Volume changes and electrostriction in the primary photoreactions of various photosynthetic systems: estimation of dielectric coefficient in bacterial reaction centers and of the observed volume changes with the Drude–Nernst equation

Photoacoustics (PA) allows the determination of enthalpy and volume changes of photoreactions in photosynthetic reaction centers on the 0.1–10 μs time scale. These include the bacterial centers from Rb. sphaeroides, PS I and PS II centers from Synechocystis and in whole cells. In vitro and in vivo PA data on PS I and PS II revealed that both the volume change (–26 A³) and reaction enthalpy (–0.4 eV) in PS I are the same as those in the bacterial centers. However the volume change in PS II is small and the enthalpy far larger, –1 eV. Assigning the volume changes to electrostriction allows a coherent explanation of these observations. One can explain the large volume decrease in the bacterial centers with an effective dielectric coefficient of ∼4. This is a unique approach to this parameter so important in estimation of protein energetics. The value of the volume contraction for PS I can only be explained if the acceptor is the super- cluster (Fe₄S₄)(Cys₄) with charge change from –1 to –2. The small volume change in PS II is explained by sub-μs electron transfer from Y_Z anion to P₆₈₀ cation, in which charge is only moved from the Y_Z anion to the Q_A with no charge separation or with rapid proton transfer from oxidized Y_Z to a polar region and thus very little change in electrostriction. At more acid pH equally rapid proton transfer from a neighboring histidine to a polar region may be caused by the electric field of the P₆₈₀ cation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Binding of carotenoids on reaction centers from Rhodopseudomonas sphaeroides R 26

The carotenoid-less reaction centers isolated from Rhodopseudomonas sphaeroides (strain R 26) bind pure all-trans spheroidene as well as spheroidenone in a nearly 1:1 molar ratio with respect to P-870. Neither β-carotene nor spirilloxanthin, both absent from wild-type Rps. sphaeroides, could be bound in appreciable amounts. Resonance Raman spectra of the carotenoidreaction center complex indicate that the carotenoid is bound as a cis isomer, its conformation being very close, although probably not identical, to that assumed by the carotenoid in the wild-type reaction centers. The electronic absorption spectra of the carotenoid-reaction center complexes are in good agreement with such a interpretation. When bound to the R 26 reaction centers, spheroidene displays light-induced absorbance changes identical in peak wavelengths and comparable in amplitudes to those observed in the wild-type reaction centers. Thus the binding of the carotenoid to the R 26 reaction centers most likely occurs at the same proteic site as in the wild-type reaction centers. This site shows selectivity towards the nature of carotenoids, and has the same sterical requirement as in the wild type, leading to the observed all-trans to cis isomerisation.     

Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: volume change, enthalpy, and entropy of electron-transfer reactions in manganese-depleted photosystem II core complexes

We have previously reported the thermodynamic data of electron transfer in photosystem I using pulsed time-resolved photoacoustics [Hou et al. (2001) Biochemistry 40, 7109-7116]. In the present work, using preparations of purified manganese-depleted photosystem II (PS II) core complexes from Synechocystis sp. PCC 6803, we have measured the DeltaV, DeltaH, and estimated TDeltaS of electron transfer on the time scale of 1 micros. At pH 6.0, the volume contraction of PS II was determined to be -9 +/- 1 A3. The thermal efficiency was found to be 52 +/- 5%, which corresponds to an enthalpy change of -0.9 +/- 0.1 eV for the formation of the state P680+Q(A-) from P680*. An unexpected volume expansion on pulse saturation of PS II was observed, which is reversible in the dark. At pH 9.0, the volume contraction, the thermal efficiency, and the enthalpy change were -3.4 +/- 0.5 A3, 37 +/- 7%, and -1.15 +/- 0.13 eV, respectively. The DeltaV of PS II, smaller than that of PS I and bacterial centers, is assigned to electrostriction and analyzed using the Drude-Nernst equation. To explain the small DeltaV for the formation of P680+Q(A-) or Y(Z*)Q(A-), we propose that fast proton transfer into a polar region is involved in this reaction. Taking the free energy of charge separation of PS II as the difference between the energy of the excited-state P680* and the difference in the redox potentials of the donor and acceptor, the apparent entropy change (TDeltaS) for charge separation of PS II is calculated to be negative, -0.1 +/- 0.1 eV at pH 6.0 (P680+Q(A-)) and -0.2 +/- 0.15 eV at pH 9.0 (Y(Z*)Q(A-)). The thermodynamic properties of electron transfer in PS II core reaction centers thus differ considerably from those of bacterial and PS I reaction centers, which have DeltaV of approximately -27 A3, DeltaH of approximately -0.4 eV, and TDeltaS of approximately +0.4 eV.     

The Contractile and Control Sites of Natural Actomyosin

The various contractile and control sites of natural actomyosin gel were studied by comparing the kinetics of ATP hydrolysis with those of gel contraction, measured as an increase in turbidity. Contraction of actomyosin gel seems to require the cooperative reaction of ATP (with Mg) at two different sites. One of these sites catalyzes the hydrolysis of ATP and most probably contributes the driving force for contraction; the binding of ATP to the other site appears to break certain links that retard movement of the gel components. At limiting concentrations of ATP, the rate of contraction seems to depend on the rate of breaking these links as well as on the rate of ATP hydrolysis. But when both sites are saturated, the rate of contraction appears to be limited only by the rate of ATP hydrolysis. In addition to these two contractile sites, there are also two different control sites. At one, the relaxing site, the binding of ATP with Mg inhibits ATP hydrolysis and gel contraction. At the other, the binding of calcium activates contraction by overcoming the inhibitory action of Mg and ATP at the relaxing site. This control system—inhibition by substrate and disinhibition by calcium—can be selectively inactivated by heat and reactivated by dithiothreitol, a disulfide-reducing agent. These observations on the isolated contractile system are discussed in relation to the contraction and relaxation of muscle.     

Volume contraction of isolated pea chloroplasts promoted by bivalent cations

1. Chloroplasts isolated from pea seedlings were incubated in sucrose–tris medium reinforced with salts of calcium, magnesium, manganese or iron, at concentrations up to 10mm. 2. Measurements of chloroplast-pellet volume and water content showed that the bivalent cations brought about a contraction in chloroplast volume and a loss of chloroplast water. This was further substantiated by density-gradient centrifugations. 3. Measurements of the light-scattering and apparent fluorescence of chloroplast suspensions confirmed this conclusion and eliminated the possibility of contraction being caused by centrifugal forces. 4. The uptake of ⁴⁵Ca²⁺ was measured and shown to be competitive with diluent Ca²⁺, Mg²⁺ or Mn²⁺ ions, indicating a mechanism of low specificity. 5. The chloroplast contraction was insensitive to light but could be made sensitive by the addition of ferric EDTA. This light-sensitivity was inhibited by added 3-(p-chlorophenyl)-1,1-dimethylurea and so probably involves the Hill reaction. 6. On the basis of these observations it is suggested that the process of contraction does not consume much energy, but that in light-activated contraction a previous step occurs that is conducive to contraction and that is energy-transducing. It is postulated that this step results in a local increase in concentration of bivalent ions, which promotes contraction.     

Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: a pulsed photoacoustic study of electron transfer in photosystem I reveals a similarity to bacterial reaction centers in both volume change and entropy

The thermodynamic properties of electron transfer in biological systems are far less known in comparison with that of their kinetics. In this paper the enthalpy and entropy of electron transfer in the purified photosystem I trimer complexes from Synechocystis sp. PCC 6803 have been studied, using pulsed time-resolved photoacoustics on the 1 micros time scale. The volume contraction of reaction centers of photosystem I, which results directly from the light-induced charge separation forming P(700+F(A)/F(B-) from the excited-state P700*, is determined to be -26 +/- 2 A3. The enthalpy of the above electron-transfer reaction is found to be -0.39 +/- 0.1 eV. Photoacoustic estimation of the quantum yield of photochemistry in the purified photosystem I trimer complex showed it to be close to unity. Taking the free energy of the above reaction as the difference of their redox potentials in situ allows us to calculate an apparent entropy change (TDeltaS) of +0.35 +/- 0.1 eV. These values of DeltaV and TDeltaS are similar to those of bacterial reaction centers. The unexpected sign of entropy of electron transfer is tentatively assigned, as in the bacterial case, to the escape of counterions from the surface of the particles. The apparent entropy change of electron transfer in biological system is significant and cannot be neglected.     

Design of a redox-linked active metal site: manganese bound to bacterial reaction centers at a site resembling that of photosystem II

Metals bound to proteins perform a number of crucial biological reactions, including the oxidation of water by a manganese cluster in photosystem II. Although evolutionarily related to photosystem II, bacterial reaction centers lack both a strong oxidant and a manganese cluster for mediating the multielectron and proton transfer needed for water oxidation. In this study, carboxylate residues were introduced by mutagenesis into highly oxidizing reaction centers at a site homologous to the manganese-binding site of photosystem II. In the presence of manganese, light-minus-dark difference optical spectra of reaction centers from the mutants showed a lack of the oxidized bacteriochlorophyll dimer, while the reduced primary quinone was still present, demonstrating that manganese was serving as a secondary electron donor. On the basis of these steady-state optical measurements, the mutant with the highest-affinity site had a dissociation constant of approximately 1 microM. For the highest-affinity mutant, a first-order rate with a lifetime of 12 ms was observed for the reduction of the oxidized bacteriochlorophyll dimer by the bound manganese upon exposure to light. The dependence of the amplitude of this component on manganese concentration yielded a dissociation constant of approximately 1 muM, similar to that observed in the steady-state measurements. The three-dimensional structure determined by X-ray diffraction of the mutant with the high-affinity site showed that the binding site contains a single bound manganese ion, three carboxylate groups (including two groups introduced by mutagenesis), a histidine residue, and a bound water molecule. These reaction centers illustrate the successful design of a redox active metal center in a protein complex.     

Reaction centers from triazine-resistant strains of Rhodopseudomonas sphaeroides: Localization of the mutation site by protein hybridization experiments

A procedure for dissociation and reconstitution of reaction centers has been used to hybridize reaction centers from three different herbicide-resistant mutant strains of Rhodopseudomonas sphaeroides with LM or H subunits derived from the native (susceptible) strains. All three mutant strains exhibited low rates of electron transfer. Hybridization of mutant reaction centers with native LM restored the high rates of electron transfer. Hybridization with native H did not. This procedure shows that the site of mutations in these mutant strains are on the LM unit.     

ATP und Ca2+ induzierte actomyosinartige Kontraktion glycerinisierter Makronuklei

Summary The addition of 2 mM-3 mM ATP to macronuclei ofParamecium bursaria suspended in a glycerol buffer medium causes a decrease in their volume up to 23% within 3 minutes. The infiltration medium must not only contain Ca²⁺, but must also be of low ionic strength for ATP to be effective. A slow, careful exchange of the glycerol medium for the contraction solution is also necessary. Ca²⁺ present alone in the standard contraction buffer can likewise induce a limited volume decrease; in the presence of low concentrations of Ca²⁺, Mg²⁺ shows no detectable effect on glycerinated nuclei. When the nuclear volume has been reduced by ATP in the presence of Ca²⁺, the addition of EGTA induces a reexpansion of the nuclei. Salyrgan, an organic mercurial, either prevents or abolishes the ATP-induced contraction. Other nucleotide triphosphates such as guanosine triphosphate (GTP), inosine triphosphate (ITP) or uridine triphosphate (UTP) likewise induce a volume decrease of the glycerinated macronuclei, but to a distinctly lesser extent than ATP.The results indicate that the volume decrease caused by the ATP-contraction solution is not a passive osmotic process. The resemblance to actomyosin contractions suggests that the volume decrease reported here might also be the result of the reaction of nuclear actomyosin and ATP.

     

Photoelectric currents across planar bilayer membranes containing bacterial reaction centers: the response under conditions of multiple reaction-center turnovers

The characteristics of the photocurrent response activated by continuous illumination of planar bilayer membranes containing bacterial reaction centers have been resolved by voltage clamp methods. The photocurrent response to a long light pulse consists of an initial spike arising from the fast, quasi-synchronous electron transfer from the reaction center bacteriochlorophyll dimer, BChl2, to the primary quinone QA. This is followed by a slow relaxation of the current to that promoted by secondary, asynchronous multiple electron transfers from the reduced cytochrome c through the reaction centers to the ubiquinone-10 pool. Currents derived from cytochrome c oxidation that occurs when cytochrome c is associated with the reaction center or when limited by diffusional interaction from solution are recognized. Changes of the ionic strength and pH in the aqueous phase, and the clamped membrane potential (+/- 150 mV), affect the electron-transfer rate between cytochrome c and BChl2. In contrast, the primary light-induced charge separation between BChl2 and QA, or electron transfer between QA on the ubiquinone pool are unaffected. During illumination of reaction center membranes supplemented with cytochrome c and a ubiquinone pool, there is a small but significant steady-state current which is considered to be caused by the re-oxidation of photoreduced quinone by molecular oxygen. In the dark, after illumination of reaction centers supplemented with cytochrome c and a ubiquinone pool, there is a small amount of reverse current resulting from the movement of charges back across the membrane. This reverse current is observed maximally after 400 ms illumination while prolonged illumination diminishes the effect. The source of this current is uncertain, but it is considered to be due to the flux of anionic semiquinone within the membrane profile; this may also be the species that interacts with oxygen giving rise to the steady-state current. It is postulated that when the reaction centers are contained in an alkane-containing phospholipid membrane, in contrast to the in vivo situation, the semiquinone anion formed in the QB site is not tightly bound to the site and can, by exchange-diffusion with the membrane-quinone pool, move away from the site and accumulate in the membrane. However, in the absence, more quantitative work superoxide anion, resulting from O2 interaction with semiquinone of QA, QB or pool cannot be excluded.     

Localization of light-induced structural changes in bacterial photosynthetic reaction centers

Light-induced structural changes in photosynthetic reaction centers from Rhodobacter sphaeroides were investigated using two approaches. Cu²⁺ was used as a paramagnetic structural probe. The EPR spectrum of Cu²⁺ incorporated into the metal-depleted reaction centers was affected by 1,10-phenanthroline, an electron transfer inhibitor substituting Q_B, which suggests a localization of Cu²⁺ in a vicinity of the Q _B ^– site. However, the spectrum was not influenced by low temperature (77 K) illumination of the sample which suggests that the copper ion position is not exactly the same as that of the iron ion. Freezing the reaction centers under illumination in the presence of potassium ferricyanide and 1,10-phenanthroline caused a change in the shape of the Cu²⁺ EPR spectrum in comparison to that of a sample frozen in darkness. These data indicate a change of the Cu²⁺ ligand symmetry owing to light-induced structural changes which are probably located near the acceptor side of the reaction center. Partial trypsinolysis of reaction centers was also used to locate the structural changes. Trypsin treatment in the dark and under illumination resulted in different peptide patterns as detected by gel electrophoresis and reverse-phase high-performance liquid chromatography. Partial amino-acid sequence analysis of a number of peptides, characteristic of either light- or dark-treated reaction centers, showed that they originated from the acceptor sides of the H and M subunits. The occurrence of light-induced structural differences in the H-subunit is consistent with the suggestion that it may be involved in regulating electron transfer in this part of the reaction center.     

Investigation on the detergent role in the function of secondary quinone in bacterial reaction centers.

In this paper are reported studies on the detergent role in isolated reaction centers (RC) from Rhodobacter sphaeroides, over a large range of lauryldimethylamino-N-oxide (LDAO) concentrations, in influencing the thermodynamics of the quinone exchange reaction as well as the protein aggregation. The occurrence of the quinone exchange reaction between the QB-binding site (where QB is the second quinone molecule of two in the RC) and the ubiquinone 0 dissolved in the different environments (water, LDAO micelles and detergent phase of the protein-detergent complex) has also been analyzed. Measurements carried out in QB-depleted RC to which exogenous quinone has been added show that the relative amplitudes of the slow and fast phase of the recombination reaction depend on this parameter. The overall amount of the restored QB-functionality is affected by the concentration of the LDAO in solution. Interpolation of the titration curves with a quadratic function obtained by simple considerations allowed the binding constant of UQ0 to the QB-binding site to be calculated. From the fitting procedure, the distribution of the quinone in the different environments present in solution was evaluated, indicating that the exchange reaction can take place only between the QB-site and the detergent phase. The dependence of the quinone pool size upon the volume of the phase in which the interacting quinone is solubilized is also discussed. The increasing difficulty in saturating the QB-pocket above the LDAO critical micellar concentration is finally related to the association of protein-detergent complexes to form large protein clusters.     

Formation of a semiquinone at the QB site by A- or B-branch electron transfer in the reaction center from Rhodobacter sphaeroides

In Rhodobacter sphaeroides reaction centers containing the mutation Ala M260 to Trp (AM260W), transmembrane electron transfer along the A-branch of cofactors is prevented by the loss of the QA ubiquinone. Reaction centers that contain this AM260W mutation are proposed to photoaccumulate the P(+)QB- radical pair following transmembrane electron transfer along the B-branch of cofactors (Wakeham, M. C., Goodwin, M. G., McKibbin, C., and Jones, M. R. (2003) Photoaccumulation of the P(+)QB- radical pair state in purple bacterial reaction centers that lack the QA ubiquinone. FEBS Lett. 540, 234-240). The yield of the P(+)QB- state appears to depend upon which additional mutations are present. In the present paper, Fourier transform infrared (FTIR) difference spectroscopy was used to demonstrate that photooxidation of the reaction center's primary donor in QA-deficient reaction centers results in formation of a semiquinone at the QB site by B-branch electron transfer. Reduction of QB by the B-branch pathway still occurs at 100 K, with a yield of approximately 10% relative to that at room temperature, in contrast to the QA- to QB reaction in the wild-type reaction center, which is not active at cryogenic temperatures. These FTIR results suggest that the conformational changes that "gate" the QA- to QB reaction do not necessarily have the same influence on QB reduction when the electron donor is the HB anion, at least in a minority of reaction centers.     

Properties of reaction centers of Rhodopseudomonas sphaeroides in dried gelatin films.: Linear dichroism and low temperature spectra

Methods of preparing dried gelatin films containing purified reaction centers of Rhodopseudomonas sphaeroides are described. The spectral properties of reaction centers in solution are essentially maintained in dried gelatin films. These films are uniform and have excellent optical properties, showing little particulate scattering at temperatures down to about 4K. Film contraction on cooling to 90K is less than 1% in linear dimension. Linear dichroism spectra are reported for films at room and low temperature. Reaction centers show a moderate amount of linear dichroism in unstretched gelatin films; the magnitude of the linear dichroism becomes much greater when the films are stretched. In stretched films, linear dichroic ratios ($\text{A}{}_{\mathrm{\parallel }}\text{A}{}_{\mathrm{\perp }}$; absorbance measured with electric vector parallel and perpendicular to stretching direction) between 1.7 and 2.2 were obtained for the 860 nm absorption band of the bacteriochlorophyll component that undergoes primary photooxidation. The relative polarizations of light-induced absorption changes of reaction centers in stretched films are similar to those reported by Vermeglio and Clayton ((1976) Biochim. Biophys. Acta 449, 500–515) and support their hypothesis that absorbance decreases, maximal near 860 and 810 nm, and an increase near 790 nm are associated with the respective disappearance and appearance of discrete bands characteristic of the reduced and oxidized bacteriochlorophyll dimer. This interpretation is also supported by the polarization of the absolute absorption spectrum near 810 and 860 nm. An absorption band near 540 nm, ascribed to the Q_x transitions of two molecules of bacteriopheophytin in the reaction center, is split at low temperatures into two bands having similar polarizations. This splitting is probably not due to exciton coupling of the two molecules, since excition theory predicts different polarizations.     

Photooxidation of mitochondrial cytochrome c by isolated bacterial reaction centers: Evidence for tight-binding and diffusional pathways

The binding of horse heart mitochondrial cytochrome c to isolated reaction centers from Rhodopseudomonas sphaeroides is described. The kinetics of photooxidation of cytochrome c following a short actinic flash is compared to the expected binding state of the cytochrome at various concentrations and at different ionic strengths. At low ionic strength a very tight binding site (K_D10^-8 M) is apparent which is nonfunctional with respect to electron donation to the bound reaction center. This tightly bound cytochrome can react with another reaction center in a diffusion limited, second order process. A weaker binding site (K_D0.3 · 10^-6 M) is also boserved which is associated with rapid, first order electron transfer from cytochrome to reaction center. Both binding processes are weakened in the presence of salt and there is no detectable binding in 100 mM NaCl. Under such conditions cytochrome oxidation is entirely a diffusional, second order process. However, analysis of the flash intensity dependence of the extent of cytochrome oxidation, by the method of van Grondelle (van Grondelle, R. (1978) Ph.D. Thesis, State University, Leiden) indicated that the cytochrome was not freely mobile even in 100 mM NaCl, at least in the sense that reduced cytochrome only slowly dissociates from unactivated reaction centers. An overall kinetic/equilibrium scheme for cytochrome c binding and photooxidation by reaction centers is presented. This is very similar to that described earlier for cytochrome c₂ (Overfield, R.E., Wraight, C.A. and DeVault, D. (1979) FEBS Lett. 105, 137–142), but the tight binding site and associated diffusion controlled oxidation is unique to cytochrome c.Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement.     

New herbicide-binding site in the photosynthetic electron-transport chain: Competitive herbicide binding at the photosystem I phylloquinone-(vitamin K1)-binding site

The binding of herbicides to the phylloquinone-(primary electron acceptor A₁)-binding site in green plant photosystem (PS) I reaction centers is shown. Dissociation constants (K_d) of various herbicides to the phylloquinone-binding site were estimated by analyzing their competitive inhibition of the reconstitution of the phylloquinone analogue, menadione (vitamin K₃), to the phylloquinone-extracted spinach PS I particles. The phylloquinone-binding site was found to bind o-phenanthroline (K_d = 1.2 × 10⁻⁴ M), but only weak binding was observed with atrazine (K_d > 10⁻² M), although both are known to bind specifically to the quinone-(Q_B)-binding site in reaction centers of purple photosynthetic bacteria or PS II. The inhibitors of the cytochrome b/c₁(ƒ) complex, myxothiazol (K_d=9.5 × 10⁻⁶ M) or antimycin A (K_d = 2.8 × 10⁻⁶ M), also strongly bound to the phylloquinone site. This is the first report showing that the PS I reaction center complex also has a herbicide-binding site, although the site is probably not sensitive in vivo to these herbicides due to its higher affinity for phylloquinone than herbicides. The inhibitor specificity of the PS I phylloquinone site is different from that of the other quinone-functioning sites in the photosynthetic or respiratory electron-transfer chain, suggesting it to have a unique structure.     

Monomeric bacteriochlorophyll is required for the triplet energy transfer between the primary donor and the carotenoid in photosynthetic bacterial reaction centers

Reaction centers from the carotenoidless mutant Rb. sphaeroides R26 were treated with sodium borohydride which is known to remove one of the accessory monomeric bacteriochlorophylls (BB). Subsequently, the carotenoid, spheroidene, was incorporated into the modified reaction centers. It is demonstrated by optical absorption and circular dichroism experiments that spheroidene, reconstituted into the sodium borohydride-treated Rb. sphaeroides R26 reaction centers, is bound in a single site, in the same environment and with the same structure as spheroidene reconstituted into untreated (native) Rb. sphaeroides R26 reaction centers. Transient optical and electron spin resonance spectroscopic data indicate that unless the accessory BB is present, the primary donor-to-carotenoid triplet energy transfer reaction is inhibited. These observations provide direct evidence for the involvement of the accessory BB in the triplet energy transfer pathway.     

A kinetic model that explains the effect of inorganic phosphate on the mechanics and energetics of isometric contraction of fast skeletal muscle

A conventional five-step chemo-mechanical cycle of the myosin–actin ATPase reaction, which implies myosin detachment from actin upon release of hydrolysis products (ADP and phosphate, Pi) and binding of a new ATP molecule, is able to fit the [Pi] dependence of the force and number of myosin motors during isometric contraction of skeletal muscle. However, this scheme is not able to explain why the isometric ATPase rate of fast skeletal muscle is decreased by an increase in [Pi] much less than the number of motors. The question can be solved assuming the presence of a branch in the cycle: in isometric contraction, when the force generation process by the myosin motor is biased at the start of the working stroke, the motor can detach at an early stage of the ATPase cycle, with Pi still bound to its catalytic site, and then rapidly release the hydrolysis products and bind another ATP. In this way, the model predicts that in fast skeletal muscle the energetic cost of isometric contraction increases with [Pi]. The large dissociation constant of the product release in the branched pathway allows the isometric myosin–actin reaction to fit the equilibrium constant of the ATPase.     

Formation of cardiomotor interrelations in rat ontogenesis

Age-related transformations and nature of short-term positive and negative oscillations of cardiac contraction rhythm were studied in comparison with periodical spontaneous somatomotor activity bursts in rats for the first 3 weeks after birth. It has been established that both accelerations and decelerations can be independent, precede motor excitation or appear as a consequence of muscle contraction. The main role in genesis of decelerations is played by parasympathetic mechanisms whose action is blocked by atropine. Activation of vagus cardiomotor centers can ether occur spontaneously by producing independent declerations or appear as reaction to motor excitation. Genesis of accelerations is connected with activity of sympathetic nervous system, as they are inhibited under conditions of desympathization or block of adrenoreceptors. The main role in realization of accelerations is played by somatosympathetic reflex, but connection with motor activity can also be effected by the principle of pre-tuning to expected movement.     

Effect of inhibitors,redox state and isoprenoid chain length on the affinity of ubiquinone for the secondary acceptor binding site in the reaction centers of photosynthetic bacteria

Quinone and inhibitor binding to Rhodopseudomonas sphaeroides (R-26 and GA) reaction centers were studied using spectroscopic methods and by direct adsorption of reaction centers onto anion exchange filters in the presence of ¹⁴C-labelled quinone or inhibitor. These measurements show that as secondary acceptor, Q_B, ubiquinone (UQ) is tightly bound in the semiquinone form and loosely bound in the quinone and quinol forms. The quinol is probably more loosely bound than the quinone. o-Phenanthroline and terbutryn, a triazine inhibitor, compete with UQ and with each other for binding to the reaction center. Inhibition by o-phenanthroline of electron transfer from the primary to the secondary quinone acceptor (Q_A to Q_B) occurs via displacement of UQ from the Q_B binding site. Displacement of UQ by terbutryn is apparently accessory to the inhibition of electron transfer. Terbutryn binding is lowered by reduction of Q_B to Q^?_B but is practically unaffected by reduction of Q_A to Q^?_A in the absence of Q_B. UQ-9 and UQ-10 have a 5- to 6-fold higher binding affinity to the Q_B site than does UQ-1, indicating that the long isoprenoid chain facilitates the binding to the Q_B site.