ENDOR and special TRIPLE resonance spectroscopy of photoaccumulated semiquinone electron acceptors in the reaction centers of green sulfur bacteria and heliobacteria

Photoaccumulation at 205 K in the presence of dithionite produces EPR signals in anaerobically prepared membranes from Chlorobium limicola and Heliobacterium chlorum that resemble the EPR spectrum of phyllosemiquinone (A1*-) photoaccumulated in photosystem I. We have used ENDOR and special TRIPLE resonance spectroscopy to demonstrate conclusively that these signals arise from menasemiquinone electron acceptors reduced by photoaccumulation. Hyperfine couplings to two protons H-bonded to the semiquinone oxygens have been identified by exchange of H. chlorum into D2O, and hyperfine couplings to the methyl group, and the methylene group of the phytyl side chain, of the semiquinone have also been assigned. The electronic structure of these menasemiquinones in these reaction centers is very similar to that of phyllosemiquinone in PSI, and shows a distorted electron spin density distribution relative to that of phyllosemiquinone in vitro. Special TRIPLE resonance spectrometry has been used to investigate the effect of detergents and oxygen on membranes of C. limicola. Triton X-100 and oxygen affect the menaquinone binding site, but n-dodecyl beta-D-maltoside preparations exhibit a relatively unaltered special TRIPLE spectrum for the photoaccumulated menasemiquinone.

Deuteration effects on the in vivo EPR spectrum of the reduced secondary photosystem I electron acceptor A1 in cyanobacteria

The photoreduction of the secondary PSI electron acceptor A1 in vivo has recently been detected via X-band EPR spectroscopy in intact spinach chloroplasts and in marine cyanobacteria Synechococcus PCC 7002 [Klughammer, C., and Pace, R. J. (1997) Biochim. Biophys. Acta 1318, 133-144]. A further study of the A1- EPR spectrum of Synechococcus PCC 7002 at room temperature with higher-field resolution revealed partially resolved hyperfine structure which was dominated by 0.4 mT splittings of three equivalent protons. The hyperfine splitting was not significantly affected by incubation of the cyanobacteria in 2H2O medium for 20 h, but was absent in fully deuterated cyanobacteria that were grown in 2H2O medium. Anisotropic g-factors consistent with a phylloquinone radical were derived by spectra simulation. Biosynthetic protonation of quinones via the CH3 donor L-methionine in deuterated cells maintained hyperfine structure in the A1- spectrum, indicating the incorporation of CH3 groups in 60% of the deuterated, photoactive A1 molecules. Conversely, biosynthetic quinone deuteration via L-[methyl-d3]methionine in protonated cells led to the loss of the 0. 4 mT splittings in 54% of the A1 molecules. These observations confirm the conclusion of Heathcote et al. [(1996) Biochemistry 35, 6644-6650] of the identity of EPR-detected, photoreduced A1- in vivo with a phylloquinone (vitamin K1) radical in PSI. The partially resolved hyperfine structure of the A1- spectrum indicates an altered spin distribution in the bound vitamin K1- radical in vivo compared to that of unbound vitamin K1- in vitro.

ENDOR and special triple resonance spectroscopy of A1.- of photosystem 1

The photoaccumulated radical state of the photosystem 1 secondary electron acceptor A1, A1.-, has been studied in spinach and the cyanobacterium Anabaena variabilis strain Met27 using electron nuclear double resonance (ENDOR) and electron--nuclear--nuclear special triple (ST) resonance spectroscopies. Spectra of A1.- in both these species are very similar. ENDOR spectra of the phylloquinone anion radical in solvent glass were also obtained. Comparison of the spectra of the in vivo and in vitro radicals shows that A1.- is a phylloquinone anion radical with a distorted electron spin density distribution. Hyperfine couplings to the A1.- methyl group and to two protons hydrogen bonded to the quinone oxygens have been identified using biosynthetic deuteration in A. variabilis. Possible hyperfine coupling to a methylene proton of the phytyl side chain of the quinone has also been identified. These results are compared with those from previous studies of protein-bound semiquinones in the light of the unusual redox potential of A1.

Photoaccumulation in photosystem I does produce a phylloquinone (A1.-) radical

Previous work has challenged the assignment of a photoaccumulated EPR signal to the phylloquinone electron acceptor in photosystem I, A1.-. Biosynthetic deuteration of the phylloquinone in the cyanobacterium Anabaena variabilis has been shown to narrow this photoaccumulated signal, demonstrating that the signal arises from A1.-. The ESP signal attributed to P700.+A1.- is also narrowed by this deuteration, showing that the photoaccumulated EPR signal and the ESP signal are monitoring the same redox component. Confirmation that the photoaccumulated EPR signal comes from deuterated phylloquinone was obtained by exchanging the deuterated for protonated phylloquinone, which broadened the photoaccumulated EPR signal.

Protein sequences and redox titrations indicate that the electron acceptors in reaction centers from heliobacteria are similar to Photosystem I

Photosynthetic reaction centers isolated from Heliobacillus mobilis exhibit a single major protein on SDS-PAGE of 47 000 Mr. Attempts to sequence the reaction center polypeptide indicated that the N-terminus is blocked. After enzymatic and chemical cleavage, four peptide fragments were sequenced from the Heliobacillus mobilis apoprotein. Only one of these sequences showed significant specific similarity to any of the protein and deduced protein sequences in the GenBank data base. This fragment is identical with 56% of the residues, including both cysteines, found in the highly conserved region that is proposed to bind iron-sulfur center FX in the Photosystem I reaction center peptide that is the psaB gene product. The similarity to the psaA gene product in this region is 48%.Redox titrations of laser-flash-induced photobleaching with millisecond decay kinetics on isolated reaction centers from Heliobacterium gestii indicate a midpoint potential of -414 mV with n=2 titration behavior. In membranes, the behavior is intermediate between n=1 and n=2, and the apparent midpoint potential is -444 mV. This is compared to the behavior in Photosystem I, where the intermediate electron acceptor A1, thought to be a phylloquinone molecule, has been proposed to undergo a double reduction at low redox potentials in the presence of viologen redox mediators.These results strongly suggest that the acceptor side electron transfer system in reaction centers from heliobacteria is indeed analogous to that found in Photosystem I. The sequence similarities indicate that the divergence of the heliobacteria from the Photosystem I line occurred before the gene duplication and subsequent divergence that lead to the heterodimeric protein core of the Photosystem I reaction center.

Identities of four low-molecular-mass subunits of the photosystem I complex from Anabaena variabilis ATCC 29413. Evidence for the presence of the psaI gene product in a cyanobacterial complex

Photosystem I (PSI) complex of Anabaena variabilis ATCC 29413 consists of at least 11 subunits, 9 of which are resolved by high resolution gel electrophoresis. N-terminal amino acid sequences of the four subunits with molecular masses of 6.8, 5.2, 4.8 and 3.5 kDa were determined. Based on the sequence homology, the 3.5 kDa subunit was revealed to correspond to PSI-I (the gene product of psaI), which had so far been detected only in higher plant PSI complexes. The 6.8 kDa protein and 4.8 kDa protein were identified as gene products of psaK and psaJ, respectively. The 5.2 kDa protein was homologous to a 4.8 kDa subunit of PSI of the thermophilic cyanobacterium Synechococcus vulcanus, suggesting that this protein is a component of PSI in cyanobacteria.

A guide to electron paramagnetic resonance spectroscopy of Photosystem II membranes

This guide is intended to aid in the detection and identification of paramagnetic species in Photosystem II membranes, by electron paramagnetic resonance spectroscopy. The spectral features and occurrence of each of the electron paramagnetic resonance signals from Photosystem II are discussed, in relation to the nature of the moiety giving rise to the signal and the role of that species in photosynthetic electron transport. Examples of most of the signals discussed are shown. The electron paramagnetic resonance signals produced by the cytochrome b6f and Photosystem I complexes, as well as the signals from other common contaminants, are also reviewed. Furthermore, references to seminal experiments on bacterial reaction centers are included. By reviewing both the spectroscopic and biochemical bases for the electron paramagnetic resonance signals of the cofactors that mediate photosynthetic electron transport, this paper provides an introduction to the use and interpretation of electron paramagnetic resonance spectroscopy in the study of Photosystem II.

Contribution of vitamin K1 to the electron spin polarization in spinach photosystem I

The electron spin polarized (ESP) electron paramagnetic resonance (EPR) signal observed in spinach photosystem I (PSI) particles was examined in preparations depleted of vitamin K1 by solvent extraction and following biological reconstitution by the quinone. The ESP EPR signal was not detected in the solvent-extracted PSI sample but was restored upon reconstitution with either protonated or deuterated vitamin K1 under conditions that also restored electron transfer to the terminal PSI acceptors. Reconstitution using deuterated vitamin K1 resulted in a line narrowing of the ESP EPR signal, supporting the conclusion that the ESP EPR signals in the reconstituted samples arise from a radical pair consisting of the oxidized PSI primary donor, P700+, and reduced vitamin K1.