1021/bi027288k.
The electrochemical midpoint potentials (E(m)'s) of 13 cytochromes, in globin (c, c(2), c(551), c(553)), four-helix bundle (c', b(562)), alpha beta roll (b(5)), and beta sandwich (f) motifs, with E(m)'s spanning 450 mV were calculated with multiconformation continuum electrostatics (MCCE). MCCE calculates changes in oxidation free energy when a heme-axial ligand complex is moved from water into protein. Calculated and experimental E(m)'s are in good agreement for cytochromes with His-Met and bis-His ligated hemes, where microperoxidases provide reference E(m)'s. In all cytochromes, E(m)'s are raised by 130-260 mV relative to solvated hemes by the loss of reaction field (solvation) energy. However, there is no correlation between E(m) and heme surface exposure. Backbone amide dipoles in loops or helix termini near the axial ligands raise E(m)'s, but amides in helix bundles contribute little. Heme propionates lower E(m)'s. If the propionic acids are partially protonated in the reduced cytochrome, protons are released on heme oxidation, contributing to the pH dependence of the E(m). In all cytochromes studied except b(5)'s and low potential globins, buried side chains raise E(m)'s. MCCE samples ionizable group protonation states, heme redox states, and side chain rotamers simultaneously. Globins show the largest structural changes on heme oxidation and four-helix bundles the least. Given the calculated protein-induced E(m) shift and measured cytochrome E(m) the five-coordinate, His heme in c' is predicted to have a solution E(m) between that of isolated bis-His and His-Met hemes, while the reference E(m) for His-Ntr ligands in cytochrome f should be near that of His-Met hemes.
利用多构象连续介质静电学（MCCE）计算了13种细胞色素的电化学中点电位（E(m)），这些细胞色素存在于珠蛋白（c、c(2)、c(551)、c(553)）、四螺旋束（c'、b(562)）、αβ折叠（b(5)）和β三明治（f）基序中，其E(m)跨度为450 mV。MCCE计算当血红素 - 轴向配体复合物从水中转移到蛋白质中时氧化自由能的变化。对于具有His - Met和双His连接血红素的细胞色素，计算得到的E(m)与实验值吻合良好，其中微过氧化物酶提供参考E(m)。在所有细胞色素中，由于反应场（溶剂化）能量的损失，E(m)相对于溶剂化血红素升高了130 - 260 mV。然而，E(m)与血红素表面暴露之间没有相关性。轴向配体附近的环或螺旋末端的主链酰胺偶极子会升高E(m)，但螺旋束中的酰胺贡献很小。血红素丙酸酯会降低E(m)。如果丙酸在还原型细胞色素中部分质子化，质子会在血红素氧化时释放，这导致了E(m)对pH的依赖性。在除b(5)和低电位珠蛋白之外的所有研究细胞色素中，埋藏的侧链会升高E(m)。MCCE同时对可电离基团的质子化状态、血红素氧化还原状态和侧链旋转异构体进行采样。珠蛋白在血红素氧化时显示出最大的结构变化，而四螺旋束的变化最小。根据计算得到的蛋白质诱导 E(m) 位移和测量的细胞色素 E(m)，预测c'中五配位的His血红素在溶液中的E(m)介于分离的双His和His - Met血红素之间，而细胞色素f中His - Ntr配体的参考E(m)应接近His - Met血红素的E(m)。
