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(KD ~ 40 µM) is significantly lower thanthat of cyt c2(KD ~ 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.