Chothia C, Lesk A M
J Mol Biol. 1985 Mar 5;182(1):151-8. doi: 10.1016/0022-2836(85)90033-6.
Analysis of cytochromes c (tuna), c2 (Rhodospirillum rubrum), c550 (Paracoccus denitrificans) and c551 (Pseudomonas aeruginosa) shows that they contain 48 residues identifiable as homologous from superposition of the structures. The other 34 to 64 residues are in loops that vary greatly in sequence, length and conformation, or in alpha-helices that are found in only some of the structures. Of the 48 homologous residues, 17 are in three segments which pack onto the haem faces. In all four structures, these segments have the same conformations, and the same locations relative to the haem. The other 31 residues are in three alpha-helices which are in contact with each other. These form the back and one side of the haem pocket. In cytochrome c551 the positions of the three alpha-helices have shifted and rotated, in comparison with cytochromes c and c2, by up to 5 A and 25 degrees relative to the haem. These shifts, facilitated by mutations at the helix-helix interfaces, are related to the reconstruction of the propionic acid side of the haem pocket described by Almassy & Dickerson (1978). Together these effects produce alternative structures for the haem pocket. This mechanism of adaptation to mutation contrasts with that observed in the globins. In the globins, mutations also produce changes in helix interfaces and shifts of packed helices, but in the globins these shifts are coupled to conserve the structure of the haem pocket.
对细胞色素c（金枪鱼）、c2（红螺菌）、c550（反硝化副球菌）和c551（铜绿假单胞菌）的分析表明，通过结构叠加，它们含有48个可确定为同源的残基。其他34至64个残基位于序列、长度和构象差异很大的环中，或仅在某些结构中出现的α螺旋中。在48个同源残基中，17个位于堆积在血红素表面的三个片段中。在所有四种结构中，这些片段具有相同的构象，并且相对于血红素具有相同的位置。其他31个残基位于相互接触的三个α螺旋中。这些形成了血红素口袋的背面和一侧。与细胞色素c和c2相比，在细胞色素c551中，三个α螺旋的位置相对于血红素最多移动了5埃，旋转了25度。这些移动由螺旋-螺旋界面处的突变促成，与Almassy和Dickerson（1978年）描述的血红素口袋丙酸侧的重构有关。这些效应共同产生了血红素口袋的替代结构。这种对突变的适应机制与在珠蛋白中观察到的不同。在珠蛋白中，突变也会导致螺旋界面的变化和堆积螺旋的移动，但在珠蛋白中，这些移动是为了保守血红素口袋的结构。
