Thermodynamic and Kinetic Characterization of Electron‐Transfer Components <i>in situ</i> in <i>Rhodopseudomonas spheroides</i> and <i>Rhodospirillum rubrum</i>

Abstract

The b ‐ and c ‐type cytochromes in chromatophores from photosynthetically grown Rhodospirillum rubrum and Rhodopseudomonas spheroides have been characterized in situ in terms of their oxidation‐reduction potential properties. In R. rubrum only one cytochrome c measured at 551—540 nm was resolved potentiometrically; the midpoint potential at pH 7.2, E m7.2 , is +293 mV. Three cytochrome species measured at 562—540 nm were resolved; they had E m7.2 values + 170 mV, −5 mV and −105 mV and were tentatively identified with cytochrome b, cc′ and b respectively; they contributed to the over‐all absorbance change in the ratio 8:25:17, respectively. In Rps. spheroides the E m7.0 of cytochrome c is +295 mV. Three cytochromes tentatively considered to be of the b ‐type have E m7.2 values of +155 mV, +50 mV and −90 mV; their approximate reduced minus oxidized maxima were determined to be 558—9 nm, 560 nm and 564 nm, respectively. The approximate relative absorbance ratio given at the reduced minus oxidized maxima was 8:21:14. The rapid oxidation‐reduction reactions of reaction‐center bacteriochlorophyll, cytochrome c and cytochromes b in Rps. spheroides measured following a 20‐nsec laser flash or a 200‐μsec xenon flash have been studied in the absence and presence of antimycin A as a function of oxidation‐reduction potential. The results indicated that the reaction‐center bacteriochlorophyll ( E m7.0 + 450 mV) serves to oxidize the cytochrome c with a midpoint potential of 295 mV, cytochrome c 295 , (half‐time t 1/2 ↗100 μsec with a slow phase t 1/2 ∼2 msec) which in turn can oxidize cytochrome b 155 t 1/2 1—2 msec) in a reaction which is antimycin‐A‐sensitive. Cytochrome b 50 becomes reduced following a flash with a t 1/2 1–1.5 msec probably via the primary electron acceptor ( E m7.0 ∼−20 mV) of the reaction‐center bacteriochlorophyll. Antimycin A stimulates the extent of cytochrome b 50 reduction following a 200‐μsec xenon flash but has no detectable effect following a 20‐nsec laser flash. A working model for electron transfer in Rps. spheroides based on the measured thermodynamic and kinetic properties is presented.