Iain McConnell

Energy Conversion in Natural and Artificial Photosynthesis

Modern civilization is dependent upon fossil fuels, a nonrenewable energy source originally provided by the storage of solar energy. Fossil-fuel dependence has severe consequences, including energy security issues and greenhouse gas emissions. The consequences of fossil-fuel dependence could be avoided by fuel-producing artificial systems that mimic natural photosynthesis, directly converting solar energy to fuel. This review describes the three key components of solar energy conversion in photosynthesis: light harvesting, charge separation, and catalysis. These processes are compared in natural and in artificial systems. Such a comparison can assist in understanding the general principles of photosynthesis and in developing working devices, including photoelectrochemical cells, for solar energy conversion.

EPR–ENDOR Characterization of (17O, 1H, 2H) Water in Manganese Catalase and Its Relevance to the Oxygen-Evolving Complex of Photosystem II

The synthesis of efficient water-oxidation catalysts demands insight into the only known, naturally occurring water-oxidation catalyst, the oxygen-evolving complex (OEC) of photosystem II (PSII). Understanding the water oxidation mechanism requires knowledge of where and when substrate water binds to the OEC. Mn catalase in its Mn(III)-Mn(IV) state is a protein model of the OECs S(2) state. From (17)O-labeled water exchanged into the di-μ-oxo di-Mn(III,IV) coordination sphere of Mn catalase, CW Q-band ENDOR spectroscopy revealed two distinctly different (17)O signals incorporated in distinctly different time regimes. First, a signal appearing after 2 h of (17)O exchange was detected with a 13.0 MHz hyperfine coupling. From similarity in the time scale of isotope incorporation and in the (17)O μ-oxo hyperfine coupling of the di-μ-oxo di-Mn(III,IV) bipyridine model (Usov, O. M.; Grigoryants, V. M.; Tagore, R.; Brudvig, G. W.; Scholes, C. P. J. Am. Chem. Soc. 2007, 129, 11886-11887), this signal was assigned to μ-oxo oxygen. EPR line broadening was obvious from this (17)O μ-oxo species. Earlier exchange proceeded on the minute or faster time scale into a non-μ-oxo position, from which (17)O ENDOR showed a smaller 3.8 MHz hyperfine coupling and possible quadrupole splittings, indicating a terminal water of Mn(III). Exchangeable proton/deuteron hyperfine couplings, consistent with terminal water ligation to Mn(III), also appeared. Q-band CW ENDOR from the S(2) state of the OEC was obtained following multihour (17)O exchange, which showed a (17)O hyperfine signal with a 11 MHz hyperfine coupling, tentatively assigned as μ-oxo-(17)O by resemblance to the μ-oxo signals from Mn catalase and the di-μ-oxo di-Mn(III,IV) bipyridine model.

Chloride Regulation of Enzyme Turnover: Application to the Role of Chloride in Photosystem II

Chloride-dependent α-amylases, angiotensin-converting enzyme (ACE), and photosystem II (PSII) are activated by bound chloride. Chloride-binding sites in these enzymes contain a positively charged Arg or Lys residue crucial for chloride binding. In α-amylases and ACE, removal of chloride from the binding site triggers formation of a salt bridge between the positively charged Arg or Lys residue involved in chloride binding and a nearby carboxylate residue. The mechanism for chloride activation in ACE and chloride-dependent α-amylases is 2-fold: (i) correctly positioning catalytic residues or other residues involved in stabilizing the enzyme-substrate complex and (ii) fine-tuning of the pKa of a catalytic residue. By using examples of how chloride activates α-amylases and ACE, we can gain insight into the potential mechanisms by which chloride functions in PSII. Recent structural evidence from cyanobacterial PSII indicates that there is at least one chloride-binding site in the vicinity of the oxygen-evolving complex (OEC). Here we propose that, in the absence of chloride, a salt bridge between D2:K317 and D1:D61 (and/or D1:E333) is formed. This can cause a conformational shift of D1:D61 and lower the pKa of this residue, making it an inefficient proton acceptor during the S-state cycle. Movement of the D1:E333 ligand and the adjacent D1:H332 ligand due to chloride removal could also explain the observed change in the magnetic properties of the manganese cluster in the OEC upon chloride depletion.

Regulation of Photosystem II Electron Transport by Bicarbonate

In oxygenic photosynthesis, carbon dioxide is fixed by ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and further reduced to carbohydrates. However, CO2, in the form of carbonate or bicarbonate, is also directly involved in the “light reactions” through structural and regulatory roles within Photosystem II (PS II). A notable feature is antagonistic interactions between bicarbonate (carbonate) and monovalent anions such as formate within PS II. Incubation of PS II-containing samples with formate results in the inhibition of electron flow activity, which can be restored only by the addition of bicarbonate. This “bicarbonate effect” influences molecular processes associated with both the electron acceptor and electron donor sides of PS II. The bicarbonate interaction on the acceptor side is located in the region of the primary and secondary quinones and contributes to the protonation states associated with quinol formation. At physiological pH, bicarbonate (carbonate) is a ligand to the non-heme iron and forms hydrogen bonds to several amino acids of the D1 and D2 proteins. Bicarbonate may stabilize, through conformational means, the reaction center proteins by protonation of certain amino acids near the secondary quinone electron acceptor. A possible functional role in vivo is that it controls PS II electron flow in order to ameliorate the impact of stress conditions leading to, for instance, photoinhibition or thermoinactivation. The role of bicarbonate on the donor of PS II has been the subject of renewed interest and bicarbonate has been suggested to play a role in the assembly of the Mn4Ca cluster during photoactivation. Additionally, a role as a catalytic base or proton transporter on the donor side of PS II has been proposed. However, while clear evidence for bicarbonate’s role on the acceptor side has been established, experiments designed to elucidate the putative role of bicarbonate on the donor side of PS II have not yet provided convincing evidence.

Substrate water binding and oxidation in photosystem II

This mini review presents a general introduction to photosystem II with an emphasis on the oxygen evolving complex. An attempt is made to summarise what is currently known about substrate interaction in the oxygen evolving complex of photosystem II in terms of the nature of the substrate, the timing and the location of its binding. As the nature of substrate water binding has a direct bearing on the mechanism of O–O bond formation in PSII, a discussion of O–O bond formation follows the summary of current opinion in substrate interaction.

Substrate Water Oxygen Exchange in Photosystem II: Insights from Mutants and Ca vs. Sr Substitution

Oxygen-18 exchange kinetics have been conducted for three point mutants associated with the oxygen evolving core of PSII and for centers that have undergone biosynthetic Sr/Ca exchange. The mutants D1-D170H, D1-E189Q are potential ligands to the Mn ions suggested from mutational and X-ray crystallographic studies. A third point mutation D1-D61N is in the second coordination sphere. In the case of the proposed Mn ligands D170H and E189Q the 18O exchange rates are perturbed only weakly, suggesting that substrate water is perhaps not associated with the metal ligation points. The D61N mutation has more significant effects, indicative of a central role of this residue in substrate delivery. The Sr substituted core PSII preparations compared to Ca show one substrate water molecule is affected by an increase in exchange, consistent with the proposed larger ionic radius of the Sr ions and weaker binding.

Insights into the photosynthetic water oxidation mechanism: Determination of the dissociation constants for the substrate water binding sites from 18O isotope exchange measurements

The exact nature of the substrate water binding sites and O-O bond formation during the oxidation of water in photosystem II are unknown. We have employed 18O water isotope exchange measurements to address this problem. For samples in the S3 state the rate of 18O water exchange can be determined by measuring the amount of 18O incorporated into the O2 produced as a function of time between the rapid injection of enriched H2 18O water and a turnover flash. At m/z = 34 (16,18O2) two distinct kinetic phases (one fast and one slow) are observed while only one phase (slow) is found at m/z = 36 (18,18O2). The data can be fit to pseudo first order kinetics, with the 18,18O2 data fit to a single exponential function and the 16,18O2 data fit to a bi-exponential function. The two distinct phases imply two separate and chemically distinct binding sites for substrate water. In a new approach each site is considered to involve an equilibrium between a bound H2 16O and H2 18O. A model constructed from this approach infers specific koff and kon rate constants (kd). These are used to derive the dissociation constants for the two substrate water binding sites. Surprisingly different values further support the concept that the two water binding sites are heterogeneous in character.