Effects of mutations near the bacteriochlorophylls in reaction centers from Rhodobacter sphaeroides

Abstract

Mutations were made in four residues near the bacteriochlorophyll cofactors of the photosynthetic reaction center from Rhodobacter sphaeroides. These mutations, L131 Leu to His and M160 Leu to His, near the dimer bacteriochlorophylls, and M203 Gly to Asp and L177 Ile to Asp, near the monomer bacteriochlorophylls, were designed to result in the placement of a hydrogen bond donor group near the ring V keto carbonyl of each bacteriochlorophyll. Perturbations of the electronic structures of the bacteriochlorophylls in the mutants are indicated by additional resolved transitions in the bacteriochlorophyll absorption bands in steady-state low-temperature and time-resolved room temperature spectra in three of the resulting mutant reaction centers. The major effect of the two mutations near the dimer was an increase up to 80 mV in the donor oxidation-reduction midpoint potential. Correspondingly, the calculated free energy difference between the excited state of the primary donor and the initial charge separated state decreased by up to 55 mV, the initial forward electron-transfer rate was up to 4 times slower, and the rate of charge recombination between the primary quinone and the donor was approximately 30% faster in these two mutants compared to the wild type. The two mutations near the monomer bacteriochlorophylls had minor changes of 25 mV or less in the donor oxidation-reduction potential, but the mutation close to the monomer bacteriochlorophyll on the active branch resulted in a roughly 3-fold decrease in the rate of the initial electron transfer.

How photosynthetic reaction centers control oxidation power in chlorophyll pairs P680, P700, and P870

“…In contrast to PSII, with an unusually high E m (P680) of 1,100-1,300 mV (1,2), the corresponding E m values in PbRC, E m (P870) ϭ 500 mV (4), and in PSI, E m (P700) ϭ 500 mV (5), are low. Part of these E m differences were associated with electronic coupling, which is weak between Chla in P D1/D2 but strong between Bchla in P L/M because of mutual overlap of BChla rings I.…”

“…By contrast, P680 ϩ⅐ in PSII is rereduced by a redoxactive tyrosine (D1-Tyr-161, Y Z ), which is subsequently reduced by electron transfer from the unique Mn 4 Ca cluster, where water is oxidized under release of atmospheric oxygen, protons, and electrons. Kinetic studies (1) and computations (2) yielded redox potentials for one-electron oxidation E m (P680) of 1,100-1,300 mV, high enough for P680 ϩ⅐ to act as an electron acceptor for the different Mn 4 Ca redox states. According to recent studies, P680 probably consists of the Chla pair P D1/D2 or the two adjacent accessory Chla, Chl D1/D2 (3).…”

Excited-State Electronic Asymmetry of the Special Pair in Photosynthetic Reaction Center Mutants: Absorption and Stark Spectroscopy

“…The special pairs in these RCs have significantly altered spectroscopic (10,11) and functional properties (12). Mutants have been designed in which the histidine residue (His L168) that is hydrogen-bonded to the peripheral conjugated carbonyl group is removed or in which non-hydrogen-bonding amino acids near to conjugated carbonyl groups of P (residues M160, L131, and M197) are replaced by histidine residues that are capable of forming hydrogen bonds (13)(14)(15)(16). The presence or absence of these hydrogen bonds leads to changes in the carbonyl stretch frequency detected by FTIR (17) and FT Raman spectroscopies (18)(19)(20).…”

New tetragonal form of reaction centers fromRhodobacter sphaeroidesand the involvement of a manganese ion at a crystal contact point

“…Wild-type reaction centers were expressed and isolated as previously described by Williams et al (1992). Prior to crystallization, samples were further purified with a tertiary amine column using FPLC (Pharmacia LKB, Uppsala, Sweden).…”