Reactions of Isocytochrome c2 in the Photosynthetic Electron Transfer Chain of Rhodobacter sphaeroides

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• 作者：Vernon C. Witthuhn Jr. ; Jiliang Gao ; Sangjin Hong ; Steven Halls ; Marc A. Rott ; Colin A. Wraight ; Antony R. Crofts ; and Timothy J. Donohue
• 刊名：Biochemistry
• 出版年：1997
• 出版时间：January 28, 1997
• 年：1997
• 卷：36
• 期：4
• 页码：903 - 911
• 全文大小：189K
• 年卷期：v.36,no.4(January 28, 1997)
• ISSN：1520-4995

文摘

Rhodobacter sphaeroides strains lacking cytochromec₂ (cyt c₂), the normalelectron donor toP₈₇₀⁺ in light-oxidized reaction center (RC)complexes, are unable to grow photosynthetically.However,spd mutations that suppress thephotosynthetic deficiency of cytc₂ mutants elevate levels of the cytc₂isoform, isocyt c₂. We monitoredphotosynthetic electron transfer in whole cells, in chromatophores,andwith purified components to ascertain if and how isocytc₂ reduced light-oxidized RC complexes.Thesestudies revealed that several fundamental aspects of photosyntheticelectron transfer were similar in strainsthat use isocyt c₂ and wild-type cells. Forexample, P₈₇₀⁺ reduction accompaniedcytochrome c oxidation.In addition, photosynthetic electron transfer was blocked by thewell-known cyt bc₁ complexinhibitorsantimycin and myxothiazol. However, even at the increased isocytc₂ levels present in these strains(~40%that of cyt c₂ in wild-type cells), there waslittle, if any, of the rapid (<5 µs) electron transfer toP₈₇₀⁺ thatis characteristic of cytochromes bound to RC complexes at the time ofthe light flash. Thus, it appearsthat isocyt c₂ function limits the invivo rate of P₈₇₀⁺ reduction.Indeed, at low ionic strength in vitro,theapparent affinity of isocyt c₂ for RC complexes(K_D ~ 40 µM) is significantly lower thanthat of cyt c₂(K_D ~ 1.0 µM). This reduced affinitydoes not appear to result from an altered mode of RC bindingbyisocyt c₂ since electrostatic interactions makesimilar overall contributions to the binding of both cytc₂and isocyt c₂ to this membrane-bound redoxpartner. Thus, sequence, structural, or localconformationaldifferences between cyt c₂ and isocytc₂ significantly alter their apparent affinitiesfor this physiologicallyrelevant redox partner.