On the simulation and interpretation of substrate-water exchange experiments in photosynthetic water oxidation

• Published in: Photosynthesis research 2024-12, Vol.162 (2), p.413-426
• Main Authors: Chernev, Petko, Aydin, A. Orkun, Messinger, Johannes
• Format: Article
• Language: English
• Subjects: Biochemistry; Biomedical and Life Sciences; calcium; data collection; Intermediates; Kinetics; Life Sciences; Manganese; Manganese - metabolism; mass spectrometry; Molecular modelling; molecular models; Oxidation; Oxidation-Reduction; oxygen; Oxygen - metabolism; Photosynthesis - physiology; Photosystem II; Photosystem II Protein Complex - metabolism; Plant Genetics and Genomics; Plant Physiology; Plant Sciences; species; Water - metabolism; Water exchange
• ISSN: 0166-8595; 1573-5079; 1573-5079
• DOI: 10.1007/s11120-024-01084-8

Water oxidation by photosystem II (PSII) sustains most life on Earth, but the molecular mechanism of this unique process remains controversial. The ongoing identification of the binding sites and modes of the two water-derived substrate oxygens (‘substrate waters’) in the various intermediates (S i states, i  = 0, 1, 2, 3, 4) that the water-splitting tetra-manganese calcium penta-oxygen (Mn 4 CaO 5 ) cluster attains during the reaction cycle provides central information towards resolving the unique chemistry of biological water oxidation. Mass spectrometric measurements of single- and double-labeled dioxygen species after various incubation times of PSII with H 2 18 O provide insight into the substrate binding modes and sites via determination of exchange rates. Such experiments have revealed that the two substrate waters exchange with different rates that vary independently with the S i state and are hence referred to as the fast (W f ) and the slow (W S ) substrate waters. New insight for the molecular interpretation of these rates arises from our recent finding that in the S 2 state, under special experimental conditions, two different rates of W S exchange are observed that appear to correlate with the high spin and low spin conformations of the Mn 4 CaO 5 cluster. Here, we reexamine and unite various proposed methods for extracting and assigning rate constants from this recent data set. The analysis results in a molecular model for substrate-water binding and exchange that reconciles the expected non-exchangeability of the central oxo bridge O5 when located between two Mn(IV) ions with the experimental and theoretical assignment of O5 as W S in all S states. The analysis also excludes other published proposals for explaining the water exchange kinetics.