Effects of chloride on paramagnetic coupling of manganese in calcium chloride-washed photosystem II preparations

Conformational changes in a Photosystem II hydrogen bond network stabilize the oxygen-evolving complex

The Mn4CaO5 oxygen-evolving complex (OEC) in Photosystem II (PSII) is assembled in situ and catalyzes water oxidation. After OEC assembly, the PsbO extrinsic subunit docks to the lumenal face of PSII and both stabilizes the OEC and facilitates efficient proton transfer to the lumen. D1 residue R334 is part of a hydrogen bond network involved in proton release during catalysis and interacts directly with PsbO. D1-R334 has recently been observed in different conformations in apo- and holo-OEC PSII structures. We generated a D1-R334G point mutant in Synechocystis sp. PCC 6803 to better understand this residue's function. D1-R334G PSII is active under continuous light, but the OEC is unstable in darkness. Isolated D1-R334G core complexes have little bound PsbO and less manganese as the wild type control. The S2 intermediate is stabilized in D1-R334G indicating that the local environment around the OEC has been altered. These results suggest that the hydrogen bond network that includes D1-R334 exists in a different functional conformation during PSII biogenesis in the absence of PsbO.

Amino acid oxidation of the D1 and D2 proteins by oxygen radicals during photoinhibition of Photosystem II

The Photosystem II reaction center is vulnerable to photoinhibition. The D1 and D2 proteins, lying at the core of the photosystem, are susceptible to oxidative modification by reactive oxygen species that are formed by the photosystem during illumination. Using spin probes and EPR spectroscopy, we have determined that both O₂^•- and HO^• are involved in the photoinhibitory process. Using tandem mass spectroscopy, we have identified a number of oxidatively modified D1 and D2 residues. Our analysis indicates that these oxidative modifications are associated with formation of HO^• at both the Mn₄O₅Ca cluster and the nonheme iron. Additionally, O₂^•- appears to be formed by the reduction of O₂ at either Pheo_D1 or Q_A Early oxidation of D1:³³²H, which is coordinated with the Mn1 of the Mn₄O₅Ca cluster, appears to initiate a cascade of oxidative events that lead to the oxidative modification of numerous residues in the C termini of the D1 and D2 proteins on the donor side of the photosystem. Oxidation of D2:²⁴⁴Y, which is a bicarbonate ligand for the nonheme iron, induces the propagation of oxidative reactions in residues of the D-de loop of the D2 protein on the electron acceptor side of the photosystem. Finally, D1:¹³⁰E and D2:²⁴⁶M are oxidatively modified by O₂^•- formed by the reduction of O₂ either by Pheo_D1^•- or Q_A^•- The identification of specific amino acid residues oxidized by reactive oxygen species provides insights into the mechanism of damage to the D1 and D2 proteins under light stress.

The extrinsic proteins of Photosystem II

In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn(4)CaO(5) cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.

The 23 and 17 kDa extrinsic proteins of photosystem II modulate the magnetic properties of the S1-state manganese cluster

An S1-state parallel polarization "multiline" EPR signal arising from the oxygen-evolving complex has been detected in spinach (PSII) membrane and core preparations depleted of the 23 and 17 kDa extrinsic polypeptides, but retaining the 33 kDa extrinsic protein. This S1-state multiline signal, with an effective g value of 12 and at least 18 hyperfine lines, has previously been detected only in PSII preparations from the cyanobacterium sp. Synechocystis sp. PCC6803 [Campbell, K. A., Peloquin, J. M., Pham, D. P., Debus, R. J., and Britt, R. D. (1998) J. Am. Chem. Soc. 120, 447-448]. It is absent in PSII spinach membrane and core preparations that either fully retain or completely lack the 33, 23, and 17 kDa extrinsic proteins. The S1-state multiline signal detected in spinach PSII cores and membranes has the same effective g value and hyperfine spacing as the signal detected in Synechocystis PSII particles. This signal provides direct evidence for the influence of the extrinsic PSII proteins on the magnetic properties of the Mn cluster.

Calcium induces binding and formation of a spin-coupled dimanganese(II,II) center in the apo-water oxidation complex of photosystem II as precursor to the functional tetra-Mn/Ca cluster

Two new intermediates are described which form in the dark as precursors to the light-induced assembly of the photosynthetic water oxidation complex (WOC) from the inorganic components. Mn2+ binds to the apo-WOC-PSII protein in the absence of calcium at a high-affinity site. By using a hydrophobic chelator to remove Mn2+ and Ca2+ from the WOC and nonspecific Fe3+, a new EPR signal becomes visible upon binding of Mn2+ to this site, characterized by six-line 55Mn hyperfine structure (DeltaHpp = 96 +/- 1 G) and effective g = 8.3. These features indicate a high-spin electronic ground state (S = 5/2) for Mn2+ and a strong ligand field with large anisotropy. This signal is eliminated if excess Ca2+ or Mg2+ is present. A second Mn2+ EPR signal forms in place of this signal upon addition of Ca2+ in the dark. The yield of this Ca-induced Mn signal is optimum at a ratio of 2 Mn/PSII, and saturates with increasing [Ca2+] >/= 8 mM, exhibiting a calcium dissociation constant of KD = 1.4 mM. The EPR signal of the Ca-induced Mn center at 25 K is asymmetric with major g value of approximately 2.04 (DeltaHpp = 380 G) and a shoulder near g approximately 3.1. It also exhibits resolved 55Mn hyperfine splitting with separation DeltaHpp = 42-45 G. These spectral features are diagnostic of a variety of weakly interacting Mn2(II, II) pairs with electronic spins that are magnetic dipolar coupled in the range of intermanganese separations 4.1 +/- 0.4 A, and commonly associated with one or two carboxylate bridges. The calcium requirement for induction of the Mn2(II,II) signal matches the value observed for steady-state O2 evolution (Michaelis constant, KM approximately 1.4 mM), and for light-induced assembly of the WOC by photoactivation. The Ca-induced Mn2(II,II) center is a more efficient electron donor to the photooxidized tyrosine radical, TyrZ+, than is the mononuclear Mn center present in the absence of Ca2+. The Ca-induced Mn2(II,II) signal serves as a precursor for photoactivation of the functional WOC and is abolished by the presence of Mg2+. Formation of the Mn2(II,II) EPR signal by addition of Ca2+ correlates with reduction of flash-induced catalase activity, indicating that calcium modulates the accessibility or reactivity of the Mn2(II,II) core with H2O2. We propose that calcium organizes the binding site for Mn ions in the apo-WOC protein and may even interact directly with the Mn2(II,II) pair via solvent or protein-derived bridging ligands.

Interaction of the 33 kDa extrinsic protein with photosystem II: rebinding of the 33 kDa extrinsic protein to photosystem II membranes which contain four, two, or zero manganese per photosystem II reaction center

The 33 kDa extrinsic protein of photosystem II acts to enhance oxygen evolution and to stabilize the manganese cluster at low chloride concentrations. Due to controversies concerning the stoichiometry of this protein [Miyao, M., & Murata, N. (1989) Biochim. Biophys. Acta 977, 315-321, versus Xu, Q., & Bricker, T. M. (1992) J. Biol. Chem. 267. 25816-25821] we have examined the rebinding of this protein to PS II membrane preparations which contain four, two, or zero manganese per photosystem II reaction center. After rebinding, immunoquantification of the 33 kDa extrinsic protein demonstrated that each of these photosystem II membrane preparations strongly bound two copies of the 33 kDa extrinsic protein per photosystem II reaction center. The first and second stoichiometric binding constants (Ka1 and Ka2) for the binding of the 33 kDa protein to PS II centers containing four manganese were 0.42 and 0.67 nM(-1), respectively. Disruption of the manganese cluster either by removal of the chloride-sensitive manganese or extraction of the manganese cluster by alkaline Tris led to a 5-6-fold decrease in Ka1 and about a 3-fold decrease in Ka2. In all cases the binding of the two copies of the 33 kDa extrinsic protein exhibited positive cooperativity with Hill coefficients ranging from 1.6 to 2.2. These findings demonstrate that damage to the manganese cluster alters the binding affinity of the 33 kDa extrinsic protein to photosystem II but does not alter the molecularity of the binding reaction.

Secondary structure of the 33 kDa, extrinsic protein of Photosystem II: a far-UV circular dichroism study

The 33 kDa extrinsic protein of Photosystem II is an important component of the oxygen-evolving apparatus which functions to stabilize the manganese cluster at physiological chloride concentrations and to lower the calcium requirement for oxygen evolution. Chou-Fasman analysis of the amino-acid sequence of this protein suggests that this component contains a high proportion of alpha-helical structure and only relatively small amounts of beta-sheet structure. A computational study using more sophisticated techniques (Beauregard, M. (1992) Environ. Exp. Bot. 32, 411-429) concluded that the protein contained little periodically ordered secondary structure. In this study, we have directly measured the relative proportions of secondary structure present in the 33 kDa protein using far-ultraviolet circular dichroism spectroscopy. Our results indicate that, in solution, this protein contains a large proportion of beta-sheet structure (38%) and relatively small amounts of alpha-helical structure (9%). A structural model of the 33 kDa protein based on a constrained Chou-Fasman analysis (Teeter, M.M. and Whitlow, M (1988) Proteins 4, 262-273) is presented.

Interaction of CPa-1 with the manganese-stabilizing protein of photosystem II: identification of domains cross-linked by 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide

The structural organization of photosystem II proteins has been investigated by use of the zero-length protein cross-linking reagent 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide and monoclonal and polyclonal antibody reagents. Photosystem II membranes were treated with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide which cross-links amino groups to carboxyl groups which are in van der Waals contact. This treatment did not affect the oxygen evolution rates of these membranes and increased the retention of oxygen evolution after CaCl2 washing. Analysis of the proteins cross-linked by this treatment indicated that two cross-linked species with apparent molecular masses of 95 and 110 kDa were formed which cross-reacted with antibodies against both the 33-kDa manganese-stabilizing protein and the chlorophyll protein CPa-1. Cleavage of the 110-kDa cross-linked species with cyanogen bromide followed by N-terminal sequence analysis was used to identify the peptide fragments of CPa-1 and the manganese-stabilizing protein which were cross-linked. Two cyanogen bromide fragments were identified with apparent molecular masses of 50 and 25 kDa. N-Terminal sequence analysis of the 50-kDa cyanogen bromide fragment indicates that this consists of the C-terminal 16.7-kDa fragment of CPa-1 and the intact manganese-stabilizing protein. This strongly suggests that the manganese-stabilizing protein is cross-linked to the large extrinsic loop domain of CPa-1. N-Terminal analysis of the 25-kDa cyanogen bromide fragment indicates that this consists of the C-terminal 16.7-kDa peptide of CPa-1 and the N-terminal 8-kDa peptide of the manganese-stabilizing protein.(ABSTRACT TRUNCATED AT 250 WORDS)

A model for the mechanism of chloride activation of oxygen evolution in photosystem II

A hypothesis is proposed to explain the function of Cl(-) in activating the oxygenevolving complex (OEC) of photosystem II (PS II), based on the results of recent (35)Cl-NMR studies. The putative mechanism involves Cl(-) binding to two types of sites. An intrinsic site is suggested to be composed of three histidyl residues (His 332 and His 337 from D1 and His 337 D2). It is proposed that Cl(-) binding to this site accelerates the abstraction of H(+) from water by raising the pKa's of the histidine imidazole groups. Cl(-) binding also stimulates the transfer of H(+) from this intrinsic site to a set of extrinsic sites on the 33 kD extrinsic polypeptide. The extrinsic Cl(-) binding sites are suggested to involve four protein domains that are linked together by salt-bridge contacts. Chloride and H(+) donated from the intrinsic site attack these intramolecular salt-bridges in a defined sequence, thereby exposing previously inaccessible Cl(-) and H(+) binding sites and stimulating the oxidation of water. This hypothesis also proposes a possible structure for the Mn active site within the D1/D2 complex. Specific amino-acid residues that are likely to participate as Mn lignads are identified on the lumenal portions of the D1 and D2 proteins that are different from those in the L and M subunits of photosynthetic bacteria; the choice of these residues is based on the metal coordination chemistry of these residues, their location within the polypeptide chain, the regularity of their spacing, and their conservation through evolution. The catalytic Mn-binding residues are suggested to be D-61, E-65, E-92, E-98, D-103; D-308, E-329, E-342 and E-333 in D1, and H-62, E-70, H-88, E-97, D-101; E-313, D-334, E-338 and E-345 in D2. Finally, this hypothesis identifies sites on both D2 and the 33 kD extrinsic polypeptide that might be involved in high- and low-affinity Ca(2+) binding.

Is functional manganese involved in hydrogen-peroxide-stimulated anomalous oxygen evolution in CACl2-washed photosystem II membranes?

When detergent-derived photosystem II (PSII) membranes are treated with CaCl2 to remove the three extrinsic proteins associated with the O2-evolving complex, the resulting membranes (CaPSII) can still catalyze water oxidation if sufficient Ca(2+) and Cl(-) are present. When CaPSII membranes are exposed to single turnover flashes on an O2 rate electrode, anomalous O2 is produced by the first two flashes. The addition of catalase to the membrane suspension completely inhibits O2 produced by the first two flashes, but not by subsequent flashes. Exogenous H2O2 stimulates anomalous O2 production by the first few flashes in CaPSII membranes, but not in control PSII membranes. Diuron (DCMU) does not inhibit H2O2-stimulated O2 production by the first flash. However, it does inhibit the O2 yield of all subsequent flashes, indicating that all flash-induced O2 signals in CaPSII membranes are dependent on photosystem II electron transport. H2O2 stimulation of O2 yields is inhibited in Tris-, heat-, and EDTA-(ethylenediaminetetraacetic acid)-treated CaPSII. In the presence of high salt, H2O2 (but not EDTA) treatment of CaPSII, extracts Mn functional in normal photosynthetic O2 evolution. The addition of exogenous Mn(2+) reconstitutes anomalous O2 production in Tris-and H2O2/EDTA-treated CaPSII preparations but only in the presence of H2O2. Anomalous H2O2-stimulated O2 production can be observed both with a Clark electrode (steady state) and an O2 rate electrode (flash sequence). The mechanism involves electron donation from H2O2, mediated by free Mn(2+), to PSII, and the 33-kDa extrinsic protein under some conditions can block this process. Since H2O2 can remove functional Mn from CaPSII membranes, its presence can convert functional Mn to the Mn(2+) mediator state required for anomalous O2 production. EDTA binds Mn in CaPSII disrupted by H2O2 and prevents anomalous O2 evolution.