Histidine is involved in coupling proton uptake to electron transfer in photosynthetic proteins.

In photosynthesis, the central step in transforming light energy into chemical energy is the coupling of light-induced electron transfer to proton uptake and release. Despite intense investigations of different photosynthetic protein complexes, including the photosystem II (PS II) in plants and the reaction center (RC) in bacteria, the molecular details of this fundamental process remain incompletely understood. In the RC of Rhodobacter (Rb.) sphaeroides, fast formation of the charge separated state, P(+)Q(A)(-), is followed by a slower electron transfer from the primary acceptor, Q(A), to the secondary acceptor, Q(B), and the uptake of a proton from the cytoplasm. The proton transfer to Q(B) takes place via a protonated water chain. Mutation of the amino acid AspL210 to Asn (L210DN mutant) near the entry of the proton pathway can disturb this water chain and consequently slow down proton uptake. Time-resolved step-scan Fourier transform infrared (FTIR) measurements revealed an intermediate X in the Q(A)(-)Q(B) to Q(A)Q(B)(-) transition. The nature of this transition remains a matter of debate. In this study, we investigated the role of the iron-histidine complex located between Q(A) and Q(B). We used time-resolved fast-scan FTIR spectroscopy to characterize the Rb. sphaeroides L210DN RC mutant marked with isotopically labeled histidine. FTIR marker bands of the intermediate X between 1120 cm(-1) and 1050 cm(-1) are assigned to histidine vibrations and indicate the protonation of a histidine, most likely HisL190, during the disappearance of the intermediate. Based on these results we propose a novel mechanism of the coupling of electron and proton transfer in photosynthesis.

Influence of a joining helix on the BLUF domain of the YcgF photoreceptor from Escherichia coli.

BLUF-domain-comprising photoreceptors sense blue light by utilizing FAD as a chromophore. The ycgF gene product of Escherichia coli is composed of a N-terminal BLUF domain and a C-terminal EAL domain, with the latter postulated to catalyze c-di-GMP hydrolysis. The linkage between these two domains involves a predominantly helical segment. Its role on the function of the YcgF photoreceptor domain was examined by characterizing BLUF domains with and without this segment and reconstituting them with either FAD, FMN or riboflavin. The stability of the light-adapted state of the YcgF BLUF domain depends on the presence of this joining, helical segment and the adenosine diphosphate moiety of FAD. In contrast to other BLUF domains, two-dimensional (1)H,(15)N and one-dimensional (1)H NMR spectra of isotope-labeled YcgF-(1-137) revealed large conformational changes during reversion from the light- to the dark-adapted state. Based on these results the function of the joining helix in YcgF during signal transfer and the role of the BLUF domain in regulating c-di-GMP levels is discussed.