Localization of Two Phylloquinones, QK and QK′, in an Improved Electron Density Map of Photosystem I at 4-Å Resolution
1999; Elsevier BV; Volume: 274; Issue: 11 Linguagem: Inglês
10.1074/jbc.274.11.7361
ISSN1083-351X
AutoresOlaf Klukas, Wolf‐Dieter Schubert, Patrick Jordan, Norbert Krauß, Petra Fromme, Horst Tobias Witt, Wolfram Saenger,
Tópico(s)Spectroscopy and Quantum Chemical Studies
ResumoAn improved electron density map of photosystem I from Synechococcus elongatus calculated at 4-Å resolution for the first time reveals a second phylloquinone molecule and thereby completes the set of cofactors constituting the electron transfer system of this iron-sulfur type photosynthetic reaction center: six chlorophyll a, two phylloquinones, and three Fe4S4 clusters. The location of the newly identified phylloquinone pair, the individual plane orientations of these molecules, and the resulting distances to other cofactors of the electron transfer system are discussed and compared with those determined by magnetic resonance techniques. An improved electron density map of photosystem I from Synechococcus elongatus calculated at 4-Å resolution for the first time reveals a second phylloquinone molecule and thereby completes the set of cofactors constituting the electron transfer system of this iron-sulfur type photosynthetic reaction center: six chlorophyll a, two phylloquinones, and three Fe4S4 clusters. The location of the newly identified phylloquinone pair, the individual plane orientations of these molecules, and the resulting distances to other cofactors of the electron transfer system are discussed and compared with those determined by magnetic resonance techniques. The electron transfer processes of oxygenic photosynthesis, as observed in cyanobacteria, eukaryotic algae, and higher plants, involve two distinct types of photosynthetic reaction centers located in the thylakoid membrane. Photosystem II catalyzes the light-driven luminal oxidation of water and the reduction of plastoquinone near the stromal side of the photosynthetic membrane. Photosystem I (PSI) 1The abbreviations used are: PSI, photosystem I; PsaA and PsaB, large, central subunits of PSI, encoded by genespsaA and psaB ; ∠ (a , b), angle between vectors a and b ; a , b-plane, crystallographic plane parallel to the membrane plane; C2(AB), axis of pseudo-2-fold symmetry relating subunits PsaA and PsaB and also respective branches of the electron transfer system; Chl a , chlorophyll a ; c-axis, crystallographic c-axis parallel to the membrane normal; eC1 and eC1′ , luminal Chl acofactors of the electron transfer system and its pseudosymmetric counterpart; eC1, pertaining to both eC1 and eC1′ ; eC2 and eC3, second and third pair of Chla cofactors of the electron transfer system; eCX, eCY, distances between named cofactor pairs (averaged value of pseudosymmetric branches); QK and QK′ , phylloquinone cofactors of the electron transfer system; F1and F2 , preliminary x-ray structural model names for FA and FB (FB and FA); P700, A0, A1, FX, FA, and FB, spectroscopically identified cofactors of the electron transfer system of PSI as follows: primary electron donor (dimer of Chl a molecules), primary (single Chl a), secondary (phylloquinone), intermediate, and two terminal (Fe4S4 clusters) electron acceptors; m , n, and o and m′, n′, and o′, α-helix nomenclature 1The abbreviations used are: PSI, photosystem I; PsaA and PsaB, large, central subunits of PSI, encoded by genespsaA and psaB ; ∠ (a , b), angle between vectors a and b ; a , b-plane, crystallographic plane parallel to the membrane plane; C2(AB), axis of pseudo-2-fold symmetry relating subunits PsaA and PsaB and also respective branches of the electron transfer system; Chl a , chlorophyll a ; c-axis, crystallographic c-axis parallel to the membrane normal; eC1 and eC1′ , luminal Chl acofactors of the electron transfer system and its pseudosymmetric counterpart; eC1, pertaining to both eC1 and eC1′ ; eC2 and eC3, second and third pair of Chla cofactors of the electron transfer system; eCX, eCY, distances between named cofactor pairs (averaged value of pseudosymmetric branches); QK and QK′ , phylloquinone cofactors of the electron transfer system; F1and F2 , preliminary x-ray structural model names for FA and FB (FB and FA); P700, A0, A1, FX, FA, and FB, spectroscopically identified cofactors of the electron transfer system of PSI as follows: primary electron donor (dimer of Chl a molecules), primary (single Chl a), secondary (phylloquinone), intermediate, and two terminal (Fe4S4 clusters) electron acceptors; m , n, and o and m′, n′, and o′, α-helix nomenclature luminally oxidizes the soluble electron donor plastocyanin (alternatively cytochromec6) and stromally reduces the extrinsic electron acceptor ferredoxin or flavodoxin. The reduced ferredoxin induces the reduction of NADP+, a reaction catalyzed by ferredoxin:NADP+ reductase. Photosystem I receives electrons from photosystem II via an intermediate plastoquinone pool, the cytochrome b6/f complex, and water soluble electron carriers. The difference in proton concentration across the thylakoid membrane, which results from the proton pumping of the plastoquinone pool and the cytochromeb6/f complex, the stromal consumption of protons by NADP+ reduction, and the luminal release of protons following water oxidation, is used by the ATP-synthase for phosphorylation of ADP to ATP (1Nugent J.H.A. Eur. J. Biochem. 1996; 237: 519-531Crossref PubMed Scopus (113) Google Scholar, 2Witt H.T. Ber. Bunsen-Ges. Phys. Chem. 1996; 100: 1923-1942Crossref Scopus (59) Google Scholar). Cyanobacterial PSI consists of 11 subunits referred to as PsaA to PsaF and PsaI to PsaM. An x-ray structural model of a cyanobacterial PSI complex from the thermophile Synechococcus elongatus has been postulated on the basis of an electron density map calculated at 4-Å resolution (3Krauß N. Schubert W.-D. Klukas O. Fromme P. Witt H.T. Saenger W. Nat. Struct. Biol. 1996; 3: 965-973Crossref PubMed Scopus (312) Google Scholar, 4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar). Despite the comparatively low resolution, it was possible to suggest an assignment of 43 α-helices to the individual subunits of PSI by correlating the information provided by the electron density map with available biochemical and biophysical data. Furthermore, the electron density map allowed the positions of 89 Chl a molecules, constituents of both the core antenna system and electron transfer system, one phylloquinone, and three iron-sulfur clusters to be modeled. The electron transfer reactions of PSI are initiated through excitation of the primary electron donor P700 positioned near the luminal side of the membrane-integral complex. Structurally, P700 consists of a chlorophyll a dimer (eC1/eC1′), whose mutually parallel dihydroporphyrin ring planes are aligned with the membrane normal. Upon excitation, P700* passes an electron to the primary electron acceptor A (probably eC2 or eC2′; see below). Spectroscopically, the first electron acceptor has been identified as A0, in all probability one (though possibly either) of the pair of Chl a monomers denotedeC3 and eC3′ in the structural model of PSI (3Krauß N. Schubert W.-D. Klukas O. Fromme P. Witt H.T. Saenger W. Nat. Struct. Biol. 1996; 3: 965-973Crossref PubMed Scopus (312) Google Scholar, 4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar). This process occurs with a rate constant of about 5·1011 s−1 (5Brettel K. Biophys. Biochim. Acta. 1997; 1318: 322-373Crossref Scopus (429) Google Scholar). The charge separation P700⨥A·̄0 is spatially extended across the membrane by electron transfer from the radical A·̄0 to the next electron acceptor spectroscopically referred to as A1; the rate constant is estimated to be 2–5·1010s−1 (for a review, see Ref. 5Brettel K. Biophys. Biochim. Acta. 1997; 1318: 322-373Crossref Scopus (429) Google Scholar). A1 is now generally agreed to be a phylloquinone (6Sétif P. Bottin H. Biochemistry. 1989; 28: 2689-2697Crossref Scopus (76) Google Scholar, 7Snyder S.W. Rustandi R. Biggins J. Norris J.R. Thurnauer M.C. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 9895-9896Crossref PubMed Scopus (45) Google Scholar). Due to the difficulty of locating the small phylloquinone molecules in low resolution electron density maps and because of the stability of the radical state P700⨥A·̄1, the position and orientation of A1 relative to the PSI holocomplex has recently received increased attention, especially by improved EPR techniques. These have, inter alia, determined the distance between A·̄1 and P700⨥ to be ∼25.4 Å (8Dzuba S.A. Hara H. Kawamori A. Iwaki M. Itoh S. Tsvetkow Y.D. Phys. Chem. Lett. 1997; 264: 238-244Crossref Scopus (49) Google Scholar, 9Zech S.G. Lubitz W. Bittl R. Ber. Bunsen-Ges. Phys. Chem. 1996; 100: 2041-2044Crossref Google Scholar, 10Bittl R. Zech S.G. Fromme P. Witt H.T. Lubitz W. Biochemistry. 1997; 36: 12001-12004Crossref PubMed Scopus (74) Google Scholar). A relative position for A1 was derived through orientation-dependent pulsed EPR measurements on PSI single crystals (10Bittl R. Zech S.G. Fromme P. Witt H.T. Lubitz W. Biochemistry. 1997; 36: 12001-12004Crossref PubMed Scopus (74) Google Scholar). Geometrically, this position was found to correspond to QK, a single phylloquinone assigned to a well defined pocket in the earlier electron density map (4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar). The assignment of this position, however, remained internally uncorroborated, since an expected pseudosymmetrically positioned second phylloquinone could not be identified at the time. The three terminal cofactors of the electron transfer system are iron-sulfur centers, FX being closest to P700, followed byF1 and F2 (we retain this nomenclature, for the present, to emphasize the remaining structural ambiguity in their assignment to the known cofactors FA and FB, although see Refs. 11Vassiliev I.R. Jung Y.-S. Yang F. Golbeck J.H. Biophys. J. 1998; 74: 2029-2035Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar, 12Dı́az-Quintana A. Leibl W. Bottin H. Sétif P. Biochemistry. 1998; 37: 3429-3439Crossref PubMed Scopus (75) Google Scholar, 13Fischer N. Hippler M. Sétif P. Jacquot J.-P. Rochaix J.-D. EMBO J. 1998; 17: 849-858Crossref PubMed Scopus (72) Google Scholar as well as Ref. 14Klukas O. Schubert W.-D. Jordan P. Krauß N. Fromme P. Witt H.T. Saenger W. J. Biol. Chem. 1999; 274: 7351-7360Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar for recent results correlating FA with F1 and FB with F2). In the following, we describe an improved model of the electron transfer system of PSI based on the present electron density map at 4-Å resolution (14Klukas O. Schubert W.-D. Jordan P. Krauß N. Fromme P. Witt H.T. Saenger W. J. Biol. Chem. 1999; 274: 7351-7360Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar). This map reveals the position of the second phylloquinone molecule and allows the spatial positioning of all 11 cofactors of the electron transfer system of PSI. The positions and orientations of individual cofactors are discussed and compared with structural information derived from spectroscopic data. The phases for the electron density map presented here were derived using essentially the same data described previously (4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar), although a new native data set with a resolution of 3.5 Å and an additional mercury derivative data set have been included (14Klukas O. Schubert W.-D. Jordan P. Krauß N. Fromme P. Witt H.T. Saenger W. J. Biol. Chem. 1999; 274: 7351-7360Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar). Using the program SHARP (15de La Fortelle E. Bricogne G. Methods Enzymol. 1997; 276: 472-494Crossref PubMed Scopus (1796) Google Scholar) instead of the earlier combination VECREF/MLPHARE (16Collaborative Computing Project 4 Acta Crystallogr. Sec. D. 1994; 50: 760-763Crossref PubMed Scopus (19667) Google Scholar, 17.Otwinowski, Z., Proceedings of the CCP4 Study Weekend: Isomorphous Replacement and Anomalous Scattering, Wolf, W., Evans, P. R., Leslie, A. G. W., 1991, 80, 86, SERC Daresbury Laboratory, Warrington, United Kingdom.Google Scholar) and including a total of five heavy atom derivative data sets, it was possible, by incorporating new minor sites, to derive a significantly improved heavy atom model. The program SOLOMON (16Collaborative Computing Project 4 Acta Crystallogr. Sec. D. 1994; 50: 760-763Crossref PubMed Scopus (19667) Google Scholar) has been employed in the solvent flattening procedure. Due to the low diffraction quality of heavy atom derivative crystals, experimentally obtained phase information is still limited to a resolution of 4 Å. Since no additional phase information at higher resolution could be achieved by phase extension using density modification techniques, the electron density map was calculated at a resolution of 4 Å. It reveals more detailed information on the polypeptide chain folding than previous maps as well as the complete cofactor set of PSI. For the detailed procedure and statistics for the determination of this electron density map, see Ref. 14Klukas O. Schubert W.-D. Jordan P. Krauß N. Fromme P. Witt H.T. Saenger W. J. Biol. Chem. 1999; 274: 7351-7360Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar. The previously reported model of the electron transfer system (3Krauß N. Schubert W.-D. Klukas O. Fromme P. Witt H.T. Saenger W. Nat. Struct. Biol. 1996; 3: 965-973Crossref PubMed Scopus (312) Google Scholar, 4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar) has been used as a basis for the present cofactor model. The Chl a head groups are visible as almost quadratically flat density pockets. The positions and orientations of the Chl a molecules are modeled by 4-fold symmetrical porphyrin moieties, since the present resolution does not permit their asymmetric features to be defined unambiguously. Similarly, the phylloquinone molecules are represented by their naphthalene moieties to interpret the corresponding elongated ellipsoidal electron density. Neither the phylloquinone side chains nor the oxygen atoms have been included in the model. Chl a cofactors of the electron transfer system were placed into the electron density using the program O (18Jones T.A. Zou J.-Y. Cowan S.W. Kjeldgaard M. Acta Crystallogr. Sec. A. 1991; 47: 110-119Crossref PubMed Scopus (12999) Google Scholar), and their positions were optimized using the real space refinement procedure as provided by this program. For Chla molecules center-to-center distances were calculated between the central Mg2+ ions, while for iron-sulfur clusters and phylloquinones the centroid of the cluster and naphthalene model, respectively, have been used. Edge-to-edge distances of cofactors important for the kinetics of electron transfer were determined between the outer atoms of the porphyrin, naphthalene, and iron-sulfur cluster models, respectively. For iron-sulfur clusters, edge-to-edge distances have been determined between the iron and sulfur atoms of the clusters, as modeled. The estimated errors for center-to-center and edge-to-edge distances are on the order of ±1 and ± 2 Å, respectively, the latter reflecting the larger uncertainties in the orientations of the planar cofactors within their molecular planes. In our previous x-ray structural model of PSI (4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar), only a tentative positional description of a single phylloquinone was included, assigned to an electron density pocket located between FX and eC3. The lack of a second, pseudosymmetrically positioned phylloquinone, however, prevented an internal corroboration of this identification. The new electron density map now reveals two such electron density structures symmetrically positioned on either side of the pseudo-2-fold rotation axis C2(AB) and located between eC3 and FX. These have been assigned to the phylloquinone electron acceptorsQK and QK′ (Fig. 1). The latter is equivalent to the position QK identified previously (4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar). Note that following our earlier convention of priming cofactors coordinated by primed α-helices, the position previously denotedQK will be renamed QK′(coordinated by α-helices m′-n′), while the new second phylloquinone will be referred to as QK(coordinated by m-n). QK (QK′) is situated slightly luminally of and close to the N terminus of α-helix n (n′) and immediately adjacent to the loop connecting α-helices m and n (m′ and n′). The corresponding electron density is clearly separated from that of the neighboring α-helices (Fig. 1). Facing away from the loop m-n (m′-n′), each phylloquinone is additionally delimited by the long loop n-o (n′-o′) connecting the C-terminal end of n (n′) to the stromal end of o (o′). In addition to QK and QK′, a significantly more symmetrical arrangement of α-helices and connecting loops on either side of the pseudo-2-fold axis C2(AB) is now apparent in it vicinity as compared with the previously published electron density map. Whereas the earlier model of the α-helix m almost passed through the position now assigned to QK, the stromal end of m now has a comparable inclination relative to the membrane normal as its pseudosymmetric partner m′. The loops m-n (m′-n′) connecting α-helix m (m′) to the “surface” α-helix n (n′) are similar in both shape and length (Fig. 2). The electron densities of both QK and QK′ are elongatedly ellipsoidal (Fig. 1). As a result, the long molecular axis of the naphthoquinone moiety may be identified with some confidence. However, the plane orientation as well as the quinone oxygen atoms remain indeterminate. As a result, the phylloquinone molecules have been modeled by their naphthalene backbones only. These naphthalene models were placed into the electron density optimizing their positions and the orientations of their long molecular axes. The molecular plane of QK was then rotated around the long axis to align the molecular plane with the vector eC1-QK, to account for the observation that the carbonyl O–O-axis is approximately aligned with the vector P700⨥-A·̄1 (19van der Est A. Prisner T. Bittl R. Fromme P. Lubitz W. Möbius K. Stehlik D. J. Phys. Chem. B. 1997; 101: 1437-1443Crossref Google Scholar). Since the electron spin density is primarily located on eithereC1 or eC1′ (20Käß H. Lubitz W. Phys. Lett. 1996; 251: 193-203Google Scholar) (although which one remains to be clarified), the procedure was repeated to align the molecular plane of QK with the vector eC1′-QK. Similarly, two plane orientations were obtained for QK′. The long molecular axes of both QK and QK′ are observed to be inclined by 13 ±5° relative to the membrane plane (equivalent to the crystallographica, b-plane). Projected onto the a, b-plane, the long molecular axis ofQK (QK′) describes an angle of 18° (60°) to the crystallographic a-axis. The axes of QK and QK′ form an angle of 42° with each other. The positions of the iron-sulfur clusters correspond to the highest electron density observed (21Krauß N. Hinrichs W. Witt I. Fromme P. Pritzkow W. Dauter Z. Betzel C. Wilson K.S. Witt H.T. Saenger W. Nature. 1993; 361: 326-331Crossref Scopus (305) Google Scholar). FX was tentatively modeled by fitting a Fe4S4 cluster into the electron density. Contouring the electron density map at 11 S.D. above the mean density reveals a tetrahedrally distorted electron density structure associated with FX (Fig. 3). The most likely explanation is, that this tetrahedron is equivalent to the arrangement of the four iron atoms of the Fe4S4cluster. Modeling a Fe4S4 cluster into this tetrahedral shape results in a good structural match, while the four sulfur atoms lie outside the contour, in agreement with their lower density of electrons. Interestingly, the derived orientation of FX upholds the 2-fold symmetry of C2(AB), a fact that had been assumed on grounds of symmetry yet had remained unsubstantiated. The observation that the gXXprincipal axis of the g tensor of reduced FX is oriented perpendicular to the thylakoid membrane (22Guigliarelli B. Guillaussier J. More C. Sétif P. Bottin H. Bertrand P. J. Biol. Chem. 1993; 268: 900-908Abstract Full Text PDF PubMed Google Scholar) now favors one of two alternative assignments of g tensor axes to the distorted cubane structure of Fe4S4 clusters. According to EPR studies on Fe4S4 model compounds (23Rius G. Lamotte B. J. Am. Chem. Soc. 1989; 111: 2464-2469Crossref Scopus (59) Google Scholar, 24Gloux J. Gloux P. Lamotte B. Mouesca J.-M. Rius G. J. Am. Chem. Soc. 1994; 116: 1953-1961Crossref Scopus (54) Google Scholar), our structural model and the EPR results are in agreement with the assignment, where each of the three principal magnetic axes is normal to one of the mutually orthogonal faces of the distorted Fe4S4 cube (25Kamlowski A. van der Est A. Fromme P. Stehlik D. Biochim. Biophys. Acta. 1997; 1319: 185-198Crossref Scopus (24) Google Scholar). The orientations of F1 and F2 have been inferred from the 2Fe4S4 ferredoxin structure fromPeptostreptococcus asaccharolyticus (26Adman E.T. Sieker L.C. Jensen L.H. J. Biol. Chem. 1976; 251: 3801-3806Abstract Full Text PDF PubMed Google Scholar) used as a model for PsaC (27.Schubert, W.-D., Klukas, O., Krauß, N., Saenger, W., Fromme, P., Witt, H. T., Photosynthesis: From Light to Biosphere: Proceedings of the 10th International Conference on Photosynthesis, Montpellier, August 20–25, 1995, Mathis, P., II, 1995, 3, 11, Kluwer Academic, Dordrecht, The Netherlands.Google Scholar). They are in agreement with those derived independently by EPR experiments on PSI single crystals (28Kamlowski A. van der Est A. Fromme P. Krauß N. Schubert W.-D. Klukas O. Stehlik D. Biochim. Biophys. Acta. 1997; 1319: 199-213Crossref PubMed Scopus (23) Google Scholar). ForF1 and F2, such tetragonally shaped density structures as observed in the case of FX are not evident as the electron density “outside” the membrane-integral region is less well defined. The electron transfer system of PSI constitutes the innermost cylindrical core of the larger, membrane-integral photosynthetic reaction center complex. A set of 10 α-helices, five from each of the two central subunits, PsaA and PsaB, tightly encloses the electron transfer system, separating it from the surrounding antenna system (4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar). The electron transfer system itself consists of two symmetrically arranged cofactor branches (Fig. 2). Now that all cofactors of the electron transfer system have been identified, this pseudosymmetry is seen to encompass the whole of the membrane-integral region, extending from the paireC1/eC1′ near the luminal side toQK/QK′ near the stromal side (Fig. 2). The iron-sulfur cluster FX is located on the pseudo-2-fold axis C2(AB), completing the symmetrical arrangement at the stromal edge of the membrane-integral subunits. Merely the two stromal iron-sulfur clusters F1and F2 (FA and FB), coordinated by the extrinsic subunit PsaC, do not adhere to this 2-fold symmetry. In the direction parallel to the membrane normal, the membrane-integral cofactors divide the membrane into four sections of roughly comparable width, here denoted eC1 - eC2, eC2 - eC3, eC3 - QK, and QK-FX. The height difference for eC1 - eC2, eC2 - eC3, eC3 - QK, and QK-FX amount to 5.9, 8.6, 7.8, and 8.8 Å, respectively, while the total distance eC1-FX is 31.1 Å. This corresponds to fractional distances of 0.19, 0.28, 0.25, and 0.28, respectively (Fig. 4a). Photovoltage measurements on oriented PSI thylakoid membranes estimated values of fractional dielectrically weighted transmembrane distances of 0.62 for P700-A0 (compare with eC1 - eC3, 0.47), 0.16 for A0-A1 (compare with eC3 - QK, 0.25), and 0.22 for A1-FX (compare with QK-FX, 0.28) (29Leibl W. Toupance B. Breton J. Biochemistry. 1995; 34: 10237-10244Crossref PubMed Scopus (44) Google Scholar, 30Hecks B. Wulf K. Breton J. Leibl W. Trissl H.-W. Biochemistry. 1994; 33: 8619-8624Crossref PubMed Scopus (57) Google Scholar). These relative distances, especially for the pair P700-A0, do not correspond to the x-ray structural model distances eC1-eC3 as well as one might have expected. Because the distances A0-A1( eC3 - QK) and A1-FX( QK-FX) are comparable (29Leibl W. Toupance B. Breton J. Biochemistry. 1995; 34: 10237-10244Crossref PubMed Scopus (44) Google Scholar), matching our observations, the distance P700-A0( eC1 - eC3) has clearly been overestimated by the photovoltage measurements relative to the other distances. Possibly, the fast rate of charge separation results in a significant error for the distance eC1 - eC3(alternatively, the dielectric constant around P700 may differ substantially from that nearer the middle of the membrane), giving rise to the observed distortion. Comparisons of structural and spectroscopic data have recently been published based on models of PSI derived at 4.5- and 4-Å resolution (4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar, 5Brettel K. Biophys. Biochim. Acta. 1997; 1318: 322-373Crossref Scopus (429) Google Scholar). These studies, however, included none or, in the latter case, a single phylloquinone position designatedQK (now renamed QK′). Here we will include the latest structural results and compare these to the available spectroscopic data. The Moser-Dutton “ruler” (31Moser C.C. Keske J.M. Warncke K. Farid R.S. Dutton L. Nature. 1992; 355: 796-802Crossref PubMed Scopus (1604) Google Scholar) (an empirical first order relationship between electron transfer rates and shortest edge-to-edge distances of the cofactors involved) provides a simple tool to estimate the “optimal” electron transfer rates from structural data, the optimal electron transfer rate being achieved when the sum of the standard reaction free energy and reorganization energy is essentially zero (32Marcus R.A. Suttin N. Biochim. Biophys. Acta. 1985; 811: 265-322Crossref Scopus (7614) Google Scholar). In Table I, the edge-to-edge distances and the optimal (i.e. fastest theoretically possible) electron transfer rates derived, using the above relationship, are listed.Table IThe averaged edge-to-edge distances between the cofactor planes and the “optimal” electron transfer rates derived using the Moser-Dutton relationship, log kET = 15 − 0.6R − 3.1(ΔG° + λ)2/λ (31Moser C.C. Keske J.M. Warncke K. Farid R.S. Dutton L. Nature. 1992; 355: 796-802Crossref PubMed Scopus (1604) Google Scholar).Structural dataSelected spectroscopic dataDistance from → toEdge-to-edge distance (±2 Å)Calculated optimal electron transfer rates, kET (−ΔG = λ)Electron transfer betweenEdge-to-edge distanceElectron transfer rates, kETÅs−1Ås−1eC 1 → eC 244.0 ·1012eC 1 → eC 313.31.0 ·107P700 → A05 ·1011 (5Brettel K. Biophys. Biochim. Acta. 1997; 1318: 322-373Crossref Scopus (429) Google Scholar)aSee Ref. 5 for original publications.eC 1→ Q K20.55.0 ·102P700 ← A14 ·103 (5Brettel K. Biophys. Biochim. Acta. 1997; 1318: 322-373Crossref Scopus (429) Google Scholar)aSee Ref. 5 for original publications., bCharge recombination rate.eC 1 → FX26.51.3 ·10−1eC 2 → eC 351.0 ·1012eC 3 → Q K4.81.3 ·1012A0 → A1≤7.8 (33Iwaki M. Kumazaki S. Yoshihar K. Erabi T. Itoh S. Phys. Chem. 1996; 100: 10802-10809Crossref Scopus (50) Google Scholar)(2Witt H.T. Ber. Bunsen-Ges. Phys. Chem. 1996; 100: 1923-1942Crossref Scopus (59) Google Scholar, 3Krauß N. Schubert W.-D. Klukas O. Fromme P. Witt H.T. Saenger W. Nat. Struct. Biol. 1996; 3: 965-973Crossref PubMed Scopus (312) Google Scholar, 4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar, 5Brettel K. Biophys. Biochim. Acta. 1997; 1318: 322-373Crossref Scopus (429) Google Scholar) ·1010 (5Brettel K. Biophys. Biochim. Acta. 1997; 1318: 322-373Crossref Scopus (429) Google Scholar)aSee Ref. 5 for original publications.Q K →FX11.31.7 ·108cThe reaction free energy is presumably not equivalent to the reorganization energy as simplifyingly assumed for the calculation of electron transfer rates in this table.A1 → FX10.7 (34Sétif P. Brettel K. Biochemistry. 1993; 32: 7846-7854Crossref PubMed Scopus (88) Google Scholar)1.5 ·107 (34Sétif P. Brettel K. Biochemistry. 1993; 32: 7846-7854Crossref PubMed Scopus (88) Google Scholar)FX →F1126.3 ·107FX → F2201.0 ·103F1 →F294.0 ·109a See Ref. 5Brettel K. Biophys. Biochim. Acta. 1997; 1318: 322-373Crossref Scopus (429) Google Scholar for original publications.b Charge recombination rate.c The reaction free energy is presumably not equivalent to the reorganization energy as simplifyingly assumed for the calculation of electron transfer rates in this table. Open table in a new tab The Chl a molecules, eC1, eC1′, eC2, eC2′, eC3, and eC3′ constitute the luminal half of the electron transfer system.eC1 and eC1′ have been identified as structural components of the spectroscopically identified primary electron donor P700; eC2 and eC2′ are referred to as the accessory chlorophylls; while either or both of eC3 and eC3′ have been assigned to the spectroscopically identified primary electron acceptor A0. As noted (4Schubert W.-D. Klukas O. Krauß N. Saenger W. Fromme P. Witt H.T. J. Mol. Biol. 1997; 272: 741-769Crossref PubMed Scopus (224) Google Scholar, 5Brettel K. Biophys. Biochim. Acta. 1997; 1318: 322-373Crossref Scopus (429) Google Scholar), the edge-to-edge dist
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