Proton/Hydrogen Transfer Affects the S-State-Dependent Microsecond Phases of P680+ Reduction during Water Splitting

• Published in: Biochemistry (Easton) 1998-03, Vol.37 (11), p.3974-3981
• Main Authors: Schilstra, Maria J, Rappaport, Fabrice, Nugent, Jonathan H. A, Barnett, Christopher J, Klug, David R
• Format: Article
• Language: English
• Subjects: Chlorophyll - metabolism; Deuterium Oxide - metabolism; Electron Transport; Hydrogen - metabolism; Kinetics; Light-Harvesting Protein Complexes; Oxidation-Reduction; Photosynthetic Reaction Center Complex Proteins - metabolism; Protons; Spinacia oleracea; Water - metabolism
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi9713815

To investigate a possible coupling between P680+ reduction and hydrogen transfer, we studied the effects of H2O/D2O exchange on the P680+ reduction kinetics in the nano- and microsecond domains. We concentrated on studying the period-4 oscillatory (i.e., S-state-related) part of the reduction kinetics, by analyzing the differences between the P680+ reduction curves, rather than the full kinetics. Earlier observations that P680+ reduction kinetics have microsecond components were confirmed:  the longest observable lifetime whose amplitude showed period-4 oscillations was 30 μs. We found that solvent isotope exchange left the nanosecond phases of the P680+ reduction unaltered. However, a significant effect on the oscillatory microsecond components was observed. We propose that, at least in the S0/S1 and S3/S0 transitions, hydrogen (proton) transfer provides an additional decrease in the free energy of the YZ +P680 state with respect to the YZP680+ state. This implies that relaxation of the state YZ +P680 is required for complete reduction of P680+ and for efficient water splitting. The kinetics of the P680+ reduction suggest that it is intraprotein proton/hydrogen rearrangement/transfer, rather than proton release to the bulk, which is occurring on the 1−30 μs time scale.