Studies on early events of photosynthetic electron transport---functions of Fe,Ca and M

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1. The number and function of Ca involved in photosystem II (PS II) electron transport were studied with oxygen evolving preparations isolated from rice and spinach. (1) PS II oxygen evolving membranes contained two Ca per PS II, which are heterogenous in binding affinity. A loosely bound Ca could be solubilized by detergent-treatments and the resulting PS II complexes containing only one Ca were highly active in oxygen evolution. Thus only one bound Ca is needed for PS II electron transport. (2) Treatment of PS II preparations with high concentrations of NaCl did not solubilize Ca, even though the treatment resulted in a strong inactivation of oxygen evolution and the lost activity was restored by added Ca. (3) A brief treatment at pH 3.0, which caused liberation of one out of the two Cabound to PS II membranes, failed to extract Ca from PS II complexes containing one Ca but induced Ca-reversible inactivation of oxygen evolution. This indicates that the acid-extractable Ca is not need … More ed for oxygen evolution. (4) The extrinsic 23 kDa protein was found to decrease the binding affinity of added Ca to pH 3.0- treated PS II preparations. (5) The above results are inconsistent with the view that NaCl- and pH 3.0-treatments inhibit oxygen evolution by extracting bound Ca and that the 23 kDa protein stabilizes or protects Ca in PS II preparation. Instead, detailed investigation on Ca concentration dependence of oxygen evolution in NaCl- and pH 3.0-treated preparations strongly suggest that the two treatments share a common mechanism of inactivation,i.e., a conformational change of the oxygen evolving complex.2. Crosslinking experiments showed that the extrinsic 33 kDa protein is located near the chlbinding 47 kDa protein and the association of the 33 kDa protein protects Mn against alkaline pH-treatment.3. Purified PS II reaction center complexes and D1-D2-cytochrome b559 complexes from the thermophilic cyanobacterium S. elongatus were found to contain one iron atom of unknown function in addition to a heme iron of cytochrome b559 and a non-heme iron locating near Q_A and Q_B. Less