The Kinetics of Reactions in and Near the Cytochrome b/f Complex of Chloroplasts. II. Cytochrome b-563 Reduction

1989 ◽  
Vol 16 (4) ◽  
pp. 353 ◽  
Author(s):  
AB Hope ◽  
J Liggins ◽  
DB Matthews
Keyword(s):  
Electron Transfer ◽  
Cytochrome B ◽  
Rate Constant ◽  
Apparent Rate Constant ◽  
Rate Limiting Step ◽  
Q Cycle ◽  
Proton Transfers ◽  
Wide Range ◽  
Dimethyl Urea ◽  
Rate Limiting

The reduction of cytochrome b-563 was measured following flash-induced electron transfer from duroquinol to methyl viologen, in the presence of 3,(3,4-dichlorophenyl)-1,1-dimethyl urea and 1 �M nonactin or 1 �M valinomycin (plus 10 mM K+). The apparent rate constant of this reduction (nonactin present) increased from about 100 s-1 to 460 s-1 as the external concentration of duroquinol was varied from 0.01 to 0.5 mM. The corresponding maximum extent of reduction of cytochrome b-563 varied from 0.13 to 0.27 molecules per b/f complex. Rate constants in the presence of valinomycin were lower at all concentrations of duroquinol by a factor of about 1.5. The mean enthalpy of activation calculated from Arrhenius plots of apparent rate constant for cytochrome b-563 reduction was 60 kJ mol-1, for temperature variation between 23 and 4°C. The above, and further data in oxidising conditions, and with added 2-n-nonyl-4-hydroxyquinoline N-oxide, together with data on proton deposition, were compared with the predictions of a kinetic model. In this model, flash-generated oxidised plastocyanin oxidised Rieske centres during random diffusion, and plastoquinol reduced the Rieske centres and cytochrome b-563 sequentially; subsequent electron and proton transfers followed those in a Q-cycle. Many observations were predicted by the model, in which the rate-limiting step was the first electron transfer from plastoquinol to the Rieske centre, subsequent steps being much faster. The rate and extent of reduction of cytochrome b-563 were fully consistent with a reaction between it and a radical form of plastoquinone formed after oxidation of the latter by Rieske centres (referred to as 'oxidant-induced reduction'), under a wide range of conditions.

scholarly journals Coassembly of SecYEG and SecA Fully Restores the Properties of the Native Translocon

2018 ◽  
Vol 201 (1) ◽  
Author(s):  
Priya Bariya ◽  
Linda L. Randall
Keyword(s):  
Lipid Bilayers ◽  
Rate Constant ◽  
Apparent Rate Constant ◽  
Membrane Vesicles ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Secretory System ◽  
Rate Limiting

ABSTRACTIn all cells, a highly conserved channel transports proteins across membranes. InEscherichia coli, that channel is SecYEG. Many investigations of this protein complex have used purified SecYEG reconstituted into proteoliposomes. How faithfully do activities of reconstituted systems reflect the properties of SecYEG in the native membrane environment? We investigated by comparing threein vitrosystems: the native membrane environment of inner membrane vesicles and two methods of reconstitution. One method was the widely used reconstitution of SecYEG alone into lipid bilayers. The other was our method of coassembly of SecYEG with SecA, the ATPase of the translocase. For nine different precursor species we assessed parameters that characterize translocation: maximal amplitude of competent precursor translocated, coupling of energy to transfer, and apparent rate constant. In addition, we investigated translocation in the presence and absence of chaperone SecB. For all nine precursors, SecYEG coassembled with SecA was as active as SecYEG in native membrane for each of the parameters studied. Effects of SecB on transport of precursors faithfully mimicked observations madein vivo. From investigation of the nine different precursors, we conclude that the apparent rate constant, which reflects the step that limits the rate of translocation, is dependent on interactions with the translocon of portions of the precursors other than the leader. In addition, in some cases the rate-limiting step is altered by the presence of SecB. Candidates for the rate-limiting step that are consistent with our data are discussed.IMPORTANCEThis work presents a comprehensive quantification of the parameters of transport by the Sec general secretory system in the threein vitrosystems. The standard reconstitution used by most investigators can be enhanced to yield six times as many active translocons simply by adding SecA to SecYEG during reconstitution. This robust system faithfully reflects the properties of translocation in native membrane vesicles. We have expanded the number of precursors studied to nine. This has allowed us to conclude that the rate constant for translocation varies with precursor species.

The Kinetics of Reactions in and Near the Cytochrome b/f Complex of Chloroplast Thylakoids. I. Proton Deposition

1988 ◽  
Vol 15 (5) ◽  
pp. 695 ◽  
Author(s):  
AB Hope ◽  
J Liggins ◽  
DB Matthews
Keyword(s):  
Rate Constant ◽  
Bimolecular Reaction ◽  
Plastoquinone Pool ◽  
Rate Limiting Step ◽  
Fixed Concentration ◽  
Wide Range ◽  
Entropy Of Activation ◽  
Electron Donation ◽  
Rate Limiting ◽  
Kinetics Of

The kinetics of proton deposition in the intrathylakoid spaces of pea chloroplasts were measured under a wide range of conditions. With duroquinol added to reduce the plastoquinone pool, and 3-(3,4-dichlorophenyl)-1,1-dimethylurea added to inhibit photosystem II, but no ionophore present, the proton deposition, attributed to plastoquinol oxidation, was biphasic. About half the deposition had an apparent rate constant (k) of 150-200 s-1, the other half about 10 s-1. Valinomycin or nonactin (<0.1 �M) plus potassium ions made the deposition almost monophasic, with k = 140 s-1. When the state of reduction of the plastoquinone pool was varied by the addition of varied concentrations of duroquinol, in the presence of 1 �M nonactin, k for proton deposition varied from about 20 (0.01 mM duroquinol) up to a maximum of 140 s-1 (0.5 mM duroquinol). When temperature was varied between 4 and 23°C, with 1 �M nonactin, an Arrhenius plot of ln(k) for proton deposition was linear; the activation enthalpy was 67 kJ mol-1, the entropy of activation, 23 J K-1 mol-1. The data are analysed in terms of a bimolecular reaction between a varying concentration of plastoquinol and a fixed concentration of oxidised Rieske centre. The results are consistent with a rate constant, for the first electron donation by plastoquinol, of 28 s-1 (the rate-limiting step), followed by a relatively fast second electron donation to cytochrome b563 (low potential), followed by deposition of two protons. The speed of the second proton deposition is dependent on the membrane potential difference.

Effect of Medium on Acid-Catalyzed Decomposition of N-(Phenylazo)-Substituted Nitrogen Heterocycles

1999 ◽  
Vol 64 (10) ◽  
pp. 1654-1672 ◽  
Author(s):  
Miroslav Ludwig ◽  
Iva Bednářová ◽  
Patrik Pařík
Keyword(s):  
Rate Constant ◽  
Principal Component ◽  
Catalyst Particle ◽  
Pivalic Acid ◽  
Linear Regression Method ◽  
Catalytic Rate Constant ◽  
Rate Limiting Step ◽  
Acid Catalyzed ◽  
Rate Limiting ◽  
Kinetics Of

Four N-(phenylazo)-substituted saturated nitrogen heterocyclics were synthesized and their structure was confirmed by 1H and 13C NMR spectroscopy. The kinetics of their acid-catalyzed decomposition were studied at various concentrations of the catalyst (pivalic acid) in 40, 30, and 20% (v/v) aqueous ethanol at 25 °C. The values obtained for the observed rate constants were processed by the non-linear regression method according to the suggested kinetic models and by the method of principal component analysis (PCA). The interpretation of the results has shown that the acid-catalyzed decomposition of the heterocyclics under the conditions used proceeds by the mechanism of general acid catalysis, the proton being the dominant catalyst particle of the rate-limiting step. The decrease in the observed rate constant at higher concentrations of the catalyst was explained by the formation of a non-reactive complex composed of the undissociated acid and the respective N-(phenylazo)heterocycle. The effect of medium and steric effect of the heterocyclic moiety on the values of catalytic rate constant are discussed.

scholarly journals Atomically manipulated proton transfer energizes water oxidation on silicon carbide photoanodes

2018 ◽  
Vol 6 (47) ◽  
pp. 24358-24366 ◽  
Author(s):  
Hao Li ◽  
Huan Shang ◽  
Yuchen Shi ◽  
Rositsa Yakimova ◽  
Mikael Syväjärvi ◽  
...  
Keyword(s):  
Silicon Carbide ◽  
Electron Transfer ◽  
Proton Transfer ◽  
Water Oxidation ◽  
Proton Coupled Electron Transfer ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting

Preferential exposure of Si-face of SiC will mechanistically shift the rate limiting step of water oxidation from sluggish proton-coupled electron transfer on C-face to a more energy-favorable electron transfer.

scholarly journals Glucose transport as rate-limiting step in the growth of Escherichia coli on glucose

1976 ◽  
Vol 156 (2) ◽  
pp. 477-480 ◽  
Author(s):  
D Herbert ◽  
H L Kornberg
Keyword(s):  
Escherichia Coli ◽  
Continuous Culture ◽  
Glucose Transport ◽  
Growth Rates ◽  
Glucose Limitation ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Wide Range ◽  
Rate Limiting

Over a wide range of growth rates, two strains of Escherichia coli growing aerobically in continuous culture under glucose limitation utilized glucose at rates identical with those at which cells harvested from the chemostats transported [14C]glucose.

scholarly journals Using pH dependence to understand mechanisms in electrochemical CO reduction

2021 ◽  
Author(s):  
Georg Kastlunger ◽  
Lei Wang ◽  
Nitish Govindarajan ◽  
Hendrik H. Heenen ◽  
Stefan Ringe ◽  
...  
Keyword(s):  
Ph Dependence ◽  
Modern Technology ◽  
Catalyst Design ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Wide Range ◽  
Alkaline Conditions ◽  
Novel Catalyst ◽  
Co Reduction ◽  
Rate Limiting

Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path towards closing the carbon cycle in modern technology. To this day, however, the reaction mechanisms towards the plethora of products are disputed, complicating the search for novel catalyst materials. In order to conclusively identify the rate-limiting steps in CO reduction on Cu, we analyzed the mechanisms on the basis of constant potential DFT kinetics and experiments at a wide range of pH values (3 - 13). We find that *CO dimerization is energetically favoured as the rate limiting step towards multi-carbon products. This finding is consistent with our experiments, where the reaction rate is nearly unchanged on an SHE potential scale, even under acidic conditions. For methane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from the first protonation step in acidic/neutral conditions to a later one in alkaline conditions. We also show, through a detailed analysis of the microkinetics, that a surface combination of *CO and *H is inconsistent with the measured current densities and Tafel slopes. Finally, we discuss the implications of our understanding for future mechanistic studies and catalyst design.

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