Rhodobacter sphaeroides electron transfer rate

Fig. 25. Electron transfer pathways in the RC isolated from Rhodobacter Sphaeroides strain R-26. (BChl)2 is dimer of the bacterioehlorophyll, BPh is the bacteriopheophytin, Q, -ubiquinone. The electron transfer rates are for the native RC with ubiquinone —10 as Q,v at room temperature. The rates given in parenthesis were determined below 100 K [194-197]...

The X-ray crystal structure of a reaction centre from Rhodobacter sphaeroides with a mutation of tyrosine M210 to tryptophan (YM210W) has been determined to have a resolution of 2.5 A (McAuley et al., 2000). It is shown that the main effect of the introduction of the bulkier tryptophan in place of the native tyrosine is a small tilt of the macrocycle of the (Bchl)L. The effect of the redox potential of the electron acceptor (Bchl) in RC from Rb. spheroides on the initial electron transfer rate and on the P (Bchl) population was investigated (Sporlein et al., 2000). Analysis of experimental... 

A remarkable feature of the Rhodopseudomonas viridis (1) and Rhodobacter sphaeroides (3-4) reaction centers is that in spite of the structural symmetry, the primary light-initiated electron transfer appears to involve only one set of chromophores. This photochemical as3nnametry is most clearly demonstrated by the two bacteriopheophytins. Only H is detected to be reduced in transient P H" states at low temperature (5,6). The absence of detectable formation of Hj in transient states suggests that the ratio of the electron transfer rates from the excited singlet P to each bacteriopheophytin, kj /kj, must exceed 10. The electron transfer time for P H reaction has been measured to be about 1 ps at 90 K (9). This suggests that the electron transfer time to Hj must exceed 10 ps. 

Boxer has written a very good review of the distance dependence of electron transfer rates in proteins.This article presents a detailed overview of theoretical and experimental studies of electron transfer processes in proteins and in modified protein systems. The major focus of this review is on the fast, primary, photoinduced processes in the reaction centers of Rhodobacter sphaeroides and Rhodopseudomonas viridis. 

Study the AG° dependence at different temperatures. This was done by Dutton and co-workers for the electron transfers from Bph to and Qa to (Bchl)2 in Rhodobacter sphaeroides photosynthetic reaction centers [139,140], which take place over about 13 A and 25 A, respectively [18], The rates were measured between 10 K and 300 K in series in which quinone substitutions provide AG° ranges of 0.5 eV and 0.8 eV for the two reactions respectively. The following conclusions were deduced from a thorough analysis of the experimental results ... 

Lin, X., Williams, J. C., Allen, J. P., and Mathis, P., 1994, Relationship between rate and free-energy difference for electron-transfer from cytochrome c(2) to the reaction center in Rhodobacter sphaeroides Biochemistry 33 13517913523. 

Table 2 Reaction of Ru-55-Cc with Rhodobacter Sphaeroides CcO mutants. The intracomplex rate constant A a for electron transfer from heme c to Cua, the dissociation constant K, and the second-order rate constant k2nd were at 5 mM, 45 mM, and 95 mM ionic strength, respectively, at pH 8 and 23...

Based on the nature of the cytochromes, there are two kinds of photosynthetic bacterial reaction centers. The first kind, represented by that of Rhodobacter sphaeroides, has no tightly bound cytochromes. For these reaction centers, as shown schematically in Fig. 2, left, the soluble cytochrome C2 serves as the secondary electron donor to the reaction center the RC also accepts electrons from the cytochrome bc complex by way ofCytc2- The rate of electron transfer from cytochrome to the reaction center is sensitive to the ionic strength of the medium. Functionally, cytochrome C2 is positioned in a cyclic electron-transport loop. In Rb. sphaeroides, Rs. rubrum and Rp. capsulata cells, the two molecules of cytochromes C2 per RC are located in the periplasmic space between the cell wall and the cell membrane. When chromatophores are isolated from the cell the otherwise soluble cytochrome C2 become trapped and held by electrostatic forces to the membrane surface at the interface with the inner aqueous phase. These cytochromes electrostatically bound to the membrane can donate electrons to the photooxidized P870 in tens of microseconds at ambient temperatures, but are unable to transfer electrons to P870 at low temperatures. 

Quantitative information about the elementary contributions of the native system structure to X. at the site of the RC from Rhodobacter sphaeroides can be obtained by comparing the electron transfer characteristics that have been already described for a family of quinones [8,13], with non-quinone, I.e. "exotic", cofactors [14,15]. The strategy to reveal specific contributions to X is to compare the rates of electron transfer mediated by quinone and exotic cofactors at comparable values of -AG°et. over the temperature range from 14 to 295 K. Figure 1 depicts the way in which alteration of the basic quinone structure could influence the appearance of the experimental rate/-AG°et relation through changes in X, In practice, the scatter in the quinone rate/-AG°et data [8], which has been attributed to small variations in V(r) for... 

The rates of the electron transfer processes in reaction centers (RC s) of photosynthetic bacteria are controlled both by the spatial and the electronic structure of the involved donor and acceptor molecules. The spatial structure of bacterial RC s has been determined by X-ray diffraction for Rhodopseudomonas (Rp.) viridis and for Rhodobacter (Rb.) sphaeroides,- The electronic structure of the transient radical species formed in the charge separation process can be elucidated by EPR and ENDOR techniques. The information is contained in the electron-nuclear hyperfine couplings (hfc s) which, after assignment to specific nuclei, yield a detailed picture of the valence electron spin density distribution in the respective molecules. 

---