Electron spin resonance characterization of Chromatium D hemes,non-heme irons and the components involved in primary photochemistry |
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Authors: | PLeslie Dutton John S Leigh |
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Institution: | Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pa. 19174 U.S.A. |
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Abstract: | The combination of redox potentiometry with low temperature electron spin resonance (ESR) spectroscopy has led to further characterization of electron transfer components of Chromatium D. These include the readily buffer-soluble cytochromes c553 and c′ and the high-potential iron-sulfur protein in the isolated state and associated with the chromatophore membrane. Buffer-insoluble cytochrome c553, cytochro—me c555, bacteriochlorophyll and the primary electron acceptor have been characterized both in the chromatophore membrane and also in a sodium dodecylsulfate detergent-solubilized subchromatophore preparation. Two iron-sulfur proteins have been revealed which are present in the chromatophore membrane but are released on treatment with sodium dodecylsulfate. They have central g values at 1.90 and 1.94 and have estimated midpoint potentials at pH 7.4 (Em7·4) at +280 mV and ?100 mV, respectively, when associated with the chromatophore.In the membrane associated state the apparent Em of cytochrome c′ is approximately 200 mV more positive than the Em values reported for the free state; this implies either that the reduced form of cytochrome c′ binds to the membrane (or to a component therein) to a degree which is > 103 times greater than that of the oxidized form or that the Em shift results from membrane solvation. In the case of the high-potential iron-sulfur protein however, its Em when associated with the chromatophore membrane is similar to that reported in the isolated state. The light-induced oxidation of the high-potential iron-sulfur protein at room temperature appears to be linked only to the oxidation of cytochrome c555; it could serve as an electron pool in equilibrium with cytochrome c555 in the cyclic electron flow system.The redox component defined in the reduced state by its gy = 1.82 and gx = 1.62 ESR spectrum satisfies the following criteria for its identification as the primary electron acceptor of P883. (a) The Em7·4 value of the g = 1.82 component is ?120 ± 25mV. (b) At ?70 mV, where the g = 1.82 component is mainly oxidized in the dark, brief illumination at low temperature which causes the irreversible oxidation of one cytochrome c553 heme, also induces the permanent reduction of the g = 1.82 component; the extent of reduction after brief illumination, given by the g = 1.82 signal height, is the same as that induced chemically at ?270 mV showing it to be fully reduced by the receipt of a single electron. (c) At more positive potentials where cytochrome c553 is oxidized and is not involved in low-temperature reactions, the light-induced low-temperature kinetics of the g = 1.82 signal are reversible; the flash-induced g = 1.82 formation and subsequent dark decay are the same as those for the flash-induced P+883 (g = 2) formation and dark decay. We suggest that until a full physical-chemical characterization is completed this g = 1.82 component be designated “photoredoxin”. |
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Keywords: | C A T computer of average transients |
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