Protein phosphorylation in regulation of photosynthesis

The primary reaction of photosynthesis is light-driven charge separation, carried out in reaction centres, which are complexes of integral membrane proteins and cofactors. The recent determination of the crystal structures of the reaction centres of two photosynthetic bacteria provides a basis for a quantitative understanding of the primary electron transfer processes of photosynthesis.

Structure and function of bacterial photosynthetic reaction centres

Photoinhibition is a state of physiological stress that occurs in all oxygen evolving photosynthetic organisms exposed to light. The primary damage occurs within the reaction center of Photosystem II (PS II). While irreversible photoinduced damage to PS II occurs at all light intensities, the efficiency of photosynthetic electron transfer decreases markedly only when the rate of damage exceeds the rate of its repair, which requires de novo PS II protein synthesis. Photoinhibition has been studied for over a century using a large variety of biochemical, biophysical and genetic methodologies. The discovery of the light induced turnover of a protein, encoded by the plastid psbA gene (the D1 protein), later identified as one of the photochemical reaction center II proteins, has led to the elucidation of the underlying mechanism of photoinhibition and to a deeper understanding of the PS II ‘life cycle.’

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Photoinhibition – a historical perspective

3. Excitation transfer in the PSU 18 3.1 Electronic excitations of BChls 22 3.1.1 Individual BChls 22 3.1.2 Rings of BChls 22 3.1.2.1 Exciton states 22 3.1.3 Effective Hamiltonian 24 3.1.4 Optical properties 25 3.1.5 The effect of disorder 26 3.2 Theory of excitation transfer 29 3.2.1 General theory 29 3.2.2 Mechanisms of excitation transfer 32 3.2.3 Approximation for long-range transfer 34 3.2.4 Transfer to exciton states 35 3.3 Rates for transfer processes in the PSU 37 3.3.1 Car U BChl transfer 37 3.3.1.1 Mechanism of Car U BChl transfer 39 3.3.1.2 Pathways of Car U BChl transfer 40 3.3.2 Efficiency of Car U BChl transfer 40 3.3.3 B800–B850 transfer 44 3.3.4 LH-II U LH-II transfer 44 3.3.5 LH-II U LH-I transfer 45 3.3.6 LH-I U RC transfer 45 3.3.7 Excitation migration in the PSU 46 3.3.8 Genetic basis of PSU assembly 49

/pdf/photosynthetic-apparatus-of-purple-bacteria-yh6dklgjn6.pdf

Photosynthetic apparatus of purple bacteria

‘Tunnelling’ is the metaphorical name given to the process, possible in quantum mechanics, but not in classical mechanics, whereby a particle can disappear from one side of a potential-energy barrier and appear on the other side without having enough kinetic energy to mount the barrier. One can think of this as a manifestation of the wave-nature of particles. The wavelength is larger if a particle is lighter. In particular electrons, being very light compared to atoms, have wavelengths as large or larger than atoms at energies found in the valence shells of molecules. Thus, they easily ooze through and around atoms and molecules. We are also concerned with the tunnelling of heavy particles: nuclei, atoms, molecules.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S003358350000175X

Quantum mechanical tunnelling in biological systems

Isolation of a reaction center fraction from Rhodopseudomonas spheroides

— A reaction-center fraction isolated from Rhodopseudomonas spheroides chromatophores exhibits light-induced changes in its optical and electron spin-resonance (ESR) spectra. In particular, a bleaching at 870nm (P870) has been found to be closely correlated with the appearance of an ESR signal with a g factor of 2.0025 and a peak-to-peak line width of 10 G. The ESR signal is indistinguishable from light-induced signals found in chromatophores or whole cells.



A careful measurement of the spin concentration showed that the ratio of the light-induced spins to bleached P870 molecules is 1.1 ± 0.1. In addition the formation and decay kinetics are identical within experimental error under a variety of experimental conditions.



Previous work has shown that P870 is a bacteriochlorophyll molecule in a specialized environment and that the bleaching signifies oxidation. The present work provides strong evidence that the photo-bleaching of P870 produces the radical cation of bacteriochlorophyll, P870+, and that this radical is the source of the ESR signal in whole cells.



The quantum yield for the bleaching of P870 in reaction centers has been measured, using actinic light of wavelengths 880, 800, 760 and 680 nm. For light absorbed at 880 or 800 nm the efficiency is close to 100 per cent. In a coupled reaction, the oxidation of mammalian cytochrome c by P870+ proceeds with nearly the same efficiency.



The above results place definite limits on the possibilities for the identity of the primary acceptor. These possibilities are discussed.

An identification of the radical giving rise to the light‐induced electron spin resonance signal in photosynthetic bacteria

Photosynthetic bacteria contain bacteriochlorophyll (BChl) that absorbs light and delivers the resulting singlet excitation energy to photochemical reaction centers.' The reaction centers, which have been isolated in a form free of lightharvesting BChl,'-3 contain a photochemically specialized BChl which in Rhodopseudomonas spheroides is called P870 after its absorption maximum near 870 nm. Excitation energy reaching the reaction centers causes oxidation (bleaching) of P870 and, presumably, reduction of an electron acceptor. Another specialized BChl, P800, is associated with P870 in the reaction centers and shows a band shift to shorter wavelengths when P870 is bleached. The oxidation of P870 occurs with a quantum efficiency close to 100 per cent whether the light is absorbed by light-harvesting BChl in chromatophores4 5 or by either P800 or P870 in reaction center preparations.6 Cells or chromatophores of photosynthetic bacteria emit BChl fluorescence when illuminated. This fluorescence comes from the light-harvesting BChl and competes with the utilization of energy at the reaction centers.\" 7 The yield of fluorescence from P870, measured in the absence of light-harvesting BChl, is small compared with that from light-harvesting BChl measured in chromatophores.8 The low fluorescence yield of P870 shows that energy reaching this pigment is quenched rapidly, and the high quantum efficiency for P870 oxidation shows that most of this quenching is due to the initiation of photochemistry. The lifetime of an excited state of a molecule is indicated by the yield of fluorescence corresponding to that state. If the only avenue for de-excitation were the radiative one, the yield would be 100 per cent and the lifetime would assume a maximum value called the intrinsic lifetime, To. If de-excitation can occur by other competing processes such as a photochemical quenching, both the lifetime r and the fluorescence quantum yield of will be reduced in proportion:

Fluorescence and photochemical quenching in photosynthetic reaction centers.

Isolation and Composition of a Photosynthetic Reaction Center Complex from Rhodopseudomonas spheroides

We have prepared photosynthetic reaction centers from Rhodo- pseudomonas spheroides and have studied the fluorescence of the photochemical electron donor, P870. The yield of this fluorescence rises at low redox potential, presumably because the photochemical electron acceptor becomes reduced and the photochemical utilization of excitation energy is then prevented. The redox titration curve for this increase in the fluorescence has a shape corresponding to the transfer of one electron. The midpoint potential is -0.05 volt, inde- pendent of the pH from 6.5 to 8.8. The amplitude of a light-induced absorbance change at 280 nanometers varies with redox potential and shows, at pH 7.5, a midpoint potential of 0.00 volt. These studies indicate that the primary photo- chemical electron acceptor is not ubiquinone and is not the substance responsible for the absorbance change at 280 nanometers. Introduction.-The chemical part of photosynthesis begins at photochemical reaction centers that receive excitation energy from the major ("light-harvest- ing") pigment components. A reaction center for bacterial photosynthesis1 has been identified as one in which a specialized bacteriochlorophyll molecule called P870 donates an electron to an unspecified acceptor. The present work is aimed at characterizing this primary photochemical electron acceptor, through studies of the fluorescence emitted by P870. We have already shown' that in reaction centers prepared from Rhodopseu- domonas spheroides, the quantum yield of P870 fluorescence corresponds to a life- time of about 10-11 seconds for the singlet excited state. This is much shorter than the intrinsic lifetime and is presumably the time needed to initiate photo- chemical electron transfer. The yield of P870 fluorescence can be increased by the addition of reduc- tants.2, ' This behavior is consistent with the interpretation that photochem- istry can occur only when the primary electron acceptor is in its oxidized form. If the acceptor is reduced, either photochemically or by a dark chemical process, the photochemical utilization of singlet excitation quanta is inhibited and the fluorescence of P870 increases. In this communication we shall describe the effect of imposed redox potential on the fluorescence of P870 and on a light- induced absorbance decrease at 280 nm that has been attributed to ubiquinone reduction. The identity of the primary electron acceptor will be discussed in the light of these results.

Dan W. Reed

Papers

Isolation of a reaction center fraction from Rhodopseudomonas spheroides

An identification of the radical giving rise to the light‐induced electron spin resonance signal in photosynthetic bacteria

Fluorescence and photochemical quenching in photosynthetic reaction centers.

Isolation and Composition of a Photosynthetic Reaction Center Complex from Rhodopseudomonas spheroides

The effect of redox potential on p870 fluorescence in reaction centers from rhodopseudomonas