ENDOR Spectroscopy Reveals Light Induced Movement of the H-Bond from Ser-L223 upon Forming the Semiquinone (QB-•) in Reaction Centers from Rhodobacter sphaeroides
| Autor: | E. C. Abresch, Marco Flores, Mark L. Paddock, Melvin Y. Okamura, C. Chang, R.A. Isaacson |
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| Rok vydání: | 2007 |
| Předmět: |
Models
Molecular Light Semiquinone Photosynthetic Reaction Center Complex Proteins Rhodobacter sphaeroides macromolecular substances Photochemistry Biochemistry Article Electron Transport Motion Protein structure Bacterial Proteins Freezing Serine Spectroscopy Hyperfine structure biology Hydrogen bond Chemistry Electron Spin Resonance Spectroscopy Quinones Hydrogen Bonding biology.organism_classification Electron transport chain Mutation Ground state |
| Zdroj: | Biochemistry. 46:8234-8243 |
| ISSN: | 1520-4995 0006-2960 |
| DOI: | 10.1021/bi7005256 |
| Popis: | Proton ENDOR spectroscopy was used to monitor local conformational changes in bacterial reaction centers (RC) associated with the electron transfer reaction DQB → D+•QB−• using mutant RCs capable of photo-reducing QB at cryogenic temperatures. The charge separated state D+•QB−• was studied in mutant RCs formed by either (i) illuminating at low temperature (77K) a sample frozen in the dark (ground state protein conformation) or (ii) illuminating at room temperature prior to and during the freezing (charge separated state protein conformation). The charge recombination rates from the two states differed greatly (>106 fold) as shown previously, indicating a structural change (Paddock et al (2006) Biochemistry 45, 14032 - 14042). ENDOR spectra of QB−• from both samples (35 GHz, 77K) showed three nearly identical sets of hyperfine couplings due to exchangeable protons that were similar to those for QB−• in native RCs indicating that in all RCs, QB−• was located at the proximal position near the metal site. In contrast, one set of H-bond couplings was observed only in the sample frozen under illumination in which the protein can relax prior to freezing. This H-bond was assigned to an interaction between the Ser-L223 hydroxyl and QB−• based on its absence in Ser L223 → Ala mutant RCs. The Ser-L223 hydroxyl H-bond was also observed in the native RCs frozen under illumination. Thus, part of the protein relaxation in response to light induced charge separation involves the formation of an H-bond between the OH group of Ser-L223 and the anionic semiquinone QB−•. This proton movement serves to stabilize the charge separated state and facilitate proton transfer to reduced QB. |
| Databáze: | OpenAIRE |
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