Mössbauer spectra of chloroplasts isolated from spinach plants grown in a mineral medium enriched with 57Fe and Mössbauer spectra of native membranes of the thermophilic cyanobacterium Synechococcus elongatus contain a broad asymmetric doublet typical of the iron-sulfur proteins of Photosystem (PS) I. Exposure of chloroplasts to temperatures of 20-70 degrees C significantly modifies the central part of the spectra. This spectral change is evidence of decreased magnitude of the quadrupole splitting. However, the thermally induced doublet (DeltaQ = 3.10 mm/sec and delta = 1.28 mm/sec) typical of hydrated forms of reduced (divalent) inorganic iron is not observed in spinach chloroplasts. This doublet is usually associated with degradation of active centers of ferredoxin, a surface-exposed protein of PS I. The Mössbauer spectra of photosynthetic membranes of spinach chloroplasts and cyanobacteria were compared using the probability distribution function of quadrupole shift (1/2 quadrupole splitting DeltaQ) of trivalent iron. The results of calculation of these functions for the two preparations showed that upon increasing the heating temperature there was a decrease in the probability of the presence of native iron-sulfur centers FX, FA, and FB (quadrupole shift range, 0.43-0.67 mm/sec) in heated preparations. This process was also accompanied by an increase in the probability of appearance of clusters of trivalent iron. This increase was found to be either gradual and continuous or abrupt and discrete in photosynthetic membranes of cyanobacteria or spinach chloroplasts, respectively. The probability of the presence of the iron-sulfur centers FX, FA, and FB in chloroplasts abruptly decreases to virtually to zero within the temperature range critical for inhibition of electron transport through PS I to oxygen. In cyanobacteria, both thermal destruction of iron-sulfur centers of PS I and functional degradation of PS I are shifted toward a higher temperature. The results of this study suggest that the same mechanism of thermal destruction of the PS I core occurs in both thermophilic and mesophilic organisms: destruction of iron-sulfur centers FX, FA, and FB, release of oxidized (trivalent) iron, and its accumulation in membrane-bound iron-oxo clusters.
A model description of the Mo «ssbauer spectrum (80 K) of native membranes of the thermophilic cyanobacterium Synechococcus elongatus is suggested on the basis of the known values of quadrupole splitting (v vE Q ) and isomer shift (N N Fe ) for the iron-containing components of the photosynthetic apparatus. Using this approach, we found that heating the membranes at 70^80 K results in a decrease of doublet amplitudes belonging to F X , F A , F B and ferredoxin and simultaneous formation of a new doublet with v vE Q = 3.10 mm/s and N N Fe = 1.28 mm/s, typical of inorganic hydrated forms of Fe 2+ . The inhibition of electron transfer via photosystem I to oxygen, catalyzed by ferredoxin, occurs within the same range of temperatures. The data demonstrate that the processes of thermoinduced Fe 2+ formation and distortions in the photosystem I electron transport in the membranes are interrelated and caused mainly by the degradation of ferredoxin. The possible role of Fe 2+ formation in the damage of the photosynthetic apparatus resulting from heating and the action of other extreme factors is discussed.z 1999 Federation of European Biochemical Societies.
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