The Mn4Ca Cluster

The structure and function of the Mn4Ca cluster has been the topic of extensive studies using biochemical, molecular biological, spectroscopic and X- ray cry-stallographic analysis (see also Chapter 5). The crystal structure of PSII at 3.0 A

resolution [ 12] yielded an overall shape like a hook for the four Mn atoms, with three Mn atoms forming the triangle of the hook and a fourth Mn located in the knob of the hook (Figure 4.4[ . This overall shape is basically similar to the Y. shaped structures reported at lower resolutions [9, 10]) and the 3 + 1 model suggested from spectroscopic studies ([57, 58], Chapter 5). However, the Ca atom was only resolved at higher resolution in the 3.5 A [11] and 3.0A [12] structures of PSII. This Ca is located at the top of the triangle formed by the three Mn atoms, leading to a pseudo-pyramid structure with the Ca and three Mn atoms occupying each corner of the pyramid (Figure 4.4). The distances between Mn atoms and those between Mn-Ca could not be determined accurately from the current crystal structure, and thus have to adopt the restraints imposed by EXAFS studies. Such studies have yielded Mn-Mn distances of 2.7 and 3.3 A, and possibly Ca-Mn distances of 3.3-3.4A (for review, see [58][. The short distance of 2.7A suggests a di-|-oxo bridge between Mn-Mn, whereas the longer distance of3.3 A suggests a mono-|-oxo bridge between Mn-Mn; however, no oxygen atoms could be identified by X-ray structure analysis due to the limited resolution.

The ligands for the Mn4Ca cluster were identified as D1-Glu189, D1-Asp170, D1-Glu333, D1-His337, D1-Asp342, and the C-terminal residue ofD1.Ala344 in the 3.0A structure (Figure 4.4[. In addition, CP43.Glu354 was also found to be close enough to coordinate the Mn atom(s). Among these residues, D1.Asp170 and D1-Asp342 were arranged as mono-dentate ligands, whereas other carboxylic

Figure 4.4 Structure of the Mn4Ca cluster and its ligand environment. Mn atoms are shown in red, Ca in yellow, TyrZ in orange, D1 in green, and CP43 in purple. The side chains of amino acid residues that may provide ligands to the Mn4Ca cluster are drawn and labeled, whereas the backbone structures of D1 and CP43 are shown in light colors.

Figure 4.4 Structure of the Mn4Ca cluster and its ligand environment. Mn atoms are shown in red, Ca in yellow, TyrZ in orange, D1 in green, and CP43 in purple. The side chains of amino acid residues that may provide ligands to the Mn4Ca cluster are drawn and labeled, whereas the backbone structures of D1 and CP43 are shown in light colors.

residues could act as bidentate ligands to the metal cluster. One of the carboxylate ligands from both D1-Glu189 and D1-Ala344 was close to Ca2+, rather than Mn, although the ligation of D1-Ala344, the C-terminal residue of D1, to Ca2+ has been questioned by FTIR studies [59, 60]. Thus, a total of 9-10 ligands were found for the four Mn atoms from the amino acids and one or two ligands for Ca-+. The remaining ligands may be provided by water molecules and bicarbonate, a molecule that was found to affect the oxygen-evolving activity - 1] . It should be mentioned here that, although the overall shape and ligands of the Mn4Ca cluster are essentially similar between the structures of 3.0 À resolution [12] and 3.5À resolution [11], there are some differences regarding the precise positions of each metal atom and also the ligand structure of each residue. Especially in the 3.5 À structure, fewer residues were reported to act as bidentate ligands. This results in a lower number of protein ligands (7-8 ligands compared with 9-10 ligands in the 3.0 À structure) for the four Mn atoms, leading to a lower saturation level for the ligands. This raises different requirements for additional ligands other than amino acids, namely water or mono- and di-|-oxo bridges in the two structures, and will probably lead to a different atomic structure of the metal cluster. In fact, a recent study using polarized extended X-ray absorption fine structure (EXAFS) measurement on PSII single crystals has yielded structural models different from either of those derived from the crystal structure analysis [61]. Thus, the precise structure of the Mn4Ca cluster needs to be determined at a higher resolution in order to solve these discrepancies.

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