Quantitative electron-paramagnetic-resonance measurements of the electron-transfer components of the photosystem-I reaction centre. The reaction-centre chlorophyll (P700), the primary electron acceptor X and bound iron-sulphur centre A

An e.p.r. spectrum of the reduced form of the electron-transport component (X), thought to be the primary electron acceptor of Photosystem I, was obtained. By using line-shape simulations of this component and the free-radical e.p.r. signal I of the oxidized reaction-centre chlorophyll (P700), it was possible to determine the ratio of the number of electron spins to which these signals correspond in Photosystem-I particles under a variety of conditions. On illumination at cryogenic temperatures of Photosystem-I preparations, in which both bound iron-sulphur centres A and B were reduced, the measured ratio of free radical to component X varied between 1.04 and 2.23, with an average value of 1.54 +/- 0.18 where a Gaussian line-shape is assumed for the component-X signal in the simulation. The error in this measurement is estimated to be up to 50%. In a similar way component X and centre A of the bound iron-sulphur protein were quantified, the ratio between these two components varying between 1.26 and 0.61 with an average value of 0.75 +/- 0.06. These results indicate that the quantitative relationship, in terms of net electron spins, between centre A, component X and P700 is of the order to be expected if component X is indeed the primary electron acceptor in Photosystem I and a component of the photosynthetic electron-transport chain.

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The site of electron transport inhibition by bentazon (3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide) in isolated chloroplasts

Canadian Journal of Botany ◽

10.1139/b82-057 ◽

1982 ◽

Vol 60 (4) ◽

pp. 409-412 ◽

Cited By ~ 8

Author(s):

Rungsit Suwanketnikom ◽

Kriton K. Hatzios ◽

Donald Penner ◽

Duncan Bell

Keyword(s):

Electron Transport ◽

Photosystem Ii ◽

Photosystem I ◽

Electron Acceptor ◽

Photosynthetic Electron Transport ◽

Primary Electron ◽

Photochemical Reactions ◽

Spinacea Oleracea ◽

Primary Electron Acceptor ◽

Isolated Chloroplasts

The effect of bentazon (3-isopropyl-1H-2,1,3-benzathiadiazin-(4)3H-one 2,2-dioxide) on various photochemical reactions of isolated spinach (Spinacea oleracea L.) chloroplasts was studied at concentrations 0, 5, 15, 45, and 135 μM. Bentazon at a concentration of 135 μM strongly inhibited uncoupled electron transport from water to ferricyanide or to methylviologen with inhibition percentages greater than 90%. Photosystem II mediated electron transport from water to oxidized diaminodurene, with 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) blocking photosystem I, was also strongly inhibited by bentazon at 135 μM but less with lower concentrations of bentazon. Photosystem I mediated transfer of electrons from diaminodurene to methylviologen, with 3,4-dichlorophenyl-1,1-dimethylurea (DCMU) blocking photosystem II, was not inhibited by bentazon at any concentration examined. Transfer of electrons from catechol to methylviologen in hydroxylamine-treated chloroplasts was inhibited by bentazon, and the inhibition percentages were again concentration dependent. The data indicate that the site of bentazon inhibition of the photosynthetic electron transport is at the reducing side of photosystem II, between the primary electron acceptor Q and plastoquinone.

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Benzofuroxan as Electron Acceptor at Photosystem I

Zeitschrift für Naturforschung C ◽

10.1515/znc-1987-11-1219 ◽

1987 ◽

Vol 42 (11-12) ◽

pp. 1265-1268

Author(s):

B. Lotina-Hennsen ◽

A . Garcia ◽

M. Aguilar ◽

M. Albores

Keyword(s):

Electron Transport ◽

Photosystem I ◽

Electron Acceptor ◽

Primary Electron ◽

Midpoint Potential ◽

Primary Electron Acceptor

The midpoint potential of BFO, the sensitivity of its photoreduction to DCM, DBMIB and KCN, and the photosystem I activity, suggest that the photoreduction of BFO in the chloroplast is at the primary electron acceptor x of photosystem I. and is irreversible. Rates of electron transport are similar in basal phosphorylating or uncoupled conditions although electron transport is coupled to photophosphorylation.

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The properties of the primary electron acceptor in the Photosystem I reaction centre of spinach chloroplasts and its interaction with P700 and the bound ferredoxin in various oxidation-reduction states

Biochemical Journal ◽

10.1042/bj1580071 ◽

1976 ◽

Vol 158 (1) ◽

pp. 71-77 ◽

Cited By ~ 61

Author(s):

M C W Evans ◽

C K Sihra ◽

R Cammack

Keyword(s):

Low Temperature ◽

Photosystem I ◽

Electron Acceptor ◽

Low Temperatures ◽

Electron Paramagnetic Resonance Spectroscopy ◽

Sodium Dithionite ◽

Primary Electron ◽

Resonance Spectroscopy ◽

Reaction Centre ◽

Primary Electron Acceptor

The properties of the component ‘X’ identified as the primary electron acceptor of Photosystem I in spinach was investigated by electron-paramagnetic-resonance spectroscopy and the complete spectrum obtained for the first time. Component ‘X’ has gx = 1.78, gy = 1.88 and gz = 2.08; it can be observed only at very low temperatures (8-13K) and high microwave powers. Component X was identified in Photosystem I particles prepared with the French press or with Triton X-100. In samples reduced with ascorbate, illumination at low temperatures results in the photo-oxidation of P700 and reduction of a bound iron-sulphur protein; this is irreversible at low temperature. In samples in which the iron-sulphur proteins are reduced by sodium dithionite, illumination at low temperature results in the oxidation of P700 and the reduction of component ‘X’; this is reversible at low temperature. The light-induced P700 signal is the same size with either ascorbate or dithionite as reducing agent, showing that all of the P700 involved in reduction of bound ferredoxin also functions in the reduction of component ‘X’.

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