Involvement of carboxyl groups of the PSII reaction center proteins [obtained from spinach leaves] in photoactivation of the apo-water-oxidizing complex

Involvement of residues of acidic amino acids in photo-ligation of manganese into the apo-water-oxidizing complex was investigated by use of 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC), a water-soluble carboxyl modifier. Treatment of Mn-depleted PSII membranes by EDC in the presence of nucleophiles induced a loss of photoactivation capability in the Mn complex and partial loss of capability of photooxidation of Mn2+, but little decrease in the DCIP photoreduction supported by diphenylcarbazide. The inhibition of diphenylcarbazide-photooxidation by submicromolar Mn2+, indicative of the intactness of high-affinity Mn-binding sites, was apparently abolished by EDC treatment. From amino acid quantitation analysis of D1 and D2 proteins and CP47 of the chemically-modified membranes, approximately three carboxyl groups of the D1 protein were found to be chemically-modified with EDC after removal of the functional Mn. These results suggest that acidic amino acids on the D1 protein are involved in photoactivation of the apo-water-oxidizing complex and probably in ligation of Mn to the water-oxidizing complex.