title: M234Glu is a component of the proton sponge in the reaction center from photosynthetic bacteria.
abstract: Bacterial reaction centers use light energy to couple the uptake of protons to the successive semi-reduction of two quinones, namely Q(A) and Q(B). These molecules are situated symmetrically in regard to a non-heme iron atom. Four histidines and one glutamic acid, M234Glu, constitute the five ligands of this atom. By flash-induced absorption spectroscopy and delayed fluorescence we have studied in the M234EH and M234EL variants the role played by this acidic residue on the energetic balance between the two quinones as well as in proton uptake. Delayed fluorescence from the P(+)Q(A)(-) state (P is the primary electron donor) and temperature dependence of the rate of P(+)Q(A)(-) charge recombination that are in good agreement show that in the two RC variants, both Q(A)(-) and Q(B)(-) are destabilized by about the same free energy amount: respectively approximately 100 +/- 5 meV and 90 +/- 5 meV for the M234EH and M234EL variants, as compared to the WT. Importantly, in the M234EH and M234EL variants we observe a collapse of the high pH band (present in the wild-type reaction center) of the proton uptake amplitudes associated with formation of Q(A)(-) and Q(B)(-). This band has recently been shown to be a signature of a collective behaviour of an extended, multi-entry, proton uptake network. M234Glu seems to play a central role in the proton sponge-like system formed by the RC protein.
authors.affiliation: Laboratoire de Chimie Physique, UMR 8000, University of Paris-Sud 11/CNRS, 91405 cedex, France.
authors.forename: Hélène
authors.initials: H
authors.lastname: Cheap
authors.forename: Sophie
authors.initials: S
authors.lastname: Bernad
authors.forename: Valérie
authors.initials: V
authors.lastname: Derrien
authors.forename: László
authors.initials: L
authors.lastname: Gerencsér
authors.forename: Julia
authors.initials: J
authors.lastname: Tandori
authors.forename: Pedro
authors.initials: P
authors.lastname: de Oliveira
authors.forename: Deborah K
authors.initials: DK
authors.lastname: Hanson
authors.forename: Péter
authors.initials: P
authors.lastname: Maróti
authors.forename: Pierre
authors.initials: P
authors.lastname: Sebban
mesh_terms: D004579:Electron Transport; D004734:Energy Metabolism; D005453:Fluorescence; D018698:Glutamic Acid / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D010788:Photosynthesis; D011522:Protons; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D011522:Protons; D018698:Glutamic Acid