Electrogenic Protonation of the Secondary Quinone Acceptor QB in Spinach Photosystem II Complexes Incorporated into Lipid Vesicles

Abstract

The generation of transmembrane electric potential difference (delta psi) in quinone acceptor complex of proteoliposomes containing core complexes of photosystem II from spinach was studied using for the measurements a direct electrometric technique. Besides the fast increase in the membrane potential associated with the electron transfer between the redox-active tyrosine 161 residue (Y(Z)) in D1 polypeptide and the primary quinone acceptor Q(A), an additional electrogenic phase with tau approximately 0.85 msec at pH 7.3 and the maximal relative amplitude of approximately 11% of the Y(Z)ox Q(A)- phase was observed after the second light flash. The sensitivity of this phase to diuron (an inhibitor of electron transfer between Q(A) and the secondary quinone acceptor Q(B)), the dependence of its amplitude on the light flash parity, and also a decrease in its rate constant with increase in pH indicated that it was due to dismutation of Q(A)- and Q(B)- with the subsequent protonation of a doubly reduced plastoquinone molecule: Q(A)- Q(B)- + 2H+ --> Q(A)Q(B)H2.

Photosysem II: where does the light-induced voltage come from?

“…No essential ∆Ψ, aside from the phase associated with electron transfer between Y Z and Q A (fast phase), is generated when the electron is transferred from Q A to Q B , since the two plastoquinones are practically equidistant from the membrane surface, as determined by PS II X-ray crystal structure data (3)(4)(5). Only following the second flash, when the formation of a doubly-reduced quinone species Q B H 2 occurs, trapping of two protons takes place and an additional electrogenic phase of a ∆Ψ with an amplitude corresponding to ∼11% of the fast phase (τ∼0.85 ms at pH 7.5) is formed (41). The following observations such as the sensitivity of this phase to diuron, an inhibitor of electron transfer between Q A and Q B , the flash-number dependence of its amplitude and the decrease of its risetime and amplitude with decreasing pH indicate that this electrogenic reaction is associated with protonation of Q B 2- (7).…”

Electrogenic reactions and dielectric properties of photosystem II

“…The study of the DW generation in proteoliposomes with PS II core particles in the presence of decyl-plastoquinone using a direct electrometrical technique showed that in addition to the fast increase in the photovoltage due to Y Z ox Q A -formation, an electrogenic phase with the maximal amplitude of *11% of the Y Z ox Q A -phase was observed after the second flash (Mamedov et al 2005). The sensitivity of this phase to diuron (an inhibitor of electron transfer between Q A and Q B ), the flash-number dependence of the phase amplitude, as well as pH-dependences of the amplitude and the rate constant indicate that the additional electrogenic phase is due to dismutation of Q A -and Q B -with the subsequent protonation of a doubly reduced plastoquinone molecule:…”