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• 作者：A. Sami Sariba//pubs.acs.org/images/entities/scedil.gif" border="0"> ; Maria Valkova-Valchanova ; Mariko K. Tokito ; Zhaolei Zhang ; Edward A. Berry ; and Fevzi Daldal
• 刊名：Biochemistry
• 出版年：1998
• 出版时间：June 2, 1998
• 年：1998
• 卷：37
• 期：22
• 页码：8105 - 8114
• 全文大小：253K
• 年卷期：v.37,no.22(June 2, 1998)
• ISSN：1520-4995

文摘

Ubihydroquinone:cytochrome (cyt) coxidoreductase (bc₁ complex and its plantcounterpartb₆f complex) is a vital component ofenergy-transducing systems in most organisms from bacteriatoeukaryotes. In the facultative phototrophic (Ps) bacteriumRhodobacter capsulatus, it is constitutedbythe cyt b, cyt c₁, and Rieske Fe-Sprotein subunits and is essential for Ps growth. Of thesesubunits, cytb has two nontransmembrane helices, cd1 andcd2, which are critical for its structure and function.Inparticular, substitution of threonine (T) at position 163 oncd1 with phenylalanine (F) or proline (P) leadsto the absence of the bc₁ complex. Here,Ps⁺ revertants of B:T163F were obtained, and theirdetailedcharacterizations indicated that position 163 is important for theassembly of the bc₁ complex bymediatingsubunit interactions at the Q_o site. The loss of thehydroxyl group at position 163 of cyt b wascompensatedfor by the gain of either a hydroxyl group at position 182 of cytb or 46 of the Fe-S protein or a sulfhydrylgroup at position 46 of cyt c₁. Examinationof the mitochondrial bc₁ complex crystalstructure [Zhang,Z., Huang, L., Shulmeister, V. M., Chi, Y.-I., Kim, K. K., Hung, L.-W.,Crofts, A. R., Berry, E. A., andKim, S.-H. (1998) Nature 392, 677-684] revealed that thecounterparts of B:G182 (i.e., G167) and F:A46(i.e., A70) are located close to B:T163 (i.e., T148), whereas the C:R46(i.e., R28) is remarkably far fromit. The revertants contained substoichiometric amounts of theFe-S protein subunit and exhibited steady-state and single-turnover, electron transfer activities lower than thatof a wild-type bc₁ complex.Interestingly, their membrane supernatants contained a smallerform of this subunit with physicochemicalproperties identical to those of its membrane-bound form.Determination of the amino-terminal aminoacid sequence of this soluble Fe-S protein revealed that it wasderived from the wild-type protein byproteolytic cleavage at V44. This work revealed for the first timethat position 163 of cyt b is importantboth for proper subunit interactions at the Q_o site and forinactivation of the bc₁ complex byproteolyticcleavage of its Fe-S protein subunit at a region apparentlyresponsible for its mobility during Q_o sitecatalysis.