Mg Coordination by Amino Acid Side Chains Is Not Required for Assembly and Function of the Special Pair in Bacterial Photosynthetic Reaction Centers

Goldsmith, J O; King, Brett; Boxer, Steven G.

doi:10.1021/bi9523365

Cited by 77 publications

(77 citation statements)

References 55 publications

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“…This is in contrast with the bacterial reaction centers where the homologous glycine replacements are practically indistinguishable from wild type (39). The largest redox effects at D1-His198 are associated with His replacement by Glu and Ser.…”

Section: Discussionmentioning

confidence: 71%

“…sphaeroides reaction centers is still the preferred binding motif over the replacement of the Bchlorophyll with Bpheophytin. Goldsmith et al (39) and Schulz et al (38) have shown that replacement of the coordinating His with Gly still results in Bchlorophyll binding but with water as the likely ligand. The same is likely true of the B-His656Gly mutation of PSI (40,41).…”

Section: Discussionmentioning

confidence: 99%

“…While such replacement appears to result from a loss of the Mg coordinate covalent bond, substitution for the coordinating His of Gly (38,39) does not lead to replacement of BChl. Despite the absence of a coordinating residue in the case of the glycine mutant, it is likely that a water molecule has taken its place (39).…”

Section: Introductionmentioning

confidence: 95%

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Site-Directed Mutations at D1-His198 and D2-His197 of Photosystem II in Synechocystis PCC 6803: Sites of Primary Charge Separation and Cation and Triplet Stabilization

et al. 2001

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Site-directed mutations were introduced to replace D1-His198 and D2-His197 of the D1 and D2 polypeptides, respectively, of the photosystem II (PSII) reaction center of Synechocystis PCC 6803. These residues coordinate chlorophylls P A and P B which are homologous to the special pair Bchlorophylls of the bacterial reaction centers that are coordinated respectively by histidines L-173 and M-200 (202). P A and P B together serve as the primary electron donor, P, in purple bacterial reaction centers. In PS II, the site-directed mutations at D1 His198 affect the P + -P-absorbance difference spectrum. The bleaching maximum in the Soret region (in WT at 433 nm) is blue-shifted by as much as 3 nm. In the D1 His198Gln mutant, a similar displacement to the blue is observed for the bleaching maximum in the Q y region (672.5 nm in WT at 80 K), whereas features attributed to a band shift centered at 681 nm are not altered. In the Y Z •-Y Z -difference spectrum, the band shift of a reaction center chlorophyll centered in WT at 433-434 nm is shifted by 2-3 nm to the blue in the D1-His198Gln mutant. The D1-His198Gln mutation has little effect on the optical difference spectrum, 3 P-1 P, of the reaction center triplet formed by P + Pheo -charge recombination (bleaching at 681-684 nm), measured at 5-80 K, but becomes visible as a pronounced shoulder at 669 nm at temperatures g150 K. Measurements of the kinetics of oxidized donor-Q A -charge recombination and of the reduction of P + by redox active tyrosine, Y Z , indicate that the reduction potential of the redox couple P + /P can be appreciably modulated both positively and negatively by ligand replacement at D1-198 but somewhat less so at D2-197. On the basis of these observations and others in the literature, we propose that the monomeric accessory chlorophyll, B A , is a long-wavelength trap located at 684 nm at 5 K. B A * initiates primary charge separation at low temperature, a function that is increasingly shared with P A * in an activated process as the temperature rises. Charge separation from B A * would be potentially very fast and form P A + B A -and/or B A + Pheo -as observed in bacterial reaction centers upon direct excitation of B A ) Proc. Natl. Acad Sci. 96, 2054-2059. The cation, generated upon primary charge separation in PSII, is stabilized at all temperatures primarily on P A , the absorbance spectrum of which is displaced to the blue by the mutations. In WT, the cation is proposed to be shared to a minor extent (∼20%) with P B , the contribution of which can be modulated up or down by mutation. The band shift at 681 nm, observed in the P + -P difference spectrum, is attributed to an electrochromic effect of P A + on neighboring B A . Because of its low-energy singlet and therefore triplet state, the reaction center triplet state is stabilized on B A at e80 K but can be shared with P A at >80 K in a thermally activated process.

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Section: Discussionmentioning

confidence: 71%

Section: Discussionmentioning

confidence: 99%

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Site-Directed Mutations at D1-His198 and D2-His197 of Photosystem II in Synechocystis PCC 6803: Sites of Primary Charge Separation and Cation and Triplet Stabilization

et al. 2001

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“…The L168HG mutant was prepared as described previously (28). The L173HL mutant was engineered with a poly(histidine) tag at the N-terminal end of the M subunit; this poly(histidine) tag has been shown to have no effect on the properties of the RC but greatly facilitates RC isolation (29).…”

Section: Methodsmentioning

confidence: 99%

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

1999

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The electronic absorption line shape and Stark spectrum of the lowest energy Q y transition of the special pair in bacterial reaction centers contain a wealth of information on mixing with charge transfer states and electronic asymmetry. Both vary greatly in mutants that perturb the chemical composition of the special pair, such as the heterodimer mutants, and in mutants that alter interactions between the special pair and the surrounding reaction center protein, such as those that add or remove hydrogen bonds. The conventional and higher-order Stark spectra of a series of mutants are presented with the aim of developing a systematic description of the electronic structure of the excited state of the special pair that initiates photosynthetic charge separation. The mutants L168HF, M197FH, L131LH and L131LH/M160LH/ M197FH are known to have different hydrogen-bonding patterns to the special pair; however, they exhibit Stark effects that are very similar to wild type. By contrast, the addition of a hydrogen bond to the M-side keto carbonyl group of the special pair in M160LH greatly affects both the absorption and Stark spectra. The heterodimer special pairs, L173HL and M202HL, exhibit much larger Stark effects than wild type, with the greatest effect in the M-side mutant. Double mutants that combine the M-side heterodimer and a hydrogen-bond addition to the L-side of the special pair decrease the magnitude of the Stark effect. These results suggest that the electronic asymmetry of the dimer can be perturbed either by the formation of a heterodimer or by adding or deleting a hydrogen bond to a keto carbonyl group. From the pattern observed, it is concluded that the charge transfer state P L + P M -has a larger influence on the excited state of the dimer in wild type than the P L -P M + charge transfer state. Furthermore, asymmetry can be varied continuously, from extreme cases in which the heterodimer and hydrogen-bond effects work together, to cases in which hydrogen bonding offsets the effects of the heterodimer, to cases in which the homodimer is perturbed by hydrogen bonds. This leads to a unified model for understanding the effects of perturbations on the electronic symmetry of the special pair, and this can be connected with perturbations on the properties of many other systems such as donor-acceptor-substituted polyenes.Photoexcitation of a strongly interacting pair of bacteriochlorophyll (BChl) 1 molecules called the special pair or P in bacterial photosynthetic reaction centers (RCs) initiates a series of light-driven charge separation reactions. The singlet excited state of P, 1 P, transfers an electron within a few picoseconds to H L , a bacteriopheophytin (BPhe) molecule that is the initial electron acceptor (1). As shown in the upper part of Figure 1, there are two similar sets of chromophores related by a local C 2 axis of symmetry that can serve as electron acceptors, yet only the chromophores on the L side (the right side as illustrated, sometimes denoted the A branch) participate in the i...

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“…Ligand replacement studies on other enzymes, for example, carbonic anhydrase (26), heme-containing proteins (27)(28)(29)(30), and the bacterial reaction center (31), suggest a biochemical method by which the proton acceptor of tyrosine D can be identified. In the above investigations, a histidine residue was replaced with glycine or alanine by site-directed mutagenesis.…”

mentioning

confidence: 99%

Chemical complementation identifies a proton acceptor for redox-active tyrosine D in photosystem II

Kim

Liang

Barry

1997

Proc. Natl. Acad. Sci. U.S.A.

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Mg Coordination by Amino Acid Side Chains Is Not Required for Assembly and Function of the Special Pair in Bacterial Photosynthetic Reaction Centers

Cited by 77 publications

References 55 publications

Site-Directed Mutations at D1-His198 and D2-His197 of Photosystem II in Synechocystis PCC 6803: Sites of Primary Charge Separation and Cation and Triplet Stabilization

Site-Directed Mutations at D1-His198 and D2-His197 of Photosystem II in Synechocystis PCC 6803: Sites of Primary Charge Separation and Cation and Triplet Stabilization

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

Chemical complementation identifies a proton acceptor for redox-active tyrosine D in photosystem II

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