ENDOR and pulsed EPR studies of photosynthetic reaction centers: Protein‐cofactor interactions

Abstract

The influence of specific protein-cofactor interactions on the electronic structure of the primary donor cation radical Pf ' and the acceptor anion radicals Q;* and Q;' in wild type and mutant reaction centers of photosynthetic bacteria is investigated by ENDOR and Pulsed EPR techniques. The results show that hydrogen bonds to the primary donor have a strong effect on the distribution of the unpaired electron over the two BChl halves of the special pair, P". A correlation between the rate of reduction of P f ' by cytochrome c2 and the spin density distribution within the dimer is found. ESEEM made it possible to detect I4N nuclear quadrupole resonances for QA' and Q;* that are assigned to nitrogen atoms of ligating amino acids (histidines and peptide backbone). The deduced hyperfine couplings indicate significant delocalization of spin density onto these residues. This indicates an active role of these ligands in the electron transfer from QA to QB.

Direct Evidence for Nitrogen Ligation to the High Stability Semiquinone Intermediate in Escherichia coli Nitrate Reductase A

“…As the latter has been shown to be much more influenced by the geometry of the N-H bond than the parameter, the relatively small value determined in this work may reflect a slightly different H-bonding pattern compared with that occurring for Q A . in bacterial reaction centers (30,35,36) or in photosystem II (25,28). Interestingly, the quadrupole characteristics determined here for the 14 N nucleus interacting with MSQ D in NarGHI are very close to the values calculated by Fritscher for an imidazole making a strong in-plane hydrogen bond with a water molecule (r O-N ϳ2.7 Å) (46).…”

“…Thus, the presence of an H-bond to a histidine residue appears to be a common feature of both PCP and MSQ D binding. This contrasts with the situation observed for the menaquinol oxidation site of the polysulfide reductase from Thermus thermophilus that has been recently characterized by cocrystallization experiments using the natural substrate menaquinone or the inhibitor PCP (27), N ⑀ Arg at the ubiquinone site Q H in cytochrome bo 3 (34,45), N ␦ His and backbone nitrogen at the Q A and Q B sites of bacterial reaction centers (30,35,36) and photosystem II (25,28,29). On the basis of our spectroscopic data and the structure of NarGHI co-crystallized with PCP, we propose that the N ␦ of the His-66 imidazole H-bonds to the semiquinone O 1 while the N ⑀ coordinates the heme b D iron.…”

Coordination and binding geometry of methyl-coenzyme M in the red1m state of methyl-coenzyme M reductase

“…Simulation of the data (HYSCORE spectra recorded at middle-and low-field positions in the EPR spectrum are given in Fig. S3 [57][58][59], whereas the NH nitrogens of histidine and proline have |e 2 qQ/h| & 1.4-1.7 MHz, g = 0.6-1.0 [60], and the NH nitrogen of guanine has |e 2 qQ/" h| = 2.63 MHz, g = 0.60. Our parameters most closely resemble those of a glutamine [61].…”

Resolving the EPR Spectra in the Cytochrome bc 1 Complex from Saccharomyces cerevisiae

“…It can provide information about the binding partners from the protein to the radical anion. Previously this method has been applied to study the stable electron acceptors of the bacterial reaction centres [15,16], as well as photosystems I and II of higher plants [17][18][19]. Indeed the specific assignments made by some of these experiments predated the Resolving the EPR Spectra in the Cytochrome bc 1 Complex of S. cerevisiae 307 resolution of the three-dimensional structures, which were later confirmed by such structural models.…”