Tuning electron transfer by ester-group of chlorophylls in bacterial photosynthetic reaction center

Role of hydrogen-bond networks on the donor side of photosynthetic reaction centers from purple bacteria

For the last decades, significant progress has been made in studying the biological functions of H-bond networks in membrane proteins, proton transporters, receptors, and photosynthetic reaction centers. Increasing availability of the X-ray crystal and cryo-electron microscopy structures of photosynthetic complexes resolved with high atomic resolution provides a platform for their comparative analysis. It allows identifying structural factors that are ensuring the high quantum yield of the photochemical reactions and are responsible for the stability of the membrane complexes. The H-bond networks are known to be responsible for proton transport associated with electron transfer from the primary to the secondary quinone as well as in the processes of water oxidation in photosystem II. Participation of such networks in reactions proceeding on the periplasmic side of bacterial photosynthetic reaction centers is less studied. This review summarizes the current understanding of the role of H-bond networks on the donor side of photosynthetic reaction centers from purple bacteria. It is discussed that the networks may be involved in providing close association with mobile electron carriers, in light-induced proton transport, in regulation of the redox properties of bacteriochlorophyll cofactors, and in stabilization of the membrane protein structure at the interface of membrane and soluble phases.

Small molecule carbon source promoting dairy wastewater treatment of Rhodospirillum rubrum by co-metabolism and the establishment of multivariate nonlinear equation

Rhodospirillum rubrum water treatment technology could recycle bio-resource. However, the inability to degrade macromolecular organics limited its wide application. This paper discussed the feasibility of small molecular carbon source promoting R. rubrum directly treating dairy machining wastewater (DMW) and accumulations for single cell protein and pigment, and establishment of a mathematical model. Six small molecules promoted the degradation of macromolecules (proteins) in DMW. They promoted protease secretion and non-growth matrix (protein) decomposition in DMW through co-metabolism. Among the molecules, 550 mg/L potassium sodium tartrate was the best, protease activity and protein removal rate were increased by 100% compared with control. Then chemical oxygen demand (COD) and protein removal rates reached 80%, the single cell protein, carotenoid and bacterial chlorophyll yields were increased 2 times. Meanwhile, carbon nitrogen ratio (C/N) and food microbial ratio (F/M) were identified as the most important factors by principal component analysis. A multivariate nonlinear equation model between COD removal rate and C/N, F/M, time was established.

Demonstration of asymmetric electron conduction in pseudosymmetrical photosynthetic reaction centre proteins in an electrical circuit

Photosynthetic reaction centres show promise for biomolecular electronics as nanoscale solar-powered batteries and molecular diodes that are amenable to atomic-level re-engineering. In this work the mechanism of electron conduction across the highly tractable Rhodobacter sphaeroides reaction centre is characterized by conductive atomic force microscopy. We find, using engineered proteins of known structure, that only one of the two cofactor wires connecting the positive and negative termini of this reaction centre is capable of conducting unidirectional current under a suitably oriented bias, irrespective of the magnitude of the bias or the applied force at the tunnelling junction. This behaviour, strong functional asymmetry in a largely symmetrical protein–cofactor matrix, recapitulates the strong functional asymmetry characteristic of natural photochemical charge separation, but it is surprising given that the stimulus for electron flow is simply an externally applied bias. Reasons for the electrical resistance displayed by the so-called B-wire of cofactors are explored.

Photosynthetic reaction centres have been proposed for applications in bioelectronics. Here, the authors examine electron transport through the reaction centre from R. sphaeroides using conductive AFM, observing asymmetric conductance along only one cofactor wire under an applied bias.

Synthesis of oligomethylene-strapped chlorophyll derivatives and optical properties of their stereoisomers in a solution

Methyl pheophorbide-a/a' derivatives covalently linked with oligomethylene chains at the 3-CH(2)OCO- and 13(2)-COO- moieties in a molecule were prepared by modifying chlorophyll-a through intramolecular ring-closing metathesis of vinyl groups. At least, a C10-length between the 3(3)- and 13(4)-positions was necessary for the cyclization and connection of a C12-strap was the most suitable to achieve the highest closure yield. The oligomethylene chain in 13(2) R-epimers derived from methyl pheophorbide-a covered the α-face of the chlorin π-plane and the strap in the corresponding 13(2) S-epimers protected the β-face. Synthetic 13(2) R-epimer with a dodecamethylene chain gave a flat chlorin π-plane, while the decamethylene chain in the 13(2) R-epimer distorted the π-system due to its shorter linkage. The distortion by strapping in the 13(2) R-epimer induced a slight blue-shift of Qy peak in dichloromethane. CD spectra of the 13(2) R-epimers were similarly dependent on the chain length, i.e., the distortion of π-plane. Visible absorption and CD spectra of all the strapped 13(2) S-epimers were almost identical and only slightly different from those of the unstrapped. The strapping in the 13(2) S-epimers shifted the Qy peak bathochromically.

Cationic state distribution over the P700 chlorophyll pair in photosystem I

The primary electron donor P700 in photosystem I is composed of two chlorophylls, P(A) and P(B). P700 forms the cationic [P(A)/P(B)](•+) state as a result of light-induced electron transfer. We obtained a P(A)(•+)/P(B)(•+) ratio of 28:72 and a spin distribution of 22:78 for the entire PSI protein-pigment complex. By considering the influence of the protein components on the redox potential for one-electron oxidation of P(A)/P(B) monomers, we found that the following three factors significantly contributed to a large P(B)(•+) population relative to P(A)(•+): 1), Thr-A743 forming a H-bond with P(A); 2), P(A) as a chlorophyll a epimer; and 3), a conserved PsaA/PsaB pair, the Arg-A750/Ser-B734 residue. In addition, 4), the methyl-ester groups of the accessory chlorophylls A(-1A)/A(-1B) significantly stabilized the cationic [P(A)/P(B)](•+) state and 5), the methyl-ester group orientations were completely different in A(-1A) and A(-1B) as seen in the crystal structure. When the methyl-ester group was rotated, the spin-density distribution over P(A)/P(B) ranged from 22:78 to 15:85.

Tyrosine deprotonation and associated hydrogen bond rearrangements in a photosynthetic reaction center

Photosynthetic reaction centers from Blastochloris viridis possess Tyr-L162 located mid-way between the special pair chlorophyll (P) and the heme (heme3). While mutation of the tyrosine does not affect the kinetics of electron transfer from heme3 to P, recent time-resolved Laue diffraction studies reported displacement of Tyr-L162 in response to the formation of the photo-oxidized P(+•), implying a possible tyrosine deprotonation event. pK(a) values for Tyr-L162 were calculated using the corresponding crystal structures. Movement of deprotonated Tyr-L162 toward Thr-M185 was observed in P(+•) formation. It was associated with rearrangement of the H-bond network that proceeds to P via Thr-M185 and His-L168.

Charge transport in the ClC-type chloride-proton anti-porter from Escherichia coli

The first chloride transporter identified in the superfamily of ClC chloride channels was from Escherichia coli (EClC) (Accardi, A., and Miller, C. (2004) Nature 427, 803-807). Pathways, energetics, and mechanism of proton and chloride translocation and their coupling are up to now unclear. To bridge the hydrophobic gap of proton transport, we modeled four stable buried waters into both subunits of the WT EClC structure. Together they form a "water wire" connecting Glu-203 with the chloride at the central site, which in turn connects to Glu-148, the hypothetical proton exit site. Assuming the transient production of hydrochloride in the central chloride binding site of EClC, the water wire could establish a transmembrane proton transport pathway starting from Glu-203 all the way downstream onto Glu-148. We demonstrated by electrostatic and quantum chemical computations that protonation of the central chloride is energetically feasible. We characterized all chloride occupancies and protonation states possibly relevant for the proton-chloride transport cycle in EClC and constructed a working model. Accordingly, EClC evolves through states involving up to two excess protons and between one and three chlorides, which was required to fulfill the experimentally observed 2:1 stoichiometry. We show that the Y445F and E203H mutants of EClC can operate similarly, thus explaining why they exhibit almost WT activity levels. The proposed mechanism of coupled chloride-proton transport in EClC is consistent with available experimental data and allows predictions on the importance of specific amino acids, which may be probed by mutation experiments.

Light-induced hydrogen bonding pattern and driving force of electron transfer in AppA BLUF domain photoreceptor

The AppA BLUF (blue light sensing using FAD) domain from Rhodobacter sphaeroides serves as a blue light-sensing photoreceptor. The charge separation process between Tyr-21 and flavin plays an important role in the light signaling state by transforming the dark state conformation to the light state one. By solving the linearized Poisson-Boltzmann equation, I calculated E(m) for Tyr-21, flavin, and redox-active Trp-104 and revealed the electron transfer (ET) driving energy. Rotation of the Gln-63 side chain that converts protein conformation from the dark state to the light state is responsible for the decrease of 150 mV in E(m) for Tyr-21, leading to the significantly larger ET driving energy in the light state conformation. The pK(a) values of protonation for flavin anions are essentially the same in both dark and light state crystal structures. In contrast to the ET via Tyr-21, formation of the W state results in generation of only the dark state conformation (even if the initial conformation is in the light state); this could explain why Trp-104-mediated ET deactivates the light-sensing yield and why the activity of W104A mutant is similar to that of the light-adapted native BLUF.

Function of redox-active tyrosine in photosystem II

Water oxidation at photosystem II Mn-cluster is mediated by the redox-active tyrosine Y(Z). We calculated the redox potential (E(m)) of Y(Z) and its symmetrical counterpart Y(D), by solving the linearized Poisson-Boltzmann equation. The calculated E(m)(Y( )/Y(-)) were +926 mV/+694 mV for Y(Z)/Y(D) with the Mn-cluster in S2 state. Together with the asymmetric position of the Mn-cluster relative to Y(Z/D), differences in H-bond network between Y(Z) (Y(Z)/D1-His(190)/D1-Asn(298)) and Y(D) (Y(D)/D2-His(189)/D2-Arg(294)/CP47-Glu(364)) are crucial for E(m)(Y(Z/D)). When D1-His(190) is protonated, corresponding to a thermally activated state, the calculated E(m)(Y(Z)) was +1216 mV, which is as high as the E(m) for P(D1/D2). We observed deprotonation at CP43-Arg(357) upon S-state transition, which may suggest its involvement in the proton exit pathway. E(m)(Y(D)) was affected by formation of P(D2)(+) (but not P(D1)(+)) and sensitive to the protonation state of D2-Arg(180). This points to an electrostatic link between Y(D) and P(D2).

Induced conformational changes upon Cd2+ binding at photosynthetic reaction centers

Cd(2+) binding at the bacterial photosynthetic reaction center (bRC) from Rhodobacter sphaeroides is known to inhibit proton transfer (PT) from bulk solvent to the secondary quinone Q(B). To elucidate this mechanism, we calculated the pK(a) for residues along the water channels connecting Q(B) with the stromal side based on the crystal structures of WT-bRC and Cd(2+)-bound bRC. Upon Cd(2+) binding, we observed the release of two protons from His-H126/128 at the Cd(2+) binding site and significant pK(a) shifts for residues along the PT pathways. Remarkably, Asp-L213 near Q(B), which is proposed to play a significant role in PT, resulted in a decrease in pK(a) upon Cd(2+) binding. The direct electrostatic influence of the Cd(2+)-positive charge on these pK(a) shifts was small. Instead, conformational changes of amino acid side chains induced electrostatically by Cd(2+) binding were the main mechanism for these pK(a) shifts. The long-range electrostatic influence over approximately 12 A between Cd(2+) and Asp-L213 is likely to originate from a set of Cd(2+)-induced successive reorientations of side chains (Asp-H124, His-H126, His-H128, Asp-H170, Glu-H173, Asp-M17, and Asp-L210), which propagate along the PT pathways as a "domino" effect.

Energetics of Proton Transfer Pathways in Reaction Centers from Rhodobacter sphaeroides

Electron transfer between the primary and secondary quinones (Q(A), Q(B)) in the bacterial photosynthetic reaction center (bRC) is coupled with proton uptake at Q(B). The protons are conducted from the cytoplasmic side, probably with the participation of two water channels. Mutations of titratable residues like Asp-L213 to Asn (inhibited mutant) or the double mutant Glu-L212 to Ala/Asp-L213 to Ala inhibit these electron transfer-coupled proton uptake events. The inhibition of the proton transfer (PT) process in the single mutant can be restored by a second mutation of Arg-M233 to Cys or Arg-H177 to His (revertant mutant). These revertant mutants shed light on the location of the main proton transfer pathway of wild type bRC. In contrast to the wild type and inhibited mutant bRC, the revertant mutant bRC showed notable proton uptake at Glu-H173 upon formation of the Q(B)- state. In all of these mutants, the pK(a) of Asp-M17 decreased by 1.4-2.4 units with respect to the wild type bRC, whereas a significant pK(a) upshift of up to 5.8 units was observed at Glu-H122, Asp-H170, Glu-H173, and Glu-H230 in the revertant mutants. These residues belonging to the main PT pathway are arranged along water channel P1 localized mainly in subunit H. bRC possesses subunit H, which has no counterpart in photosystem II. Thus, bRC may possess alternative PT pathways involving water channels in subunit H, which becomes active in case the main PT pathway is blocked.