Demetrios F. Ghanotakis1, Gerald T. Babcock2, Charles F. Yocum1
1Division of Biological Sciences, The University of Michigan, Ann Arbor, MI 48109-1048 USA
2Department of Chemistry, Michigan State University, East Lansing, MI 48824-1322 USA
Tóm tắt
Exposure of highly resolved Photosystem II preparations to 2 M NaCl produces an 80% inhibition of oxygen‐evolution activity concomitant with extensive loss of two water‐soluble polypeptides (23 and 17 kDa). Addition of Ca2+ to salt‐washed PS II membranes causes an acceleration in the decay of Z⨥, the primary donor to P‐680+, and we show here that this acceleration is due to reconstitution of oxygen‐evolution activity by Ca2+. Other cations (Mg2+, Mn2+, Sr2+) are much less effective in restoring oxygen evolution. On the basis of these observations we propose that Ca2+, perhaps in concert with the 23 kDa polypeptide, is an essential cofactor for electron transfer from the ‘S’‐states to Z on the oxidizing side of PS II.
