Alternative complex III (ACIII) is a multi-subunit quinol:electron acceptor oxidoreductase that couples quinol oxidation with transmembrane proton translocation in bacterial respiratory and photosynthetic electron transport chains. Four ACIII cryoelectron microscopy (cryo-EM) structures are known. However, the effects of cryo-EM versus X-ray crystallography structure determination on ACIII structure are unclear. Here, we report a 3.25 Å crystal structure of photosynthetic ACIII from Chloroflexus aurantiacus (CaACIIIp), revealing eight subunits (ActA–G and I) with four iron-sulfur clusters and six c-type hemes, a menaquinol-binding site, and two proton translocation passages. Structural comparisons with the previously reported cryo-EM structures reveal slight local conformational changes in the solvent-exposed regions of ActB, ActD, ActG, and the transmembrane (TM) helix of subunit I. The regions conferring structural flexibility possess low sequence conservation across species. However, the core functional modules containing the menaquinol-binding pocket, redox centers, and proton translocation passages remain unchanged, preserving the enzyme’s activity.

中文翻译：

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来自光养细菌 Chloroflexus aurantiacus 的替代复合物 III 的晶体结构


替代复合物 III （ACIII） 是一种多亚基喹醇：电子受体氧化还原酶，可将喹醇氧化与细菌呼吸和光合电子传递链中的跨膜质子易位偶联。已知四种 ACIII 冷冻电子显微镜 （cryo-EM） 结构。然而，冷冻电镜与 X 射线晶体学结构测定对 ACIII 结构的影响尚不清楚。在这里，我们报道了来自 Chloroflexus aurantiacus （CaACIIIp） 的光合 ACIII 的 3.25 Å 晶体结构，揭示了八个亚基（ActA-G 和 I），具有四个铁硫簇和六个 c 型血红素、一个甲萘喹结合位点和两个质子易位通道。与先前报道的冷冻电镜结构的结构比较显示，ActB、ActD、ActG 的溶剂暴露区域和亚基 I 的跨膜 （TM） 螺旋存在轻微的局部构象变化。赋予结构灵活性的区域在物种间具有低序列保守性。然而，包含甲萘喹结合口袋、氧化还原中心和质子易位通道的核心功能模块保持不变，保留了酶的活性。