Electrostatic Interaction between Redox Cofactors in Photosynthetic Reaction Centers

Abstract

Intramolecular electron transfer within proteins is an essential process in bioenergetics. Redox cofactors are embedded in proteins, and this matrix strongly influences their redox potential. Several cofactors are usually found in these complexes, and they are structurally organized in a chain with distances between the electron donor and acceptor short enough to allow rapid electron tunneling. Among the different interactions that contribute to the determination of the redox potential of these cofactors, electrostatic interactions are important but restive to direct experimental characterization. The influence of interaction between cofactors is evidenced here experimentally by means of redox titrations and time-resolved spectroscopy in a chimeric bacterial reaction center (Maki, H., Matsuura, K., Shimada, K., and Nagashima, K. V. P. (2003) J. Biol. Chem. 278, 3921-3928) composed of the core subunits of Rubrivivax gelatinosus and the tetraheme cytochrome of Blastochloris viridis. The absorption spectra and orientations of the various cofactors of this chimeric reaction center are similar to those found in their respective native protein, indicating that their local environment is conserved. However, the redox potentials of both the primary electron donor and its closest heme are changed. The redox potential of the primary electron donor is downshifted in the chimeric reaction center when compared with the wild type, whereas, conversely, that of its closet heme is upshifted. We propose a model in which these reciprocal shifts in the midpoint potentials of two electron transfer partners are explained by an electrostatic interaction between them.Proteins exert a fine electrochemical tuning of the redox potential of the cofactors they bind in order to perform the various electron transfer reactions that are involved in biological processes. As a famous example, the redox potentials (E m ) 1 of c-type cytochromes are tuned by more than 500 mV (see Ref.1 and references therein). The physicochemical basis of a such wide range of modulations has been rationalized by various authors who all agree that the protein medium has the unique property of providing a dielectric environment in which the redox cofactors are embedded with an intricate charge or dipole distribution (1-3). From a general standpoint, the free energy difference between the oxidized and reduced forms of any redox cofactor in a protein is the sum of several terms. Among these are ⌬G conf and ⌬G el . ⌬G conf accounts for any conformational change that the change in the redox state of the cofactor may induce (including proton or ion binding or release). ⌬G el results from the electrostatic potential at the cofactors resulting from the individual charged groups and the permanent dipoles within the protein. Estimating the respective values of these different terms is a difficult task, yet their sum is readily accessible experimentally because it can be obtained by comparing the absolute values of the E m in solution and in the protein. The latter may be obta...

Structural and spectropotentiometric analysis of Blastochloris viridis heterodimer mutant reaction center

“…5, revealed the increase of redox midpoint potential in the modified organism. The titration curve in the WT could be satisfactorily fitted with a Nernst curve with E m =516.9±5.3 mV, in agreement with previous measurements [51,73,74]. The reverse titration was possible only for the WT RCs.…”

“…This has implications for ET between the B. viridis cytochrome and the R. gelatinosus RC, as the proximal heme in the B. viridis cytochrome would be expected to have essentially the same midpoint potential as the primary donor in R. gelatinosus . In the expressed complex the change in potential was more pronounced; the primary donor was found to have an even lower midpoint potential than the WT while the heme had a higher one, indicating a significant alteration in electrostatic interactions [51]. …”

Study of the high-potential iron sulfur protein in Halorhodospira halophila confirms that it is distinct from cytochrome c as electron carrier

“…As shown above, the rate of electron donation from the cytochrome subunit to P ϩ strongly decelerates upon oxidation of the lowpotential pair of hemes. Similar effects have been observed for other purple bacterial reaction centers (38). A third, intermediate kinetic phase is present in all traces shown.…”

The electron distribution in the “activated” state of cytochrome c oxidase

“…Because interactions between the redox sites are not considered (see 35 , 36 ), the values differ from those obtained in redox titrations, which typically yield a higher midpoint potential for the catalytic site heme (here heme a 3 ) than for the intermediate electron acceptor (here heme a ) 35 , 36 , although in many bacterial oxidases the difference between the midpoint potentials is reversed 37 . Furthermore, because a single electron equilibrates among the five redox sites, the situation differs from a redox titration 38 . The purpose of the exercise to calculate the apparent redox potentials is not to provide the values, only to indicate that the apparent redox potential of Cu B is not higher than that of any other site after reaction of the Cyt c O with O 2 .…”