Structural Changes in the Mn(4)Ca Cluster And the Mechanism of Photosynthetic Water Splitting
Photosynthetic water oxidation, where water is oxidized to dioxygen, is a fundamental chemical reaction that sustains the biosphere. This reaction is catalyzed by a Mn{sub 4}Ca complex in the photosystem II (PS II) oxygen-evolving complex (OEC): a multiprotein assembly embedded in the thylakoid membranes of green plants, cyanobacteria, and algae. The mechanism of photosynthetic water oxidation by the Mn{sub 4}Ca cluster in photosystem II is the subject of much debate, although lacking structural characterization of the catalytic intermediates. Biosynthetically exchanged Ca/Sr-PS II preparations and x-ray spectroscopy, including extended x-ray absorption fine structure (EXAFS), allowed us to monitor Mn-Mn and Ca(Sr)-Mn distances in the four intermediate S states, S{sub 0} through S{sub 3}, of the catalytic cycle that couples the one-electron photochemistry occurring at the PS II reaction center with the four-electron water-oxidation chemistry taking place at the Mn{sub 4}Ca(Sr) cluster. We have detected significant changes in the structure of the complex, especially in the Mn-Mn and Ca(Sr)-Mn distances, on the S{sub 2}-to-S{sub 3} and S{sub 3}-to-S{sub 0} transitions. These results implicate the involvement of at least one common bridging oxygen atom between the Mn-Mn and Mn-Ca(Sr) atoms in the O-O bond formation. Because PS II cannot advance beyond the S{sub 2} state in preparations that lack Ca(Sr), these results show that Ca(Sr) is one of the critical components in the mechanism of the enzyme. The results also show that Ca is not just a spectator atom involved in providing a structural framework, but is actively involved in the mechanism of water oxidation and represents a rare example of a catalytically active Ca cofactor.
- Research Organization:
- Stanford Linear Accelerator Center (SLAC)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 953052
- Report Number(s):
- SLAC-REPRINT-2009-232
- Journal Information:
- Proc. Nat. Acad. Sci.105:1879,2008, Journal Name: Proc. Nat. Acad. Sci.105:1879,2008 Journal Issue: 6 Vol. 105; ISSN 0027-8424; ISSN PNASA6
- Country of Publication:
- United States
- Language:
- English