Comparative study of thermal degradation of iron-sulfur proteins in spinach chloroplasts and membranes of thermophilic cyanobacteria: Mossbauer spectroscopy

Mossbauer spectra of chloroplasts isolated from spinach plants grown in a m ineral medium enriched with Fe-57 and Mossbauer 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 degreesC significantl y modifies the central part of the spectra. This spectral change is evidenc e of decreased magnitude of the quadrupole splitting. However, the thermall y 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 spi nach chloroplasts. This doubler is usually associated with degradation of a ctive centers of ferredoxin, a surface-exposed protein of PS I. The Mossbau er spectra of photosynthetic membranes of spinach chloroplasts and cyanobac teria were compared using the probability distribution function of quadrupo le shift (1/2 quadrupole splitting DeltaQ) of trivalent iron. The results o f 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 F-X, F-A, and F-B (quadrupole shift range, 0.43-0.67 mm/sec) in heated preparations. This process was al so accompanied by an increase in the probability of appearance of clusters of trivalent iron. This increase was found to be either gradual and continu ous or abrupt and discrete in photosynthetic membranes of cyanobacteria or spinach chloroplasts, respectively. The probability of the presence of the iron-sulfur centers F-X, F-A, and F-B 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 d estruction of iron-sulfur centers of PS I and functional degradation of PS I are shifted toward a higher temperature. The results of this study sugges t that the same mechanism of thermal destruction of the PS I core occurs in both thermophilic and mesophilic organisms: destruction of iron-sulfur cen ters F-X, F-A, and F-B, release of oxidized (trivalent) iron, and its accum ulation in membrane-bound iron-ore clusters.