BBA - Bioenergetics (v.1767, #10)

Entropy production and the Second Law in photosynthesis by Robert S. Knox; William W. Parson (1189-1193).
An assertion that the primary photochemistry of photosynthesis can violate the Second Law of thermodynamics in certain efficient systems has been put forward by Jennings et al., who maintain their position strongly despite an argument to the contrary by Lavergne. We identify a specific omission in the calculation of Jennings et al. and show that no violation of the Second Law occurs, regardless of the photosynthetic efficiency.
Keywords: Entropy; Photosynthesis; Second law of thermodynamics;

Entropy consumption in primary photosynthesis by Robert C. Jennings; Erica Belgio; Anna Paola Casazza; Flavio M. Garlaschi; Giuseppe Zucchelli (1194-1197).
Knox and Parson have objected to our previous conclusion on possible negative entropy production during primary photochemistry, i.e., from photon absorption to primary charge separation, by considering a pigment system in which primary photochemistry is not specifically considered. This approach does not address our proposal. They suggest that when a pigment absorbs light and passes to an excited state, its entropy increases by hν/T. This point is discussed in two ways: (i) from considerations based on the energy gap law for excited state relaxation; (ii) using classical thermodynamics, in which free energy is introduced into the pigment (antenna) system by photon absorption. Both approaches lead us to conclude that the excited state and the ground state are isoentropic, in disagreement with Knox and Parson. A discussion on total entropy changes specifically during the charge separation process itself indicates that this process may be almost isoentropic and thus our conclusions on possible negentropy production associated with the sequence of reactions which go from light absorption to the first primary charge separation event, due to its very high thermodynamic efficiency, remain unchanged.
Keywords: Entropy consumption; Negentropy; Photon absorption; Primary photochemistry; Energy gap law; Photosystem;

On “Entropy consumption in primary photosynthesis” by Jennings et al. by Robert S. Knox; William W. Parson (1198-1199).
The preceding paper [R.C. Jennings, E. Belgio, A.P. Casazza, F.M. Garlaschi, G. Zucchelli, Entropy consumption in primary photosynthesis, Biochim. Biophys. Acta (in press)] is criticized on several grounds.
Keywords: Entropy; Photosynthesis; Second law of thermodynamics;

Mechanism and energetics of proton translocation by the respiratory heme-copper oxidases by Mårten Wikström; Michael I. Verkhovsky (1200-1214).
Recent time-resolved optical and electrometric experiments have provided a sequence of events for the proton-translocating mechanism of cytochrome c oxidase. These data also set limits for the mechanistic, kinetic, and thermodynamic parameters of the proton pump, which are analysed here in some detail. The analysis yields limit values for the pK of the “pump site”, its modulation during the proton-pumping process, and suggests its identity in the structure. Special emphasis is made on side-reactions that may short-circuit the pump, and the means by which these may be avoided. We will also discuss the most prominent proton pumping mechanisms proposed to date in relation to these data.
Keywords: Electron transfer; Proton transfer; Proton pump; Cell respiration;

Human mitochondrial complex I assembly: A dynamic and versatile process by Rutger O. Vogel; Jan A.M. Smeitink; Leo G.J. Nijtmans (1215-1227).
One can but admire the intricate way in which biomolecular structures are formed and cooperate to allow proper cellular function. A prominent example of such intricacy is the assembly of the five inner membrane embedded enzymatic complexes of the mitochondrial oxidative phosphorylation (OXPHOS) system, which involves the stepwise combination of > 80 subunits and prosthetic groups encoded by both the mitochondrial and nuclear genomes. This review will focus on the assembly of the most complicated OXPHOS structure: complex I (NADH:ubiquinone oxidoreductase, EC 1.6.5.3). Recent studies into complex I assembly in human cells have resulted in several models elucidating a thus far enigmatic process. In this review, special attention will be given to the overlap between the various assembly models proposed in different organisms. Complex I being a complicated structure, its assembly must be prone to some form of coordination. This is where chaperone proteins come into play, some of which may relate complex I assembly to processes such as apoptosis and even immunity.
Keywords: Mitochondria; Oxidative phosphorylation; NADH:ubiquinone oxidoreductase; Complex I; Assembly; Chaperones;

Marcus treatment of endergonic reactions: A commentary by Antony R. Crofts; Stuart Rose (1228-1232).
Two forms of the equation for expression of the rate constant for electron transfer through a Marcus-type treatment are discussed. In the first (exergonic) form, the Arrhenius exponential term was replaced by its classical Marcus term; in the second (endergonic) form, the forward rate constant was replaced by the reverse rate constant (the forward rate constant in the exergonic direction), which was expanded to an equivalent Marcus term and multiplied by the equilibrium constant. When the classical Marcus treatment was used, these two forms of the rate equation give identical curves relating the logarithm of the rate constant to the driving force. The Marcus term for the relation between activation free-energy, ΔG #, reorganization energy, λ, and driving force, ΔG o, derived from parabolas for the reactant and product states, was identical when starting from exergonic or endergonic parabolas. Moser and colleagues introduced a quantum mechanical correction factor to the Marcus term in order to fit experimental data. When the same correction factor was applied in the treatment for the endergonic direction by Page and colleagues, a different curve was obtained from that found with the exergonic form. We show that the difference resulted from an algebraic error in development of the endergonic equation.
Keywords: Marcus theory; Moser–Dutton treatment; Endergonic reaction; Rate constant;

Endogenous probes of light-induced transthylakoid proton motive force (pmf), membrane potential (Δψ) and ΔpH were used in vivo to assess in Arabidopsis the lumen pH responses of regulatory components of photosynthesis. The accumulation of zeaxanthin and protonation of PsbS were found to have similar pK a values, but quite distinct Hill coefficients, a feature allowing high antenna efficiency at low pmf and fine adjustment at higher pmf. The onset of “energy-dependent’ exciton quenching (q E) occurred at higher lumen pH than slowing of plastoquinol oxidation at the cytochrome b 6 f complex, presumably to prevent buildup of reduced electron carriers that can lead to photodamage. Quantitative comparison of intrinsic probes with the electrochromic shift signal in situ allowed quantitative estimates of pmf and lumen pH. Within a degree of uncertainly of ∼ 0.5 pH units, the lumen pH was estimated to range from ∼ 7.5 (under weak light at ambient CO2) to ∼ 5.7 (under 50 ppm CO2 and saturating light), consistent with a ‘moderate pH’ model, allowing antenna regulation but preventing acid-induced photodamage. The apparent pK a values for accumulation of zeaxanthin and PsbS protonation were found to be ∼ 6.8, with Hill coefficients of about 4 and 1 respectively. The apparent shift between in vitro violaxanthin deepoxidase protonation and zeaxanthin accumulation in vivo is explained by steady-state competition between zeaxanthin formation and its subsequent epoxidation by zeaxanthin epoxidase. In contrast to tobacco, Arabidopsis showed substantial variations in the fraction of pmf (0.1–0.7) stored as Δψ, allowing a more sensitive qE response, possible as an adaptation to life at lower light levels.
Keywords: Chloroplast; Cytochrome b6f complex; Electrochromic shift; In vivo spectroscopy; Non-photochemical quenching; Regulation of photosynthesis;

In yeast, Ca2+ and octylguanidine interact with porin (VDAC) preventing the mitochondrial permeability transition by Manuel Gutiérrez-Aguilar; Victoriano Pérez-Vázquez; Odile Bunoust; Stéphen Manon; Michel Rigoulet; Salvador Uribe (1245-1251).
In yeast, Ca2+ and long chain alkylguanidines interact with mitochondria modulating the opening of the yeast mitochondrial unspecific channel. Mammalians possess a similar structure, the mitochondrial permeability transition pore. The composition of these pores is under debate. Among other components, the voltage-dependent anion channel has been proposed as a component of either pore. In yeast from an industrial strain, octylguanidine and calcium closed the yeast mitochondrial unspecific channel. Here, the effects of the cations Ca2+ or octylguanidine and the voltage-dependent anion channel effector decavanadate were evaluated in yeast mitochondria from either a wild type or a voltage-dependent anion channel deletion laboratory strain. It was observed that in the absence of voltage-dependent anion channel, the yeast mitochondrial unspecific channel was desensitized to Ca2+, octylguanidine or decavanadate but remained sensitive to phosphate. It is thus suggested that in yeast mitochondria, the voltage-dependent anion channel has a cation binding site where Ca2+ and octylguanidine interact, conferring cation sensitivity to the yeast mitochondrial unspecific channel.
Keywords: Ca2+; Permeability transition; Porin; Yeast mitochondria; YMUC;

The role of PGR5 in the redox poising of photosynthetic electron transport by Beena Nandha; Giovanni Finazzi; Pierre Joliot; Simon Hald; Giles N. Johnson (1252-1259).
The pgr5 mutant of Arabidopsis thaliana has been described as being deficient in cyclic electron flow around photosystem I, however, the precise role of the PGR5 protein remains unknown. To address this issue, photosynthetic electron transport was examined in intact leaves of pgr5 and wild type A. thaliana. Based on measurements of the kinetics of P700 oxidation in far red light and re-reduction following oxidation in the presence of DCMU, we conclude that this mutant is able to perform cyclic electron flow at a rate similar to the wild type. The PGR5 protein is therefore not essential for cyclic flow. However, cyclic flow is affected by the pgr5 mutation under conditions where this process is normally enhanced in wild type leaves, i.e. high light or low CO2 concentrations resulted in enhancement of cyclic electron flow. This suggests a different capacity to regulate cyclic flow in response to environmental stimuli in the mutant. We also show that the pgr5 mutant is affected in the redox poising of the chloroplast, with the electron transport chain being substantially reduced under most conditions. This may result in defective feedback regulation of photosynthetic electron transport under some conditions, thus providing a rationale for the reduced efficiency of cyclic electron flow.
Keywords: Cyclic electron transport; Non photochemical quenching; Photosynthesis;

Mitochondrial dysfunction in rat with nonalcoholic fatty liver by Giuseppe Petrosillo; Piero Portincasa; Ignazio Grattagliano; Giacoma Casanova; Mariagiuseppa Matera; Francesca M. Ruggiero; Domenico Ferri; Giuseppe Paradies (1260-1267).
Mitochondrial dysfunction and oxidative stress play a central role in the pathophysiology of nonalcoholic fatty liver disease (NAFLD). This study aimed to elucidate the mechanism(s) responsible for mitochondrial dysfunction in nonalcoholic fatty liver. Fatty liver was induced in rats with a choline-deficient (CD) diet for 30 days. We examined the effect of CD diet on various parameters related to mitochondrial function such as complex I activity, oxygen consumption, reactive oxygen species (ROS) generation and cardiolipin content and oxidation. The activity of complex I was reduced by 35% in mitochondria isolated from CD livers compared with the controls. These changes in complex I activity were associated with parallel changes in state 3 respiration. Hydrogen peroxide (H2O2) generation was significantly increased in mitochondria isolated from CD livers. The mitochondrial content of cardiolipin, a phospholipid required for optimal activity of complex I, decreased by 38% as function of CD diet, while there was a significantly increase in the level of peroxidized cardiolipin. The lower complex I activity in mitochondria from CD livers could be completely restored to the level of control livers by exogenously added cardiolipin. This effect of cardiolipin could not be replaced by other phospholipids nor by peroxidized cardiolipin. It is concluded that CD diet causes mitochondrial complex I dysfunction which can be attributed to ROS-induced cardiolipin oxidation. These findings provide new insights into the alterations underlying mitochondrial dysfunction in NAFLD.
Keywords: Nonalcoholic fatty liver; Mitochondria; Complex I; ROS; Cardiolipin;