Photosynthetic electrogenic events in native membranes of Chloroflexus aurantiacus. Flash-induced charge displacements in the acceptor quinone complex of the photosynthetic reaction center

Light-dependent reduction of cystine disulfide bonds results in activation of several of the enzymes of photosynthetic carbon metabolism within the chloroplast. Tertiary structure modeling suggests that the redox-sensitivity of the chloroplast malate dehydrogenase (EC 1.1.1.82) is due to disulfide crosslinking of the carbon substrate and nucleotide-binding domains. Consistent with this suggestion, introduction of Cys residues in opposition to one another on the two domains of the Escherichia coli enzyme results in redox-sensitivity [Muslin EH et al. (1995) Biophys J 68: 2218-2223]. We have now substituted Cys residues into the bacterial malate dehydrogenase (EC 1.1.1.37) in positions that correspond more exactly to those postulated to be responsible for the redox-sensitivity of the chloroplast enzyme. The introduction of one pair of Cys residues renders the enzyme redox-sensitive, but the introduction of the alternate pair does not. Energy minimization calculations suggest that the difference in redox-sensitivity is consistent with differences in the energy required for formation of the disulfide bond.     

Bacteriorhodopsin-mediated photoelectric responses in lipid/water systems

Summary Bacteriorhodopsin-mediated photopotential generation has been studied in two kinds of lipid/water systems: (1) decane solution of asolectin was used as the lipid phase; (2) a mixture of bacteriorhodopsin sheets and hexane solution of phosphatidyl choline was applied onto a water surface to form a monolayer, and then the monolayer was covered with a 0.3-mm decane layer. In both cases, illumination was found to induce formation of an electric potential difference, with the bulk water phase being found negative when measured with a vibrating electrode. In the latter, but not in the former, system small amounts of a protonophorous uncoupler were found to stimulate the photoresponse. Large amounts of the uncoupler proved depressing in both systems. Phenyldicarbaundecaborane anion (PCB^–) was shown to substitute for the uncoupler, being much more potent both as an activator and as an inhibitor of the photoresponse. In both studied systems, gramicidin A inhibits the photoresponse, the effect being greatly potentiated by K⁺, Na⁺ or H⁺ ions.In the system decane solution of asolectin/water, an Ag/AgCl electrode immersed into the lipid phase can be used instead of a vibrating electrode. All the measured features of the photoelectric responses observed with any of these electrodes were found to be quite similar to those inherent in a phospholipid-impregnated collodion film adsorbing bacteriorhodopsin sheets on one of its surfaces.A scheme is discussed built on the assumption that photopotentials in all the studied systems are due to an uphill light-dependent transport of H⁺ ions from the bulk water phase to a water cavity localized between a bacteriorhodopsin sheet and the surface of the bulk lipid phase. Thus, the above lipid/water systems containing bacteriorhodopsin are composed of four, rather than two, phases, as was supposed previously.Bacteriorhodopsin-mediated photopotential generation has been studied also in the decane/water system without phospholipids. This system with bacteriorhodopsin sheets added to the water phase demonstrates a light-dependent photoelectric response reaching 1.5 V, which can be measured only by a vibrating electrode. The photoresponse starts after a lag period of several seconds. Switching off the light results in the reversal of the light-induced electric potential change. The off-effect also has a lag period. The action spectrum of the photoresponse shows at least two maxima: a smaller at 560 nm and a larger at <420 nm. Free retinal can substitute for bacteriorhodopsin in the studied system. All the above effects disappear if, instead of air, argon is used. In the system decane solution of asolectin/water, a slow photoelectric response of this type can be demonstrated at neutral pH in the presence of gramicidin and at pH 4 without gramicidin. A suggestion is put forward that the slow photoelectric response is due to an interface Volta-potential change induced by a product of photooxidation of bacteriorhodopsin and/or free retinal released from bacteriorhodopsin.     

Photosynthetic electrogenic events in native membranes ofChloroflexus aurantiacus. Flash-induced charge displacements within the reaction center-cytochromec 554 complex

The thermophilic phototrophChloroflexus aurantiacus possesses a photosynthetic reaction center (RC) containing a pair of menaquinones as primary (Q_A) and secondary (Q_B) electron acceptors and a bacteriochlorophyll dimer (P) as a primary donor. A tetraheme cytochromec ₅₅₄ with two high(H)- and two low(L)-potential hemes operates as an immediate electron donor for P. The following equilibrium E_m,7 values were determined by ESR for the hemes in whole membrane preparations: 280 mV (H1), 150 mV (H2), 95 mV (L1) and 0 mV (L2) (Van Vliet et al. (1991) Eur. J. Biochem. 199: 317–323). Partial electrogenic reactions induced by a laser flash inChl. aurantiacus chromatophores adsorbed to a phospholipid-impregnated collodion film were studied electrometrically at pH 8.3. The photoelectric response included a fast phase of generation ( < 10 ns, phase A). It was ascribed to the charge separation between P⁺ and Q_A ^– as its amplitude decreased both at high and low E_h values (E_m,high=360±10 mV, estimated E_m,low\s-160 mV) in good agreement with E_m values for P/P⁺ and Q_A/Q_A ^– redox couples. A slower kinetic component appeared upon reduction of the cytochromec ₅₅₄ hemes (phase C). With H1 reduced before the flash the amplitude of phase C was equal to 15–20% of that of phase A and its rise time was 1.2–1.3 s: we attribute this phase to the electrogenic electron transfer from H1 to P⁺. Pre-reduction of H2 decreased the value to about 700–800 ns and increased the amplitude of phase C to 30–35% of that of phase A. Pre-reduction of L1 further accelerated phase C (up to of 500 ns) and induced a reverse electrogenic phase with of 12 s and amplitude equal to 10% of phase A. Upon pre-reduction of L2 the rise time of phase C was decreased to about 300 ns and its amplitude decreased by 30%. The acceleration in the onset of phase C is explained by the acceleration of the rate-limiting H1 P electrogenic reaction after reduction of the other hemes due to their electrostatic influence; a P-H1-(L1-L2)-H2 alignment of redox centers with an approximately rhombic arrangement of the cytochromec ₅₅₄ hemes is proposed. The observed reverse phase is ascribed to the post-flash charge redistribution between the hemes. Redox titration of the amplitude of phase C yielded the E_m,8.3 values of H1, H2 and L2 hemes: 340±10 mV for H1, 160±20 mV for H2 and –40±40 mV for L2.     

Ni-Supported Pd Nanoparticles with Ca Promoter: A New Catalyst for Low-Temperature Ammonia Cracking

In this paper we report a new nanometallic, self-activating catalyst, namely, Ni-supported Pd nanoparticles (Pd_NPs/Ni) for low temperature ammonia cracking, which was prepared using a novel approach involving the transfer of nanoparticles from the intermediate carrier, i.e. nano-spherical SiO₂, to the target carrier technical grade Ni (t-Ni) or high purity Ni (p-Ni) grains. The method that was developed allows a uniform nanoparticle size distribution (4,4±0.8 nm) to be obtained. Unexpectedly, the t-Ni-supported Pd NPs, which seemed to have a surface Ca impurity, appeared to be more active than the Ca-free (p-Ni) system. A comparison of the novel Pd_NPs/Ni catalyst with these reported in the literature clearly indicates the much better hydrogen productivity of the new system, which seems to be a highly efficient, flexible and durable catalyst for gas-phase heterogeneous ammonia cracking in which the TOF reaches a value of 2615 mmol_H2/g_Pd min (10,570 mol_NH3/mol_Pd(NP) h) at 600°C under a flow of 12 dm³/h (t-Ni).     

Characterising the biophysical,economic and social impacts of soil carbon sequestration as a greenhouse gas removal technology

To limit warming to well below 2°C, most scenario projections rely on greenhouse gas removal technologies (GGRTs); one such GGRT uses soil carbon sequestration (SCS) in agricultural land. In addition to their role in mitigating climate change, SCS practices play a role in delivering agroecosystem resilience, climate change adaptability and food security. Environmental heterogeneity and differences in agricultural practices challenge the practical implementation of SCS, and our analysis addresses the associated knowledge gap. Previous assessments have focused on global potentials, but there is a need among policymakers to operationalise SCS. Here, we assess a range of practices already proposed to deliver SCS, and distil these into a subset of specific measures. We provide a multidisciplinary summary of the barriers and potential incentives towards practical implementation of these measures. First, we identify specific practices with potential for both a positive impact on SCS at farm level and an uptake rate compatible with global impact. These focus on: (a) optimising crop primary productivity (e.g. nutrient optimisation, pH management, irrigation); (b) reducing soil disturbance and managing soil physical properties (e.g. improved rotations, minimum till); (c) minimising deliberate removal of C or lateral transport via erosion processes (e.g. support measures, bare fallow reduction); (d) addition of C produced outside the system (e.g. organic manure amendments, biochar addition); (e) provision of additional C inputs within the cropping system (e.g. agroforestry, cover cropping). We then consider economic and non‐cost barriers and incentives for land managers implementing these measures, along with the potential externalised impacts of implementation. This offers a framework and reference point for holistic assessment of the impacts of SCS. Finally, we summarise and discuss the ability of extant scientific approaches to quantify the technical potential and externalities of SCS measures, and the barriers and incentives to their implementation in global agricultural systems.     

Editor's choice: Functional classification of 15 million SNPs detected from diverse chicken populations

Next-generation sequencing has prompted a surge of discovery of millions of genetic variants from vertebrate genomes. Besides applications in genetic association and linkage studies, a fraction of these variants will have functional consequences. This study describes detection and characterization of 15 million SNPs from chicken genome with the goal to predict variants with potential functional implications (pfVars) from both coding and non-coding regions. The study reports: 183K amino acid-altering SNPs of which 48% predicted as evolutionary intolerant, 13K splicing variants, 51K likely to alter RNA secondary structures, 500K within most conserved elements and 3K from non-coding RNAs. Regions of local fixation within commercial broiler and layer lines were investigated as potential selective sweeps using genome-wide SNP data. Relationships with phenotypes, if any, of the pfVars were explored by overlaying the sweep regions with known QTLs. Based on this, the candidate genes and/or causal mutations for a number of important traits are discussed. Although the fixed variants within sweep regions were enriched with non-coding SNPs, some non-synonymous-intolerant mutations reached fixation, suggesting their possible adaptive advantage. The results presented in this study are expected to have important implications for future genomic research to identify candidate causal mutations and in poultry breeding.     

Orientation of reaction center complexes fromRhodobacter sphaeroides in proteoliposomes and the effect ofo-phenanthroline on electrogenesis during primary photochemical reaction

The orientation ofRhodobacter sphaeroides reaction center complexes (RC complexes) in proteoliposomal membranes was investigated by a direct electrometric method. Conditions were found that allow monitoring of only that RC complex fraction that is oriented with its donor side to the inner part of the proteoliposome. It is shown thato-phenanthroline, an inhibitor of electron transfer between primary (Q_A) and secondary (Q_B) quinone acceptors, can also inhibit the photoinduced Q_A reduction. The efficiency of this inhibition depends on the concentration of added ubiquinone. It is assumed that the laser flash-inducedo-phenanthroline inhibition of primary dipole (P-870⁺ · Q _A ^– ) formation is of a competitive nature.