Possible ligation of the Mn cluster in Photosystem II by the carboxylterminus of the D1 polypeptide: an FTIR study

摘要

The catalytic site of photosynthetic oxygen evolution contains a (Mn)_4-Ca cluster that interacts closely with a redox-active tyrosine residue known as Y_z [for review see (Britt, 1996; Hoganson & Babcock, 2000; Debus, 2000; Pecoraro & Hsieh, 2000;Tommos & Babcock, 2000; Renger, 2001)]. The (Mn)_4 cluster accumulates oxidizing equivalents in response to photoinduced electron transfer reactions within PSII, then catalyzes the oxidation of two molecules of water, releasing one molecule of O_2 as a by-product. Understanding the mechanism of water oxidation requires understanding the ligation environment of the (Mn)_4-Ca cluster and how this environment changes as oxidizing equivalents are accumulated. To provide this information, we are combining spectroscopic characterizations involving EPR, FTIR, and time-resolvedoptical absorption and fluorescence spectroscopies with site-directed mutagenesis and isotopic labeling. Our early studies, conducted with intact cells, suggested that Asp170, His332, Glu333, His337, and Asp342 of the D1 polypeptide are potential ligands of Mn and that Asp59, Asp61, and Asp342 of the D1 polypeptide are potentialligands of Ca (Chu et al., 1994a,b; 1995a,b). Our recent efforts, conducted with purified PSII particles, have shown that D1-His332 almost certainly ligates the assembled (Mn)_4 cluster (Debus et al., 2001) and that D1-Asp170 either ligates Mn directly or participates in a hydrogen bond to the (Mn)_4-Ca cluster (Chu et al., 2001).