Localization of Two Phylloquinones, QK and QK′, in an Improved Electron Density Map of Photosystem I at 4-Å Resolution

Abstract

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 Fe 4 S 4 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) 1 luminally oxidizes the soluble electron donor plastocyanin (alternatively cytochrome c 6 ) and stromally reduces the extrinsic electron acceptor ferredoxin or flavodoxin. The reduced ferredoxin induces the reduction of NADP ϩ , a reaction catalyzed by ferredoxin: NADP ϩ reductase. Photosystem I receives electrons from photosystem II via an intermediate plastoquinone pool, the cytochrome b 6 /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 cytochrome b 6 /f complex, the stromal consumption of protons by NADP ϩ reduction, and the luminal release of protons following water oxidation, is used by the ATP-synthase for phosphorylation of ADP to ATP (1, 2).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 (3, 4). Despite the comparatively low resolution, it was possible to suggest an assignment of 43 ␣-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 ironsulfur 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 (eC 1 /eC 1 ), 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 eC 2 or eC 2 ; see below). Spectros...

Recruitment of a Foreign Quinone into the A1 Site of Photosystem I

“…Recent transient Qband EPR of P700 ϩ A 1 Ϫ in PS I single crystals 2 specify an angle of 65 Ϯ 20°between the quinone plane and the plane made up of the crystalline c axis (membrane normal) and the quinone carbonyl bond direction. This is in good agreement with the most recent x-ray structure (10,11). Although the x-ray structure is not sufficiently resolved to identify the amino acids in the binding pocket, electron spin echo envelope modulation studies (12) and a model based on all available structural data (8) suggest that a tryptophan residue may be close to the semiquinone radical in PS I (12).…”

Evidence for two active branches for electron transfer in photosystem I

“…The structural data on PS I suggested that the most likely region for binding the phylloquinones was the stretch of highly conserved residues in PsaA and PsaB forming the stromal ␣-helices n and nЈ (2). Additionally, magnetic resonance data indicated that an aromatic nitrogen-containing amino acid was close to the phylloquinone A 1 Ϫ radical (23).…”

“…The mutations in PsaA and PsaB were designed by using the crystal structures of PS I and the related purple bacterial reaction center as a guide (2,6,22). The structural data on PS I suggested that the most likely region for binding the phylloquinones was the stretch of highly conserved residues in PsaA and PsaB forming the stromal ␣-helices n and nЈ (2).…”

Mutations in Both Sides of the Photosystem I Reaction Center Identify the Phylloquinone Observed by Electron Paramagnetic Resonance Spectroscopy

“…ENDOR spectroscopy of the photo-accumulated A 1 Ϫ radical detected an altered electron spin density distribution compared with phyllosemiquinone in vitro (17,18), and it has been suggested that the quinone oxygen(s) are hydrogen-bonded (17), although strong H-bonding would be difficult to reconcile with A 1 's low redox potential and the ability of different quinone compounds to insert into the A 1 site in alternate orientations (21). EPR analysis of photo-accumulated A 1 Ϫ in oriented PS1 (15) and of the P 700 ϩ A 1 Ϫ radical pair in PS1 crystals allowed prediction of the phylloquinone site in the three-dimensional structure (22), and electron densities in this region and the symmetry-related region have been assigned to the two phylloquinones (23). These sites are in an area where the 10th transmembrane ␣-helices of PsaA and PsaB are linked to an ␣-helix parallel to the membrane plane (Fig.…”