Effects of Carboxyl Amino Acid Modification on the Properties of the High-Affinity, Manganese-Binding Site in Photosystem II

Our previous work using the 'diphenylcarbazide (DPC)-inhibition assay' has identified four amino acid (two carboxyls and two histidyls) ligands to four Mn²⁺ bound with high affinity on Tris-washed photosystem II (PSII) membrane fragments [Preston and Seibert (1991) Biochemistry 30, 9615-9624, 9625-9633]. One of the ligands binds a photooxidizable Mn, specifically, and the others bind either nonphotooxidizable Mn²⁺, Zn²⁺, or Co²⁺ [Ghirardi et al. (1996) Biochemistry 35, 1820-1828]. The current paper shows the following: (a) the high-affinity photooxidizable Mn, which donates to they oxidized primary PSII donor (Yz·), is bound to a carboxyl residue with a K(M) 1.5 μM or K(d) 0.94 μM in the absence of DPC, and a K(i) = 1.3 μM in the presence of DPC (both steady-state and flash approaches were used); (b) if this carboxyl is chemically modified using 1-ethyl-3-[3- (dimethylamino)propyl]-carbodiimide hydrochloride (EDC), Mn²⁺ is photooxidized at a lower affinity (K(d) = 25 μM) site that does not involve carboxyl ligands; (c) low-affinity Mn is photooxidized (possibly by Y(D)·, the oxidized form of the alternative PSII donor) with a K(M) = 220 μM at a completely different site that also requires a carboxyl ligand; (d) photooxidation of high-affinity DPC by Y(z) with a K(M) of 40-42 μM or K(d) of 49-58 μM occurs at a site that does not require carboxyl residues; (e) photooxidation of low-affinity DPC with a K(M) = 1200 μM occurs at a site (possibly near Y(D)) that is not affected by carboxyl modification with EDC. Due to the similarities between the binding of the high-affinity photooxidizable Mn to EDC-treated membranes and to PSII complexes from Asp 170D1 mutants [Nixon and Diner (1992) Biochemistry 31,942-948], we identify its carboxyl residue ligand as Asp170 on D1, one of the reaction-center proteins. The second carboxyl ligand identified using the DPC-inhibition assay binds Mn (but not a photooxidizable one), Zn, or Co ions. At least one of the two histidyl ligands (either His337 on D1 or another unidentified histidyl) that bind nonphotooxidizable, high-affinity Mn²⁺ also binds Zn²⁺ and Co²⁺.