Jegerschold C, Arellano JB, Schroder WP, Vankan PJM, Baron M, Styring S
Copper(Ii) Inhibition of Electron-Transfer through Photosystem-Ii Studied by Epr Spectroscopy
Biochemistry: 1995 34:12747-12754
EPR spectroscopy was applied to investigate the inhibition of electron transport in photosystem II by Cu2+ ions. Our results show that Cu2+ has inhibitory effects on both the donor and the acceptor side of photosystem II. In the presence of Cu2+, neither EPR signal IIvery (fast) nor signal IIfast, which both reflect oxidation of tyrosine(Z), could be induced by illumination. This shows that Cu2+ inhibits electron transfer from tyrosine(Z) to the oxidized primary donor P680(+). Instead of tyrosine(Z) oxidation, illumination results in the formation of a new radical with g = 2.0028 +/- 0.0002 and a spectral width of 9.5 +/- 0.3 G. At room temperature, this radical amounts to one spin per PS IT reaction center. Incubation of photosystem II membranes with cupric ions also results in release of the 16 kDa extrinsic subunit and conversion of cytochrome b(559) to the low-potential form. On the acceptor side, QA can still be reduced by illumination or chemical reduction with dithionite. However, incubation with Cu2+ results in loss of the normal EPR signal from Q(A)(-) which is coupled to the non-heme Fe2+ on the acceptor side (the Q(A)(-)-Fe2+ EPR signal). Instead, reduction of QA results in the formation of a free radical spectrum which is 9.5 G wide and centered at g = 2.0044. This signal is attributed to Q(A)(-) which is magnetically decoupled from the nonheme iron. This suggests that Cu2+ displaces the Fe2+ or severly alters its binding properties. The inhibition of tyrosine(Z) is reversible upon removal of the copper ions with EDTA while the modification of Q(A) was found to be irreversible.
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