Temperature- and Hydration-Dependent Protein Dynamics in Photosystem II of Green Plants Studied by Quasielastic Neutron Scattering

Abstract

Protein dynamics in hydrated and vacuum-dried photosystem II (PS II) membrane fragments from spinach has been investigated by quasielastic neutron scattering (QENS) in the temperature range between 5 and 300 K. Three distinct temperature ranges can be clearly distinguished by active type(s) of protein dynamics: (A) At low temperatures (T < 120 K), the protein dynamics of both dry and hydrated PS II is characterized by harmonic vibrational motions. (B) In the intermediate temperature range (120 < T < 240 K), the total mean square displacement <u2>total slightly deviates from the predicted linear behavior. The QENS data indicate that this deviation, which is virtually independent of the extent of hydration, is due to a partial onset of diffusive protein motions. (C) At temperatures above 240 K, the protein flexibility drastically changes because of the onset of diffusive (large-amplitude) protein motions. This dynamical transition is clearly hydration-dependent since it is strongly suppressed in dry PS II. The thermally activated onset of protein flexibility as monitored by QENS is found to be strictly correlated with the temperature-dependent increase of the electron transport efficiency from Q(A)(-) to QB (Garbers et al. (1998) Biochemistry 37, 11399-11404). Analogously, the freezing of protein mobility by dehydration in dry PS II appears to be responsible for the blockage of Q(A)(-) reoxidation by Q(B) at hydration values lower than 45% r.h. (Kaminskaya et al. (2003) Biochemistry 42, 8119-8132). Similar effects were observed for reactions of the water-oxidizing complex as outlined in the Discussion section.

The functional role of protein dynamics in photosynthetic reaction centers investigated by elastic and quasielastic neutron scattering

“…PS II membrane fragments hydrated at 90% r.h. [10]. The QENS spectra could be fit with two Lorentzian components corresponding to relaxation times of about 19 ps and 0.8 ps, respectively, at all temperatures.…”

“…QENS studies of PS II membrane fragments were initially carried out varying sample temperature [10] and hydration [11]. For this purpose, PS II membrane fragments were prepared from spinach thylakoids as described in detail in [10].…”

“…For this purpose, PS II membrane fragments were prepared from spinach thylakoids as described in detail in [10]. The samples were hydrated using D 2 O in order to suppress the hydration water scattering and equilibrated at well defined r.h.-values using vapors of saturated salt solutions.…”

“…3, the dynamical transition in PS II membrane fragments is hydration dependent and found to be absent in dry and fast (QISF 2 , triangles) quasielastic components obtained from QENS data of PS II membrane fragments hydrated at 90% r.h. according to [10]. The three temperature ranges A, B, and C are discussed in the text.…”

“…So far, ENS and QENS have been used to study the temperature and hydration dependence of molecular dynamics in PS II membranes of green plants [10][11][12][13][14] as well as that of bacterial RCs [15,16]. This review provides a short overview about the results of these studies.…”