Optimal Oxidation-Reduction Potentials and Endogenous Co-factors in Bacterial Photophosphorylation^*

Abstract

Rhodospirillum rubnim cell suspensions, disrupted by sonication, yield chromatophores which can be depleted by washing to give minimal photophosphorylation and dark-phosphorylation rates. Photophosphorylation by such chromatophores can be restored to a maximal rate by an optimal concentration of ascorbate (4.7 X 10-8 m) added externally. The same maximal rate is obtained by addition of phenazine methosulfate or by addition of endogenous co-factors, either lumped together in a "supernatant fraction" (chromatophore washings) or as single purified components (flavins, heme proteins). Further, the amounts of chromatophore washings which produce the characteristic maximal rate of photophosphorylation have reducing power equivalent to ascorbate at a concentration of 2.2 X 10~8 m. Cytochrome c2 and riboflavin exhibit special effects which show that the former has substrate specificity as a terminal acceptor in the electron-transport system which supports photophosphorylation, while the latter is apparently active at the photophosphorylation site as the co-factor for a pyridine nucleotide-linked flavoenzyme heme protein reductase. All of the results found are in accord with the suggestion that a circular chain of respiratory catalysts is present which is coupled to phosphorylation optimally at an effective intrachromatophore oxidationreduction potential of ~0.0 ± 0.01 volts.

Photophosphorylation in Chloroplasts

“…(81), who showed with ferredoxin cyclic phosphorylation that ATP was formed only while the ferredoxin was in an intermediate redox level. These observations extend to the chloroplast system and point up the previously emphasized im portance of the maintenance of a redox balance or "poising" in bacterial cyclic photophosphorylation (42,48,74,104,169). Many apparent discor dant observations previously noted may have been a result of insufficient appreciation of the importl;l-nce of the maintenance of a proper redox level during the reaction.…”

Observations on the Respiratory System of Rhodospirillum rubrum*

“…The place assigned cytochrome c2-between RHP or flavin-mediated RHP-reductase and bacteriochlorophyll-is based on its high oxidation potential (midpoint potential, pH 7, approximately 330 mv)-and evidence based on our studies of photophosphorylation (Horio and Kamen, 1962) which show clearly it is at the terminal end of the electron transport chain coupled to photophosphorylation.…”

“…Chromatium contains large amounts of RHP (Bartsch and Kamen, 1960), and, as a strict anaerobe, has no requirement for an oxidase, although it shows weak oxidase activity in cell-free extracts (Kamen and Vernon, 1954). Horio and Kamen (1962) have shown that in R. rubrum chromatophores RHP acts as an intermediate in phosphorylation coupled to the photoactivated transport of electrons.…”

Action of phenazine methyl sulfate, inhibitors, and uncouplers on the light-induced proton transport by cells ofRhodospirillum rubrum

“…In the cyclic photosynthetic electron transport of R. rubrum, PMS could accept electrons from an early reductant such as flavoprotein since o-phenanthroline, which inhibits the cyclic photophosphorylation in the physiological system, does not inhibit the PMS-mediated phosphorylation [25]. Although ubiquinone-10, flavoprotein, and rhodospirillum heme protein have been shown to be involved in cyclic electron transport and photophosphorylation in R. rubrum, the exact sequence of these electron transport carriers is not known [1,11,21].…”