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Photosynthetic reaction centers from heliobacteria (HbRC) and green sulfur bacteria (GsbRC) are homodimeric proteins and share a common ancestor with photosystem I (PSI), classified as type I reaction centers. Using the HbRC crystal structure, we calculated the redox potential ( E m ) values in the electron-transfer branches, solving the linear Poisson-Boltzmann equation and considering the protonation states of all titratable sites in the entire protein-pigment complex. E m (A -1 ) for bacteriochlorophyll g at the secondary site in HbRC (-1157 mV) is as low as E m (A -1 ) for chlorophyll a in PSI (-1173 mV). E m (A 0 /HbRC) is at the same level as E m (A 0 /GsbRC) and is 200 mV higher than E m (A 0 /PSI) due to the replacement of PsaA-Trp697/PsaB-Trp677 in PSI with PshA-Arg554 in HbRC. In contrast, E m (F X ) for the Fe 4 S 4 cluster in HbRC (-420 mV) is significantly higher than E m (F X ) in GsbRC (-719 mV) and PSI (-705 mV) due to the absence of acidic residues that correspond to PscA-Asp634 in GsbRC and PsaB-Asp575 in PSI. It seems likely that type I reaction centers have evolved, adopting (bacterio)chlorophylls suitable for their light environments while maintaining electron-transfer cascades.

Photosynthetic reaction centers from a green sulfur bacterium (GsbRC), the PscA/PscA proteins, and photosystem I (PSI), PsaA/PsaB proteins, share structural similarities. Here, we report the redox potential ( E m ) values of GsbRC by solving the linear Poisson-Boltzmann equation and considering the protonation states of all titratable sites in the entire GsbRC protein and identify the factors that shift the E m values with respect to PSI. The E m values for one-electron reduction of the accessory (A -1 ) and adjacent (A 0 ) chlorophylls in GsbRC are 100-250 mV higher than those in PSI, whereas the E m values for the Fe 4 S 4 cluster (F X ) are at the same level. The PsaA-Trp697/PsaB-Trp677 pair in PSI, which forms the A 1 -quinone binding site, is replaced with PscA-Arg638 in GsbRC. PsaB-Asp575 in PSI, which is responsible for the E m difference between A 1A and A 1B quinones in PSI, is absent in GsbRC. These discrepancies also contribute to the upshift in E m (A -1 ) and E m (A 0 ) in GsbRC with respect to PSI. It seems likely that the upshifted E m for chlorophylls in GsbRC ultimately originates from the characteristics of the electrostatic environment that corresponds to the A 1 site of PSI.

Photosystem I has two active electron-transfer pathways. However, electron transfer occurs primarily along one of the two branches (A-branch) irrespective of the similar protein environments. Here, we report the origin of the A-branch electron transfer, considering the electronic coupling of the pigments and the electrostatic interaction with the protein environments. In the chlorophyll pair [P A P B ], the electronic coupling between P A and P B is large (85 meV) for the highest occupied molecular orbital, forming the electronically coupled dimer [P A P B ] and serving as an initial electron donor. In contrast, the coupling for the lowest unoccupied molecular orbital is small (15 meV), leading to charge transfer from P B to P A upon the [P A P B ] excitation. The electronic coupling between [P A P B ] and the accessory chlorophyll in the A-branch is significantly larger than that in the B-branch. These results indicate that the asymmetry of the electron-transfer activity originates from P A as a chlorophyll epimer.

Using the cyanobacterial and plant photosystem I (PSI) crystal structures and by considering the protonation states of all titratable residues, redox potentials (E m ) of the two phylloquinones-A 1A and A 1B -were calculated. The calculated E m values were E m (A 1A ) = -773 mV and E m (A 1B ) = -818 mV for the plant PSI structure and E m (A 1A ) = -612 mV and E m (A 1B ) = -719 mV for the cyanobacterial PSI structure. Our analysis of the PSI crystal structures suggested that the side-chain orientations of Lys-B542 and Gln-B678 in the cyanobacterial crystal structure differ from these side-chain orientations in the plant crystal structure. Quantum mechanical/molecular mechanical calculations indicated that the geometry of the cyanobacterial PSI crystal structure was best described as the conformation where Asp-B575 is protonated and A 1A is reduced to A 1A •- form ( Rutherford, A. W., Osyczka, A., Rappaport, F. ( 2012 ) FEBS Lett. 586 , 603 - 616 ). Reorienting the Lys-B542 and Gln-B678 side-chains and rearranging the H-bond pattern of the water cluster near Asp-B575 lowered the E 1A form ( Rutherford, A. W., Osyczka, A., Rappaport, F. ( 2012 ) FEBS Lett. 586 , 603 - 616 ). Reorienting the Lys-B542 and Gln-B678 side-chains and rearranging the H-bond pattern of the water cluster near Asp-B575 lowered the E m to E m (A 1A ) = -718 mV and E m (A 1B ) = -795 mV. It seems possible that PSI has two conformations: the high-potential A 1A form and the low-potential A 1A form.

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., (Copyright © 2011 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

Electrostatic calculations have predicted that the partial negative charge associated with D575PsaB plays a significant role in modulating the midpoint potentials of the A1A and A1B phylloquinones in photosystem I. To test this prediction, the side chain of residue 575PsaB was changed from negatively charged (D) to neutral (A) and to positively charged (K). D566PsaB, which is located at a considerable distance from either A1A or A1B, and should affect primarily the midpoint potential of FX, was similarly changed. In the 575PsaB variants, the rate of electron transfer from A1A to FX is observed to decrease slightly according to the sequence D/A/K. In the 566PsaB variants, the opposite effect of a slight increase in the rate is observed according to the same sequence D/A/K. These results are consistent with the expectation that changing these residues will shift the midpoint potentials of nearby cofactors to more positive values and that the magnitude of this shift will depend on the distance between the cofactors and the residues being changed. Thus, the midpoint potentials of A1A and A1B should experience a larger shift than will FX in the 575PsaB variants, while FX should experience a larger shift than will either A1A or A1B in the 566PsaB variants. As a result, the driving energy for electron transfer from A1A and A1B to FX will be decreased in the former and increased in the latter. This rationalization of the changes in kinetics is compared with the results of electrostatic calculations. While the altered amino acids shift the midpoint potentials of A1A, A1B, and FX by different amounts, the difference in the shifts between A1A and FX or between A1B and FX is small so that the overall effect on the electron transfer rate between A1A and FX or between A1B and FX is predicted to be small. These conclusions are borne out by experiment.

The absence of the PsaC subunit in the photosystem I (PSI) complex (native PSI complex) by mutagenesis or chemical manipulation yields a PSI core (P700-F(X) core) that also lacks subunits PsaD and PsaE and the two iron-sulfur clusters F(A) and F(B), which constitute an integral part of PsaC. In this P700-F(X) core, the redox potentials (E(m)) of the two quinones A(1A/B) and the iron-sulfur cluster F(X) as well as the corresponding protonation patterns are investigated by evaluating the electrostatic energies from the solution of the linearized Poisson-Boltzmann equation. The B-side specific Asp-B558 changes its protonation state significantly upon isolating the P700-F(X) core, being mainly protonated in the native PSI complex but ionized in the P700-F(X) core. In the P700-F(X) core, E(m)(A(1A/B)) remains practically unchanged, whereas E(m)(F(X)) is upshifted by 42 mV. With these calculated E(m) values, the electron transfer rate from A(1) to F(X) in the P700-F(X) core is estimated to be slightly faster on the A(1A) side than that of the wild type, which is consistent with kinetic measurements.

The redox potentials of the two electron transfer (ET) active quinones in the central part of photosystem I (PSI) were determined by evaluating the electrostatic energies from the solution of the Poisson-Boltzmann equation based on the crystal structure. The calculated redox potentials are -531 mV for A1A and -686 mV for A1B. From these results we conclude the following. (i) Both branches are active with a much faster ET in the B-branch than in the A-branch. (ii) The measured lifetime of 200-290 ns of reduced quinones agrees with the estimate for the A-branch and corroborates with an uphill ET from this quinone to the iron-sulfur cluster as observed in recent kinetic measurements. (iii) The electron paramagnetic resonance spectroscopic data refer to the A-branch quinone where the corresponding ET is uphill in energy. The negative redox potential of A1 in PSI is primarily because of the influence from the negatively charged FX, in contrast to the positive shift on the quinone redox potential in bacterial reaction center and PSII that is attributed to the positively charged non-heme iron atom. The conserved residue Asp-B575 changes its protonation state after quinone reduction. The difference of 155 mV in the quinone redox potentials of the two branches were attributed to the conformation of the backbone with a large contribution from Ser-A692 and Ser-B672 and to the side chain of Asp-B575, whose protonation state couples differently with the formation of the quinone radicals.