Isotopic labelling of photosystem II in Thermosynechococcus elongatus

Further evidence that interfacial water is the main "driving force" of protein dynamics: a neutron scattering study on perdeuterated C-phycocyanin

The fundamental role of hydration water (also called interfacial water) is widely recognized in protein flexibility, especially in the existence of the so-called protein "dynamical transition" at around 220 K. In the present study, we take advantage of perdeuterated C-phycocyanin (CPC) and elastic incoherent neutron scattering (EINS) to distinguish between protein dynamics and interfacial water dynamics. Powders of hydrogenated (hCPC) and perdeuterated (dCPC) CPC protein have been hydrated, respectively, with D(2)O or H(2)O and measured by EINS to separately probe protein dynamics (hCPC/D(2)O) and water dynamics (dCPC/H(2)O) at different time- and length-scales. We find that "fast" (<20 ps) local mean-square displacements (MSD) of both protein and interfacial water coincide all along the temperature range, with the same dynamical transition temperature at ~220 K. On higher resolution (<400 ps), two different types of motions can be separated: (i) localized motions with the same amplitude for CPC and hydration water and two transitions at ~170 and ~240 K for both; (ii) large scale fluctuations exhibiting for both water molecules and CPC protein a single transition at ~240 K, with a significantly higher amplitude for the interfacial water than for CPC. Moreover, by comparing these motions with bulk water MSD measured under the same conditions, we show no coupling between bulk water dynamics and protein dynamics all along the temperature range. These results show that interfacial water is the main "driving force" governing both local and large scale motions in proteins.

Probing the coupling between proton and electron transfer in photosystem II core complexes containing a 3-fluorotyrosine

The catalytic cycle of numerous enzymes involves the coupling between proton transfer and electron transfer. Yet, the understanding of this coordinated transfer in biological systems remains limited, likely because its characterization relies on the controlled but experimentally challenging modifications of the free energy changes associated with either the electron or proton transfer. We have performed such a study here in Photosystem II. The driving force for electron transfer from Tyr(Z) to P(680)(*+) has been decreased by approximately 80 meV by mutating the axial ligand of P(680), and that for proton transfer upon oxidation of Tyr(Z) by substituting a 3-fluorotyrosine (3F-Tyr(Z)) for Tyr(Z). In Mn-depleted Photosystem II, the dependence upon pH of the oxidation rates of Tyr(Z) and 3F-Tyr(Z) were found to be similar. However, in the pH range where the phenolic hydroxyl of Tyr(Z) is involved in a H-bond with a proton acceptor, the activation energy of the oxidation of 3F-Tyr(Z) is decreased by 110 meV, a value which correlates with the in vitro finding of a 90 meV stabilization energy to the phenolate form of 3F-Tyr when compared to Tyr (Seyedsayamdost et al. J. Am. Chem. Soc. 2006, 128,1569-1579). Thus, when the phenol of Y(Z) acts as a H-bond donor, its oxidation by P(680)(*+) is controlled by its prior deprotonation. This contrasts with the situation prevailing at lower pH, where the proton acceptor is protonated and therefore unavailable, in which the oxidation-induced proton transfer from the phenolic hydroxyl of Tyr(Z) has been proposed to occur concertedly with the electron transfer to P(680)(*+). This suggests a switch between a concerted proton/electron transfer at pHs < 7.5 to a sequential one at pHs > 7.5 and illustrates the roles of the H-bond and of the likely salt-bridge existing between the phenolate and the nearby proton acceptor in determining the coupling between proton and electron transfer.