Photosynthetic oxygen evolution by photosystem II
Research in the Haddy lab is on the catalytic production of oxygen by photosystem II (PSII) during photosynthesis by plants and cyanobacteria. The evolution of O2 occurs as a result of the biological conversion of solar energy into chemical energy. Because of its key role in the natural energy cycle of the earth, catalytic O2 production in plants is an important model for artificial solar energy systems. In photosystem II, H2O is oxidized to O2 at a Mn4Ca cluster with electrons donated to the electron transport process driven by light absorption within the thylakoid membranes of chloroplasts. In our laboratory, we are interested in how this complex catalytic reaction is promoted by the Ca2+ ion of the Mn4Ca cluster and two nearby Cl- ions. To carry out this research, we combine biochemical techniques such as protein purification and enzyme kinetics assays with physical techniques, particularly electron paramagnetic resonance (EPR) spectroscopic. Projects in our lab have been funded by the National Science Foundation, the Dreyfus Foundation, and the Research Corporation.
Photosystem II is a large integral-membrane protein complex located in the thylakoid membrane of chloroplasts. At its center PSII contains the light-absorbing reaction center, a cluster of chlorophyll molecules at which primary charge separation occurs after it has entered an excited state. The charge separation within PSII, along with a similar event within photosystem I, promotes electron transport through numerous redox active centers, eventually producing the transmembrane proton gradient that is required for the synthesis of ATP. At the site of O2 evolution, where electron transport begins, the Mn4Ca cluster passes through five oxidation states during its catalytic cycle. In addition, the site includes a redox active tyrosine residue (Tyr Z), other coordinating amino acid residues, and two nearby Cl- ions. Although much is known about the structure of PSII, thanks to several X-ray crystallography studies, the mechanism by which H2O is converted into O2 is not well understood. For example, while the oxidation states of the Mn ions are approximately known for the various stages of catalysis, the nature of the O-O bond formation step is largely unknown. Similarly while it is evident that the directly bound Ca2+ ion and the nearby Cl- ion have important roles in coordinating the step-wise oxidation of the Mn ions and the transfer of electrons from H2O, the details are poorly understood. The Ca2+ ion, although somewhat labile, appears to have a critical role in electron transfer to the Tyr Z residue. The Cl- ion, although more distantly bound, is required for electron transfer in the higher oxidation states. Both ions may participate in proton movement or a hydrogen bond network, function in charge neutralization, influence the coupling of the Mn4Ca cluster, or a combination of these possibilities.
Our studies have the eventual goal of leading to a better understanding of how Ca2+ and Cl- ions affect the function of the Mn4Ca cluster in the catalytic cycle that produces O2. Using enzyme kinetics studies, we have characterized the function of Cl- by replacing it with anions that can activate or inhibit O2 evolution. For example, studies of inhibition kinetics have characterized fluoride and azide anions as competitive inhibitors of chloride activation. Iodide and nitrite have been found to be activators at low concentrations but inhibitors at higher concentrations. Other enzyme kinetics studies have examined the Ca2+ requirement of PSII, its dependence on Cl- as a co-activator, and the effects of decreased pH. Electron paramagnetic resonance spectroscopy, which detects unpaired electrons, is used to observe the Mn4Ca cluster and the nearby tyrosine radical, Tyr Z, which accepts electron from the Mn4Ca cluster. The signals can be used to determine the oxidation state of the Mn4Ca cluster and the degree of inhibition of electron transfer to Tyr Z, among other things. By combining enzyme kinetics analyses with EPR spectroscopy, the effects of Cl- and Ca2+ are understood at a molecular level.
Haddy, A. “Mn and photosynthetic systems” in Encyclopedia of Metalloproteins (R. H. Kretsinger, V. N. Uversky, and E. A. Permyakov, Eds.), 2013, Springer Publishers
Haddy, A.; Ore, B.M. "An alternative method for calcium depletion of the oxygen evolving complex of photosystem II as revealed by the dark-stable multiline EPR signal" Biochemistry 2010 49, 3805-3814
Kuntzleman, T; Haddy, A. "Fluoride Inhibition of Photosystem II and the Effect of Removal of the PsbQ Subunit" Photosynthesis Research 2009 102, 7-19
Haddy, A. "EPR Spectroscopy of the Manganese Cluster of Photosystem II" (Review) Photosynthesis Research 2007 92, 357-368
Bryson, D.I.; Doctor, N.; Johnson, R.; Baranov, S.; Haddy, A. "Characteristics of Iodide Activation and Inhibition of Oxygen Evolution by Photosystem II" Biochemistry 2005 44, 7354-7360Haddy, A.; Lakshmi, K.V.; Brudvig, G.W.; Frank, H.A. "Q-band EPR of the S2 state of Photosystem II confirms an S=5/2 origin of the X-band g=4.1 signal" Biophysical Journal 2004 87, 2885-2896