Substrate water binding and oxidation in photosystem II

Weak red light plays an important role in awakening the photosynthetic machinery following desiccation in the subaerial cyanobacterium Nostoc flagelliforme

The subaerial cyanobacterium Nostoc flagelliforme can survive for years in the desiccated state and light exposure may stimulate photosynthetic recovery during rehydration. However, the influence of light quality on photosynthetic recovery and the underlying mechanism remain unresolved. Exposure of field collected N. flagelliforme to light intensity ≥2 μmol photons m^-2 s^-1 showed that the speed of photosystem II (PSII) recovery was in the following order: red > green > blue ≈ violet light. Decreasing the light intensity showed that weak red light stimulated PSII recovery during rehydration. The chlorophyll fluorescence transient and oxygen evolution activity indicated that the oxygen evolution complex (OEC) was the activated site triggered by weak red light. The damaged D1 protein accumulated in the thylakoid membrane during dehydration and is degraded and resynthesized during dark rehydration. PsbO interaction with the thylakoid membrane was induced by weak red light. Thus, weak red light plays an important role in triggering OEC photoactivation and the formation of functional PSII during rehydration. In its arid habitats, weak red light could stimulate the awakening of dormant N. flagelliforme after absorbing water from nighttime dew or rain to maximize growth during the early daylight hours of the dry season.

An enzyme kinetics study of the pH dependence of chloride activation of oxygen evolution in photosystem II

Oxygen evolution by photosystem II (PSII) involves activation by Cl^- ion, which is regulated by extrinsic subunits PsbQ and PsbP. In this study, the kinetics of chloride activation of oxygen evolution was studied in preparations of PSII depleted of the PsbQ and PsbP subunits (NaCl-washed and Na₂SO₄/pH 7.5-treated) over a pH range from 5.3 to 8.0. At low pH, activation by chloride was followed by inhibition at chloride concentrations >100 mM, whereas at high pH activation continued as the chloride concentration increased above 100 mM. Both activation and inhibition were more pronounced at lower pH, indicating that Cl^- binding depended on protonation events in each case. The simplest kinetic model that could account for the complete data set included binding of Cl^- at two sites, one for activation and one for inhibition, and four protonation steps. The intrinsic (pH-independent) dissociation constant for Cl^- activation, K _S, was found to be 0.9 ± 0.2 mM for both preparations, and three of the four pK _as were determined, with the fourth falling below the pH range studied. The intrinsic inhibition constant, K _I, was found to be 64 ± 2 and 103 ± 7 mM for the NaCl-washed and Na₂SO₄/pH7.5-treated preparations, respectively, and is considered in terms of the conditions likely to be present in the thylakoid lumen. This enzyme kinetics analysis provides a more complete characterization of chloride and pH dependence of O₂ evolution activity than has been previously presented.

Interaction of methanol with the oxygen-evolving complex: atomistic models, channel identification, species dependence, and mechanistic implications

Methanol has long being used as a substrate analogue to probe access pathways and investigate water delivery at the oxygen-evolving complex (OEC) of photosystem-II. In this contribution we study the interaction of methanol with the OEC by assembling available spectroscopic data into a quantum mechanical treatment that takes into account the local channel architecture of the active site. The effect on the magnetic energy levels of the Mn₄Ca cluster in the S₂ state of the catalytic cycle can be explained equally well by two models that involve either methanol binding to the calcium ion of the cluster, or a second-sphere interaction in the vicinity of the "dangler" Mn4 ion. However, consideration of the latest ¹³C hyperfine interaction data shows that only one model is fully consistent with experiment. In contrast to previous hypotheses, methanol is not a direct ligand to the OEC, but is situated at the end-point of a water channel associated with the O4 bridge. Its effect on magnetic properties of plant PS-II results from disruption of hydrogen bonding between O4 and proximal channel water molecules, thus enhancing superexchange (antiferromagnetic coupling) between the Mn3 and Mn4 ions. The same interaction mode applies to the dark-stable S₁ state and possibly to all other states of the complex. Comparison of protein sequences from cyanobacteria and plants reveals a channel-altering substitution (D1-Asn87 versus D1-Ala87) in the proximity of the methanol binding pocket, explaining the species-dependence of the methanol effect. The water channel established as the methanol access pathway is the same that delivers ammonia to the Mn4 ion, supporting the notion that this is the only directly solvent-accessible manganese site of the OEC. The results support the pivot mechanism for water binding at a component of the S₃ state and would be consistent with partial inhibition of water delivery by methanol. Mechanistic implications for enzymatic regulation and catalytic progression are discussed.

Split electron paramagnetic resonance signal induction in Photosystem II suggests two binding sites in the S2 state for the substrate analogue methanol

Illuminating a photosystem II sample at low temperatures (here 5-10 K) yields so-called split signals detectable with continuous wave-electron paramagnetic resonance (CW-EPR). These signals reflect the oxidized, deprotonated radical of D1-Tyr161 (YZ(•)) in a magnetic interaction with the CaMn4 cluster in a particular S state. The intensity of the split EPR signals are affected by the addition of the water substrate analogue methanol. This was previously shown by the induction of split EPR signals from the S1, S3, and S0 states [Su, J.-H. et al. (2006) Biochemistry 45, 7617-7627.]. Here, we use two split EPR signals induced from photosystem II trapped in the S2 state to further probe the binding of methanol in an S state dependent manner. The signals are induced with either visible or near-infrared light illumination provided at 5-10 K where methanol cannot bind or unbind from its site. The results imply that the binding of methanol not only changes the magnetic properties of the CaMn4 cluster but also the hydrogen bond network in the oxygen evolving complex (OEC), thereby affecting the relative charge of the S2 state. The induction mechanisms for the two split EPR signals are different resulting in two different redox states, S2YZ(•) and S1YZ(•) respectively. The two states show different methanol dependence for their induction. This indicates the existence of two binding sites for methanol in the CaMn4 cluster. It is proposed that methanol binds to MnA with high affinity and to MnD with lower affinity. The molecular nature and S-state dependence of the methanol binding to each respective site are discussed.

Probing the N-terminal sequence of spinach PsbO: evidence that essential threonine residues bind to different functional sites in eukaryotic photosystem II

The N-terminal ¹E-⁶L domain of the manganese-stabilizing protein (PsbO) from spinach prevents non-specific binding of the subunit to photosystem II (PSII) and deletions of the ¹E-⁷T or ¹E-¹⁵T sequences from the PsbO N-terminus reduce or impair, respectively, functional binding of PsbO to PSII (Popelkova et al., Biochemistry 42:6193-6200, 2003). The work presented here provides deeper insights into the interaction of PsbO with PSII. The data show that a single mutation, ¹⁵T → A in mature PsbO from spinach reduces the stoichiometry of its functional binding from two to one subunit per PSII and decreases reconstitution of activity to about 45 % of the wild-type control. Replacement of the ¹E-⁶L domain with ⁶M in the T15A PsbO mutant has no additional negative effect on recovery of O₂ evolution activity, but it significantly weakens both functional and nonspecific binding of the truncated mutant to PSII. These results suggest that the ¹⁵T side-chain by itself is essential for binding of one of two PsbO subunits to eukaryotic PSII and that specific PSII-binding sites for PsbO are distinguishable; one PSII-binding site does not require PsbO-¹⁵T and probably interacts with the other N-terminal domain of PsbO. Identity of the latter domain is revealed by a requirement for the presence of the ¹E-⁶L sequence that is shown here to be necessary for high-affinity binding of PsbO to PSII. When combined with previous results, the data presented here lead to a more detailed model for PsbO binding in eukaryotic PSII.

pH optimum of the photosystem II H₂O oxidation reaction: effects of PsbO, the manganese-stabilizing protein, Cl- retention, and deprotonation of a component required for O₂ evolution activity

Hydroxide ion inhibits Photosystem II (PSII) activity by extracting Cl(-) from its binding site in the O(2)-evolving complex (OEC) under continuous illumination [Critchley, C., et al. (1982) Biochim. Biophys. Acta 682, 436]. The experiments reported here examine whether two subunits of PsbO, the manganese-stabilizing protein, bound to eukaryotic PSII play a role in protecting the OEC against OH(-) inhibition. The data show that the PSII binding properties of PsbO affect the pH optimum for O(2) evolution activity as well as the Cl(-) affinity of the OEC that decreases with an increasing pH. These results suggest that PsbO functions as a barrier against inhibition of the OEC by OH(-). Through facilitation of efficient retention of Cl(-) in PSII [Popelkova, H., et al. (2008) Biochemistry 47, 12593], PsbO influences the ability of Cl(-) to resist OH(-)-induced release from its site in the OEC. Preventing inhibition by OH(-) allows for normal (short) lifetimes of the S(2) and S(3) states in darkness [Roose, J. L., et al. (2011) Biochemistry 50, 5988] and for maximal steady-state activity by PSII. The data presented here indicate that activation of H(2)O oxidation occurs with a pK(a) of ∼6.5, which could be a function of deprotonation of one or more amino acid residues that reside near the OEC active site on the D1 and CP43 intrinsic subunits of the PSII reaction center.

Probing the topography of the photosystem II oxygen evolving complex: PsbO is required for efficient calcium protection of the manganese cluster against dark-inhibition by an artificial reductant

The photosystem II (PSII) manganese-stabilizing protein (PsbO) is known to be the essential PSII extrinsic subunit for stabilization and retention of the Mn and Cl(-) cofactors in the oxygen evolving complex (OEC) of PSII, but its function relative to Ca(2+) is less clear. To obtain a better insight into the relationship, if any, between PsbO and Ca(2+) binding in the OEC, samples with altered PsbO-PSII binding properties were probed for their potential to promote the ability of Ca(2+) to protect the Mn cluster against dark-inhibition by an exogenous artificial reductant, N,N-dimethylhydroxylamine. In the absence of the PsbP and PsbQ extrinsic subunits, Ca(2+) and its surrogates (Sr(2+), Cd(2+)) shield Mn atoms from inhibitory reduction (Kuntzleman et al., Phys Chem Chem Phys 6:4897, 2004). The results presented here show that PsbO exhibits a positive effect on Ca(2+) binding in the OEC by facilitating the ability of the metal to prevent inhibition of activity by the reductant. The data presented here suggest that PsbO may have a role in the formation of the OEC-associated Ca(2+) binding site by promoting the equilibrium between bound and free Ca(2+) that favors the bound metal.

An alternative method for calcium depletion of the oxygen evolving complex of photosystem II as revealed by the dark-stable multiline EPR signal

The dark-stable multiline EPR signal of photosystem II (PSII) is associated with a slow-decaying S(2) state that is due to Ca(2+) loss from the oxygen evolving complex. Formation of the signal was observed in intact PSII in the presence of 100-250 mM NaCl at pH 5.5. Both moderately high NaCl concentration and decreased pH were required for its appearance in intact PSII. It was estimated that only a portion of oxygen evolving complexes was responsible for the signal (about 20% in 250 mM NaCl), based on the loss of the normal S(2)-state multiline signal. The formation of the dark-stable multiline signal in intact PSII at pH 5.5 could be reversed by addition of 15 mM Ca(2+) in the presence of moderately high NaCl, confirming that it was the absence of Ca(2+) that led to its appearance. Formation of the dark-stable multiline signal in NaCl-washed PSII, which lacks the PsbP (23 kDa) and PsbQ (17 kDa) subunits, was observed in about 80% of the sample in the presence of 150 mM NaCl at pH 5.5, but some signal was also observed under normal buffer conditions. In both intact and NaCl-washed PSII, the S(2)Y(Z). signal, which is also characteristic of Ca(2+) depletion, appeared upon subsequent illumination. Formation of the dark-stable multiline signal took place in the absence of Ca(2+) chelator or polycarboxylic acids, indicating that the signal did not require their direct binding as has been proposed previously. The conditions used here were milder than those used to produce the signal in previous studies and included a preillumination protocol to maximize the dark-stable S(2) state. Based on these conditions, it is suggested that Ca(2+) release occurred through protonation of key residues that coordinate Ca(2+) at low pH, followed by displacement of Ca(2+) with Na(+) by mass action at the moderately high NaCl concentration.