THE INHIBITORY MECHANISM OF CU(II) ON THE PHOTOSYSTEM-II ELECTRON-TRANSPORT FROM HIGHER-PLANTS

In a previous paper, we reported that Cu(II) inhibited the photosynthetic electron transfer at the level of the pheophytin-Q(A)-Fe domain of the Photosystem II reaction center. In this paper we characterize the underlying mechanism of Cu(II) inhibition. Cu(II)-inhibition effect was more sensitive with high pH values. Double-reciprocal plot of the inhibition of oxygen evolution by Cu(II) is shown and its corresponding inhibition constant, K(i), was calculated. Inhibition by Cu(II) was non-competitive with respect to 2,6-dichlorobenzoquinone and 3-(3,4-dichlorophenyl)-1,1-dimethylurea and competitive with respect to protons. The non-competitive inhibition indicates that the Cu(II)-binding site is different from that of the 2,6-dichlorobenzoquinone electron acceptor and 3-(3,4-dichlorophenyl)-1,1-dimethylurea sites, the Q(B) niche. On the other hand, the competitive inhibition with respect to protons may indicate that Cu(II) interacts with an essential amino acid group(s) that can be protonated or deprotonated in the inhibitory-binding site.