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 all (1996) Biochemistry 35, 1820-1828]. The current paper shows the following: (a) the high-affinity photooxidizable Mn, which donates to the oxidized primary PSII donor (Y-z(.)), is bound to a carboxyl residue with a K-M = 1.5 mu M or K-d = 0.44 mu M in the absence of DPC, and a K-i = 1.3 mu M 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 mu M) 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 mu M 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 mu M or K-d of 49-58 mu M occurs at a site that does not require carboxyl residues; (e) photooxidation of low-affinity DPC with a K-M = 1200 mu M 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+.