Oxidation-reduction potentials of bound iron-sulfur proteins of photosystem I

Electron paramagentic resonance studies of photosynthetic electron transport: photoreduction of ferredoxinand membrane-bound iron-sulfur centers

Electron paramagnetic resonance spectrometry was used to investigate, at physiological temperatures, light-induced electron transport from membrane-bound iron-sulfur components (bound ferredoxin) to soluble ferredoxin and NADP(+) in membrane fragments (from the blue-green alga, Nostoc muscorum) that had high rates of electron transport from water to NADP(+) and from an artificial electron donor, reduced dichlorophenolindophenol (DCIPH(2)) to NADP(+). Illumination at 20 degrees resulted in the photoreduction of membrane-bound iron-sulfur centers A and B. Photoreduction by water gave electron paramagnetic resonance signals of both centers A and B; photoreduction by DCIPH(2) was found to generate a strong electron paramagnetic signal of only center B. When water was the reductant, the addition and photoreduction of soluble ferredoxin generated additional signals characteristics of soluble ferredoxin without causing a decrease in the amplitude of the signals due to centers A and B. The further addition of NADP(+) (and its photoreduction) greatly diminished signals due to the bound iron-sulfur centers and to soluble ferredoxin. An outflow of electrons from center B to soluble ferredoxin and NADP(+) was particularly pronounced when DCIPH(2) was the reductant. These observations provide the first evidence for a light-induced electron transport between membrane-bound iron-sulfur centers and ferredoxin-NADP(+). The relationship of these observations to current concepts of photosynthetic electron transport is discussed.

Appearance of Membrane-bound Iron-Sulfur Centers and the Photosystem I Reaction Center during Greening of Barley Leaves

Dark-grown barley (Hordeum vulgare) etioplasts were examined for their content of membrane-bound iron-sulfur centers by electron paramagnetic resonance spectroscopy at 15K. They were found to contain the high potential iron-sulfur center characterized (in the reduced state) by an electron paramagnetic resonance g value of 1.89 (the "Rieske" center) but did not contain any low potential iron-sulfur centers. Per mole of cytochrome f, dark-grown etioplasts and fully developed chloroplasts had the same content of the Rieske center. During greening of etioplasts under continuous light, low potential bound iron-sulfur centers appear. In addition, the photosystem I reaction center, as measured by the photooxidation of P700 at 15K, also became functional; during greening the appearance of a photoreducible low potential iron-sulfur center paralleled the appearance of P700 photoactivity.These findings indicate the close association of the low potential iron-sulfur centers with the photosystem I reaction center; they also support the concept that the development of stable charge separation in the photosystem I reaction center requires, in addition to P700, a low potential iron-sulfur center.

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

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'.

The isolation and characterization of a new iron-sulfur protein from photosynthetic membranes

A new iron-sulfur protein, distinct from the soluble chloroplast ferredoxin, was isolated from chloroplast membranes. The isolated protein, purified to homogeneity, had a molecular weight of about 8000 and 4 atoms of iron and 4 inorganic sulfides per mole. Its absorption spectrum had a broad absorbance band in the 400 nm region, a shoulder at approximately 310 nm, and a peak around 280 nm. The absorbance ratio A(400) to A(280) was 0.55. The electron paramagnetic resonance spectrum (measured at 12 degrees K) of the reduced protein was similar to that of other reduced iron-sulfur proteins, showing a major resonance line at g = 1.94. The isolated protein, when photoreduced by spinach chloroplasts, can in turn transfer electrons to mammalian cytochrome c. However, the photoreduced protein cannot replace soluble ferredoxin in NADP(+) reduction because of its apparent inability to interact with the chloroplast enzyme, ferredoxin-NADP(+) reductase. The relation of the isolated iron-sulfur protein to the bound ferredoxin that acts as the primary electron acceptor in Photosystem I is discussed.

Primary photochemical reactions in chloroplast photosynthesis

Photosynthesis begins with the absorption of light energy and this absorbed energy is transferred to special sites, termed reaction centres. At these sites, the light energy is transformed into chemical products through an oxidation-reduction reaction that generates the primary reactants, an oxidized pigment molecule (P+) and a reduced electron acceptor (A–) (Clayton, 1972). The subsequent reactions of these species in the dark ultimately results in the formation of chemical products required for the fixation of CO2. In this essay we will discuss the nature of the primary reactants generated in the light reactions of chloroplast photosynthesis, stressing recent advances in the identification and characterization of such reactants.