Comparative Study of Thermal Degradation of Iron-Sulfur Proteins in Spinach Chloroplasts and Membranes of Thermophilic Cyanobacteria: Mössbauer Spectroscopy

A. A. Novakova^1*, E. A. Khval'kovskaya¹, A. Yu. Aleksandrov², T. Yu. Kiseleva¹, L. N. Davletshina², B. K. Semin², I. I. Ivanov², Yu. N. Kaurov², and A. B. Rubin²

¹School of Physics, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 932-8820; E-mail: aan@runar.phys.msu.ru

²School of Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-1115; E-mail: davlet@mars.biophys.msu.ru; semin@mars.biophys.msu.ru; ivanov@mars.biophys.msu.ru; rubin@mars.biophys.msu.ru

^* To whom correspondence should be addressed.

Received July 14, 2000; Revision received October 10, 2000

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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°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 F_X, F_A, and F_B (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 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 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 F_X, F_A, and F_B, release of oxidized (trivalent) iron, and its accumulation in membrane-bound iron-oxo clusters.

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KEY WORDS: Mössbauer spectroscopy, spinach, thermophilic cyanobacteria, photosynthesis, Photosystem I, iron-sulfur centers, thermal destruction

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