Nov 11, 1999

Proton transfer from the bulk to the bound ubiquinone Q(B) of the reaction center in chromatophores of Rhodobacter sphaeroides: retarded conveyance by neutral water

Proceedings of the National Academy of Sciences of the United States of America
O A GoptaArmen Y Mulkidjanian

Abstract

The mechanism of proton transfer from the bulk into the membrane protein interior was studied. The light-induced reduction of a bound ubiquinone molecule Q(B) by the photosynthetic reaction center is accompanied by proton trapping. We used kinetic spectroscopy to measure (i) the electron transfer to Q(B) (at 450 nm), (ii) the electrogenic proton delivery from the surface to the Q(B) site (by electrochromic carotenoid response at 524 nm), and (iii) the disappearance of protons from the bulk solution (by pH indicators). The electron transfer to Q(B)(-) and the proton-related electrogenesis proceeded with the same time constant of approximately 100 microseconds (at pH 6.2), whereas the alkalinization in the bulk was distinctly delayed (tau approximately 400 microseconds). We investigated the latter reaction as a function of the pH indicator concentration, the added pH buffers, and the temperature. The results led us to the following conclusions: (i) proton transfer from the surface-located acidic groups into the Q(B) site followed the reduction of Q(B) without measurable delay; (ii) the reprotonation of these surface groups by pH indicators and hydronium ions was impeded, supposedly, because of their slow diffusion in the surface ...Continue Reading

  • References23
  • Citations18

Citations

Mentioned in this Paper

Buffers
Health Center
Thermodynamics
Ubiquinone (Lab Procedure)
Drug Delivery Systems
Tissue Membrane
Proton Therapy
Photosynthetic Reaction Centers
Bacteriorhodopsins
Carotenoids

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