Crystal structure of the yeast cytochrome bc(1) complex with its bound substrate cytochrome c

Small diffusible redox proteins facilitate electron transfer in respiration and photosynthesis by alternately binding to integral membrane proteins. Specific and transient complexes need to be formed between the redox partners to ensure fast turnover. In respiration, the mobile electron carrier cytochrome c shuttles electrons from the cytochrome bc(1) complex to cytochrome c oxidase. Despite extensive studies of this fundamental step of energy metabolism, the structures of the respective electron transfer complexes were not known. Here we present the crystal structure of the complex between cytochrome c and the cytochrome bc(1) complex from Saccharomyces cerevisiae. The complex was crystallized with the help of an antibody fragment, and its structure was determined at 2.97-Å resolution. Cytochrome c is bound to subunit cytochrome c(1) of the enzyme. The tight and specific interactions critical for electron transfer are mediated mainly by nonpolar forces. The close spatial arrangement of the c-type hemes unexpectedly suggests a direct and rapid heme-to-heme electron transfer at a calculated rate of up to 8.3 × 10(6) s(−1). Remarkably, cytochrome c binds to only one recognition site of the homodimeric multisubunit complex. Interestingly, the occupancy of quinone in the Qi site is higher in the monomer with bound cytochrome c, suggesting a coordinated binding and reduction of both electron-accepting substrates. Obviously, cytochrome c reduction by the cytochrome bc(1) complex can be regulated in response to respiratory conditions.

可扩散氧化还原蛋白（diffusible redox proteins）可通过交替结合整合膜蛋白（integral membrane proteins），在呼吸作用与光合作用中介导电子传递。氧化还原伴侣（redox partners）之间需形成特异性且瞬时的复合物，以保障电子传递的快速周转。在呼吸作用中，可移动电子载体细胞色素c（cytochrome c）会将电子从细胞色素bc₁复合物（cytochrome bc(1) complex）传递至细胞色素c氧化酶（cytochrome c oxidase）。尽管学界已对这一能量代谢的核心步骤开展了大量研究，但对应电子传递复合物的结构此前仍未被解析。
本研究解析了酿酒酵母（Saccharomyces cerevisiae）来源的细胞色素c与细胞色素bc₁复合物所形成复合物的晶体结构。该复合物借助抗体片段（antibody fragment）实现结晶，其结构以2.97埃的分辨率完成解析。细胞色素c结合于该酶的细胞色素c₁亚基（cytochrome c₁）。介导电子传递所需的紧密且特异性相互作用，主要通过非极性作用力实现。两类c型血红素（c-type hemes）的紧密空间排布出乎意料地表明，二者之间可发生直接且快速的血红素-血红素电子传递，经计算其速率最高可达8.3×10⁶ s⁻¹。
值得注意的是，细胞色素c仅结合同源二聚体多亚基复合物的一个识别位点。有趣的是，结合了细胞色素c的单体中，Qi位点的醌（quinone）占据率更高，这表明两种电子接受底物的结合与还原过程存在协同调控。显然，细胞色素bc₁复合物对细胞色素c的还原过程可响应呼吸作用的状态进行调控。