1. Manganese  Oxygen Generation and Detoxification
Chapter 16
Manganese  Oxygen Generation and Detoxification
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FIGURE 16.1 Cofactors in electron transport in PSII, as determined by X-ray crystallography (Ferreira et al., 2004) (PDB 1S5L). With the exception of the metal-cluster of the oxygen evolving centre (OES), the haem of cyt b559 and a redox active b-carotene molecule, all the cofactors are arranged around a pseudo-2-fold axis passing between the chlorophylls PD1 and PD2 and the nonhaem iron. Side chains of the D1 and D2 proteins are in yellow and orange, respectively, chlorophyll in green, pheophytin blue, plastoquinones QA and QB in magenta, haem of cyt b559 red. The phytyl tails of the chlorophylls and pheophytins have been removed for clarity. The atoms of the water splitting catalyst are manganese (magenta), calcium (blue-green), and oxygen (red). Also shown are the nonhaem iron (red) and its bicarbonate ligand. Distances are in ångstroms. Red arrows indicate electron pathway of the active branch while cyt b599, a carotenoid (brown), and ChlzD2 form a protective branch. (From Murray & Barber, Copyright 2007 with permission from Elsevier.)
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FIGURE 16.2 The S-state cycle model of O2 generation. (Adapted from Voet & Voet, 2004: pp )
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FIGURE 16.3 (a,b) The water-splitting site as reported by Ferreira et al. (2004). (a) The Mn4Ca2+ cluster positioned within the Mnanomalous difference map with amino acid side chains. (b) Schematic of the amino acid ligation pattern for the model in (a) with distances less than 2.8Å shown by connecting lines. (c,d) Remodelling the water-splitting site using the native electron density maps of Ferreira et al. (2004) and Loll et al. (2005) and Mn-anomalous difference map of Ferreira et al. (2004), keeping the Mn3Ca2+O4 cubane of Ferreira et al. but with Mn4 linked to it via a single 3.3 Å mono--oxo bridge. (c) Structure of the water-splitting site, assuming a single mono-  -oxo bridge between Mn4 (dangler Mn) and Mn3 of the Mn3Ca2+O4 cubane fitted into the Mn-anomalous difference map by real-space refinement. (d) Schematic of the amino acid ligation pattern for model in (c) with distance less than 3 Å shown by connecting lines. The Mn-anomalous difference map is shown in red and contoured at 5 sigma. The arrow indicates the direction of the normal to the membrane plane. (From Barber & Murray, Copyright 2008 with permission from the Royal Society.)
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FIGURE 16.4 (a,b) Possible mechanisms for formation of dioxygen during the S4-S0 transition. (Adapted rom Barber, 2008.)
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FIGURE 16.5 The protein fold of Mn- or Fe-SODs (left) and the active site (right). (From Miller, Copyright 2004 with permission from Elsevier.)
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FIGURE 16.6 Lactobacillus plantarum Mn catalase: (a) stereo view of the secondary structure  the di-Mn unit as red spheres and (b) the detailed geometry of the di-Mn centre. (From Barynin et al., Copyright 2001 with permission from Cell Press.)
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FIGURE 16.7 Catalytic reaction cycle for manganese catalase turnover. (Adapted from Whittaker, Barynin, Igarashi, & Whittaker, 2003.)
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FIGURE 16.8 (a) A ribbon plot of the arginase trimer. The binuclear manganese cluster is represented by a pair of spheres in each monomer. (b) The binuclear manganese cluster of arginase. Metal coordination interactions are indicated by green dotted lines, and the hydrogen bond between the metal-bridging hydroxide ion (red sphere) and Asp 128 is indicated by a white dotted line. Mn2+A is coordinated with square pyramidal geometry, leaving a vacant coordination site that permits octahedral coordination geometry as a means of transition state stabilisation in catalysis. Mn2+B is coordinated with octahedral geometry. (Reprinted with permission from Kanyo, Scolnick, Ash, and Christianson, Copyright 1996 Nature Publishing Group.)
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FIGURE 16.9 Structure-based arginase mechanism. (Left) Schematic representation of the arginase mechanism; R-amino and R-carboxylate substrate groups are omitted for clarity. (Middle, right) Stereo view of the arginase mechanism. Protein atoms are colour-coded as follows: C) gray, O) red, and N) blue; ligand atoms are colour-coded the same with the exception: C) yellow; water molecules appear as red spheres. Manganese coordination interactions are designated by red dashed lines and hydrogen bonds by black dashed lines. (From Cox et al., Copyright 2001 with permission from the American Chemical Society.)
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SCHEME 16.1 L-arginine catabolism by arginase and NO synthase.
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