Remarkable Affinity and Selectivity for Cs+ and Uranyl (UO2 2+) Binding to the Manganese Site of the Apo-Water Oxidation Complex of Photosystem II

• Published in: Biochemistry (Easton) 1999-06, Vol.38 (22), p.7200-7209
• Main Authors: Ananyev, Gennady M, Murphy, Andrew, Abe, Yuriko, Dismukes, G. Charles
• Format: Article
• Language: English
• Subjects: Alkalies - chemistry; Apoproteins - antagonists & inhibitors; Apoproteins - chemistry; Apoproteins - metabolism; Binding Sites - drug effects; Cations, Divalent - chemistry; Cations, Monovalent - chemistry; Cesium - chemistry; Cesium - metabolism; Electron Transport - drug effects; Half-Life; Kinetics; Manganese - antagonists & inhibitors; Manganese - chemistry; Manganese - metabolism; Oxidation-Reduction - drug effects; Photolysis; photosynthesis; Photosynthetic Reaction Center Complex Proteins - antagonists & inhibitors; Photosynthetic Reaction Center Complex Proteins - chemistry; Photosynthetic Reaction Center Complex Proteins - metabolism; Photosystem II Protein Complex; Spinacia oleracea; Tyrosine - antagonists & inhibitors; Uranium Compounds - chemistry; Uranium Compounds - metabolism; Water - chemistry; Water - metabolism
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi990023u

The size and charge density requirements for metal ion binding to the high-affinity Mn2+ site of the apo-water oxidizing complex (WOC) of spinach photosystem II (PSII) were studied by comparing the relative binding affinities of alkali metal cations, divalent metals (Mg2+, Ca2+, Mn2+, Sr2+), and the oxo-cation UO2 2+. Cation binding to the apo-WOC-PSII protein was measured by:  (1) inhibition of the rate and yield of photoactivation, the light-induced recovery of O2 evolution by assembly of the functional Mn4Ca1Cl x , core from its constituent inorganic cofactors (Mn2+, Ca2+, and Cl-); and by (2) inhibition of the PSII-mediated light-induced electron transfer from Mn2+ to an electron acceptor (DCIP). Together, these methods enable discrimination between inhibition at the high- and low-affinity Mn2+ sites and the Ca2+ site of the apo-WOC-PSII. Unexpectedly strong binding of large alkali cations (Cs+ ≫ Rb+ > K+ > Na+ > Li+) was found to smoothly correlate with decreasing cation charge density, exhibiting one of the largest Cs+/Li+ selectivities (≥5000) for any known chelator. Both photoactivation and electron-transfer measurements at selected Mn2+ and Ca2+ concentrations reveal that Cs+ binds to the high-affinity Mn2+ site with a slightly greater affinity (2−3-fold at pH 6.0) than Mn2+, while binding about 104-fold more weakly to the Ca2+-specific site required for reassembly of functional O2 evolving centers. In contrast to Cs+, divalent cations larger than Mn2+ bind considerably more weakly to the high-affinity Mn2+ site (Mn2+ ≫ Ca2+ > Sr2+). Their affinities correlate with the hydrolysis constant for formation of the metal hydroxide by hydrolysis of water:  Me2+ aq → [MeOH]+ aq + H+ aq. Along with the strong stimulation of the rate of photoactivation by alkaline pH, these metal cation trends support the interpretation that [MnOH]+ is the active species that forms upon binding of Mn2+ aq to apo-WOC. Further support for this interpretation is found by the unusually strong inhibition of Mn2+ photooxidation by the linear uranyl cation (UO2 2+). The intrinsic binding constant for [MnOH]+ to apo-WOC was determined using a thermodynamic cycle to be K = 4.0 × 1015 M-1 (at pH 6.0), consistent with a high-affinity, preorganized, multidentate coordination site. We propose that the selectivity for binding [MnOH]+, a linear low charge-density monocation, vs symmetrical Me2+ dications is functionally important for assembly of the WOC by enabling:  (1) discrimination ag