BBA - Bioenergetics (v.1827, #4)

In photosystem II membrane fragments with oxidized cytochrome (Cyt) b559 reduction of Cyt b559 by plastoquinol formed in the membrane pool under illumination and by exogenous decylplastoquinol added in the dark was studied. Reduction of oxidized Cyt b559 by plastoquinols proceeds biphasically comprising a fast component with a rate constant higher than (10 s)− 1, named phase I, followed by a slower dark reaction with a rate constant of (2.7 min)− 1 at pH 6.5, termed phase II. The extents of both components of Cyt b559 reduction increased with increasing concentrations of the quinols, with that, maximally a half of oxidized Cyt b559 can be photoreduced or chemically reduced in phase I at pH 6.5. The photosystem II herbicide dinoseb but not 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) competed with the quinol reductant in phase I. The results reveal that the two components of the Cyt b559 redox reaction reflect two redox equilibria attaining in different time domains. One-electron redox equilibrium between oxidized Cyt b559 and the photosystem II-bound plastoquinol is established in phase I of Cyt b559 reduction. Phase II is attributed to equilibration of Cyt b559 redox forms with the quinone pool. The quinone site involved in phase I of Cyt b559 reduction is considered to be the site regulating the redox potential of Cyt b559 which can accommodate quinone, semiquinone and quinol forms. The properties of this site designated here as QD clearly suggest that it is distinct from the site QC found in the photosystem II crystal structure.► Reduction of Cyt b559 of PS II membrane fragments by plastoquinol is biphasic. ► The data reveal in PS II a quinol binding site interacting with cytochrome b559. ► The quinone site of PS II regulates the redox potential of cytochrome b559.
Keywords: Photosystem II; Cyt b559; Plastoquinone; Quinone binding site;

Solution structure of a mutant of the triheme cytochrome PpcA from Geobacter sulfurreducens sheds light on the role of the conserved aromatic residue F15 by Joana M. Dantas; Leonor Morgado; P. Raj Pokkuluri; David L. Turner; Carlos A. Salgueiro (484-492).
Extracellular electron transfer is one of the physiological hallmarks of Geobacteraceae. Most of the Geobacter species encode for more than 100 c-type cytochromes which are, in general, poorly conserved between individual species. An exception to this is the PpcA family of periplasmic triheme c-type cytochromes, which are the most abundant proteins in these bacteria. The functional characterization of PpcA showed that it has the necessary properties to couple electron/proton transfer, a fundamental step for ATP synthesis. The detailed thermodynamic characterization of a PpcA mutant, in which the strictly conserved residue phenylalanine 15 was replaced by leucine, showed that the global redox network of cooperativities among heme groups is altered, preventing the mutant from performing a concerted electron/proton transfer. In this work, we determined the solution structure of PpcA F15L mutant in the fully reduced state using NMR spectroscopy by producing 15N-labeled protein. In addition, pH-dependent conformational changes were mapped onto the structure. The mutant structure obtained is well defined, with an average pairwise root-mean-square deviation of 0.36 Å for the backbone atoms and 1.14 Å for all heavy atoms. Comparison between the mutant and wild-type structures elucidated the contribution of phenylalanine 15 in the modulation of the functional properties of PpcA.► PpcA couples e/H+ transfer and the process is controlled by a conserved residue. ► Solution structure and pH-linked conformational changes of PpcAF15L were determined. ► Structural rearrangements had an effect in lowering the reduction potentials. ► Structural rearrangements altered the properties of the redox-Bohr center.
Keywords: Geobacter; Multiheme cytochromes; NMR; Structure–function; Site-directed mutagenesis;

Spectroscopic insights into the decreased efficiency of chlorosomes containing bacteriochlorophyll f by Gregory S. Orf; Marcus Tank; Kajetan Vogl; Dariusz M. Niedzwiedzki; Donald A. Bryant; Robert E. Blankenship (493-501).
Chlorosomes are light-harvesting antenna complexes that occur in green photosynthetic bacteria which have only been shown naturally to contain bacteriochlorophyll (BChl) c, d, or e as the principal light-harvesting pigments. BChl f has long been thought to be an obvious fourth member of the so-called Chlorobium chlorophylls, because it possesses a C-7 formyl group like BChl e and lacks a methyl group at C-20 like BChl d. In organisms that synthesize BChl c or e, the bchU gene product encodes the enzyme that methylates the C-20 position of these molecules. A bchU null mutant of the green sulfur bacterium Chlorobaculum limnaeum strain 1677T, which normally synthesizes BChl e, has recently been generated via insertional inactivation, and it produces chlorosomes containing BChl f [Vogl et al., 2012]. In this study, chlorosomes containing BChl f and monomeric BChl f in pyridine were characterized using a variety of spectroscopic techniques, including fluorescence emission and excitation spectroscopy, fluorescence lifetime and quantum yield determinations, and circular dichroism. These spectroscopic measurements, as well as Gaussian simulation of the data, show that chlorosomes containing BChl f are less efficient in energy transfer than those with BChl e. This can primarily be attributed to the decreased spectral overlap between the oligomeric BChl f (energy donor) fluorescence emission and the BChl a (energy acceptor) absorption in the chlorosome baseplate. This study allows us to hypothesize that, if they exist in nature, BChl f-containing organisms most likely live in rare high-light, anoxic conditions devoid of Chl a, d, or BChl e filtering.K. Vogl, M. Tank, G.S. Orf, R.E. Blankenship, D.A. Bryant, Bacteriochlorophyll f: properties of chlorosomes containing the “forbidden chlorophyll,” Front. Microbiol. 3 (2012) 298.► We spectroscopically characterize chlorosomes containing bacteriochlorophyll f. ► Bacteriochlorophyll f-containing chlorosomes display some unusual fluorescence properties. ► Bacteriochlorophyll f-containing chlorosomes show inefficient energy transfer. ► We examine the consequences of bacteriochlorophyll f utilization by green bacteria.
Keywords: Green sulfur bacteria; Bacteriochlorophyll; Chlorobaculum limnaeum; Chlorosomes; Photosynthesis; Förster Resonance Energy Transfer;

Electron transfer between multihaem cytochromes c 3 from Desulfovibrio africanus by Pedro O. Quintas; Márcia S. Oliveira; Teresa Catarino; David L. Turner (502-506).
The tetrahaem type I cytochromes c 3 from Desulfovibrionaceae shuttle electrons from a periplasmic hydrogenase to transmembrane electron transfer complexes. In D. africanus, it is believed that the electrons are received by another tetrahaem cytochrome c 3, denoted type II, which is associated with the membrane complex. Thermodynamic measurements show that the type I cytochrome c 3 has the potential to transfer two electrons at a time. This study uses two-dimensional NMR to investigate the exchange of electrons between type I and type II cytochromes c 3 at equilibrium in intermediate stages of oxidation. The results indicate that the two proteins are physiological partners but that only single-electron transfers occur in solution.► TpIc 3 is a soluble tetrahaem cytochrome from sulphate reducing bacteria. ► 2D NMR exchange was used to quantify intermolecular electron transfer rates. ► Only single-electron transfers were detected in each collisional event. ► The self exchange rate is about 30 times slower than the diffusion limit. ► Electron transfer between TpIc 3 and TpIIc 3 from D. africanus is diffusion limited.
Keywords: Sulphate-reducing bacterium; Multiheme cytochrome; Electron transfer; 2D NMR exchange spectroscopy; Marcus theory; Diffusion control;

Spectroscopic and functional characterization of cyanobacterium Synechocystis PCC 6803 mutants on the cytoplasmic-side of cytochrome b 559 in photosystem II by Yi-Fang Chiu; Yung-Han Chen; Mercedes Roncel; Preston L. Dilbeck; Jine-Yung Huang; Shyue-Chu Ke; José M. Ortega; Robert L. Burnap; Hsiu-An Chu (507-519).
We performed spectroscopic and functional characterization on cyanobacterium Synechocystis PCC6803 with mutations of charged residues of the cytoplasmic side of cytochrome (Cyt) b 559 in photosystem II (PSII). All of the mutant cells grew photoautotrophically and assembled stable PSII. However, R7Eα, R17Eα and R17Lβ mutant cells grew significantly slower and were more susceptible to photoinhibition than wild-type cells. The adverse effects of the arginine mutations on the activity and the stability of PSII were in the following order (R17Lβ > R7Eα > R17Eα and R17Aα). All these arginine mutants exhibited normal period-four oscillation in oxygen yield. Thermoluminescence characteristics indicated a slight decrease in the stability of the S3QB /S2QB charge pairs in the R7Eα and R17Lβ mutant cells. R7Eα and R17Lβ PSII core complexes contained predominantly the low potential form of Cyt b 559. EPR results indicated the displacement of one of the two axial ligands to the heme of Cyt b 559 in R7Eα and R17Lβ mutant reaction centers. Our results demonstrate that the electrostatic interactions between these arginine residues and the heme propionates of Cyt b 559 are important to the structure and redox properties of Cyt b 559. In addition, the blue light-induced nonphotochemical quenching was significantly attenuated and its recovery was accelerated in the R7Lα and R17Lβ mutant cells. Furthermore, ultra performance liquid chromatography–mass spectrometry results showed that the PQ pool was more reduced in the R7Eα and R17Lβ mutant cells than wild-type cells in the dark. Our data support a functional role of Cyt b 559 in protection of PSII under photoinhibition conditions in vivo.Display Omitted► Structure and redox properties of Cyt b 559 were altered in mutant photosystem II. ► Blue light-induced nonphotochemical quenching was perturbed in Cyt b 559 mutants. ► The PQ pool was more reduced in R7Eα and R17Lβ mutant cells in the dark.
Keywords: Photosystem II; Cytochrome b 559; Photoinhibition; Chlorophyll a fluorescence; Electron paramagnetic resonance; Thermoluminescence;

Fluorescence microspectroscopy observations were used to study the processes of cell differentiation and assemblies of photosynthesis proteins in Zea mays leaves under the greening process. The observations were done at 78 K by setting the sample in a cryostat to avoid any undesired progress of the greening process during the measurements. The lateral and axial spatial resolutions of the system were 0.64 μm and 4.4 μm, respectively. The study revealed the spatial distributions of protochlorophyllide (PChld) in both the 632-nm-emitting and 655-nm-emitting forms within etiolated Zea mays leaves. The sizes of the fluorescence spots attributed to the former were larger than those of the latter, validating the assignment of the former and latter to the prothylakoid and prolamellar bodies, respectively. In vivo microspectroscopy observations of mature Zea mays leaves confirmed the different photosystem II (PS I)/photosystem I (PS II) ratio between the bundle sheath (BS) and mesophyll (MS) cells, which is specific for C4-plants. The BS cells in Zea mays leaves 1 h after the initiation of the greening process tended to show fluorescence spectra at shorter wavelength side (at around 679 nm) than the MS cells (at around 682 nm). The 679-nm-emitting chlorophyll-a form observed mainly in the BS cells was attributed to putative precursor complexes to PS I. The BS cells under 3-h greening showed higher relative intensities of the PS I fluorescence band at around 735 nm, suggesting the reduced PS II amount in the BS cells in this greening stage.► Pigment distributions in etioplasts are observed with a microscope at 77 K. ► Distributions of different chlorophyll species during the greening are observed. ► The 655-nm emission is assigned to prolamellar body. ► 1-h greening accumulates different chlorophyll species between BS and MS cells.
Keywords: Prolamellar body; Prothylakoid; C4-plant; Protochlorophyllide;

Two types of fucoxanthin-chlorophyll-binding proteins I tightly bound to the photosystem I core complex in marine centric diatoms by Yohei Ikeda; Atsushi Yamagishi; Masayuki Komura; Takehiro Suzuki; Naoshi Dohmae; Yutaka Shibata; Shigeru Itoh; Hiroyuki Koike; Kazuhiko Satoh (529-539).
Intact fucoxanthin (Fucox)-chlorophyll (Chl)-binding protein I-photosystem I supercomplexes (FCPI-PSIs) were prepared by a newly developed simple fast procedure from centric diatoms Chaetoceros gracilis and Thalassiosira pseudonana to study the mechanism of their efficient solar energy accumulation. FCPI-PSI purified from C. gracilis contained 252 Chl a, 23 Chl c, 56 Fucox, 34 diadinoxanthin + diatoxanthin, 1 violaxanthin, 21 ß-carotene, and 2 menaquinone-4 per P700. The complex showed a high electron transfer activity at 185,000 μmol mg Chl a − 1  · h− 1 to reduce methyl viologen from added cytochrome c 6. We identified 14 and 21 FCP proteins in FCPI-PSI of C. gracilis and T. pseudonana, respectively, determined by N-terminal and internal amino acid sequences and liquid chromatography–tandem mass spectrometry (LC–MS/MS) analyses. PsaO and a red lineage Chla/b-binding-like protein (RedCAP), Thaps3:270215, were also identified. Severe detergent treatment of FCPI-PSI released FCPI-1 first, leaving the FCPI-2-PSI-core complex. FCPI-1 contained more Chl c and showed Chl a fluorescence at a shorter wavelength than FCPI-2, suggesting an excitation-energy transfer from FCPI-1 to FCPI-2 and then to the PSI core. Fluorescence emission spectra at 17 K in FCPI-2 varied depending on the excitation wavelength, suggesting two independent energy transfer routes. We formulated a model of FCPI-PSI based on the biochemical assay results.► A simple fast procedure from two centric diatoms was developed. ► PSI complexes have a high transport activity. ► A red lineage Chla/b-binding-like protein (RedCAP) was found. ► Two types of fucoxanthin-chlorophyll-binding proteins I in diatom PSI is proposed.
Keywords: Photosystem I; Fucoxanthin-chlorophyll-binding protein I; Diatom; Chaetoceros gracilis; Thalassiosira pseudonana;