Marуti P., Gerencsйr L.
Department of Biophysics University of Szeged, Hungary, Egyetem utca 2. Szeged, Hungary H-6722
The reaction center (RC) protein of photosynthetic bacteria performs
the primary photochemistry by coupling light-induced electron transfer
to vectorial proton transfer across the membrane [1]. It plays a central
role in photosynthetic energy conversion by facilitating the light-induced
double reduction and protonation of a bound quinone molecule, QB. The first
electron transfer to QB does not involve direct protonation of the quinone
molecule; however, the nearby amino acid residues change their protonation
state in response to the change in the electrostatic field associated with
the formation of QB-. The second electron transfer is coupled with direct
protonation of the quinone leading to formation of quinol, QBH2. The quinol
dissociates from the protein and releases its protons on the periplasmic
side of the membrane, resulting in the formation of a proton gradient that
drives ATP synthesis. The vacant QB site of the protein becomes occupied
by a free quinone molecule from the membrane pool, resetting the photocycle
for an additional turnover.
The lecture will focus on the kinetic analysis of the photocycle of
isolated RC protein driven either by single saturating flashes [7] or by
continuous illumination of high intensity [8]. Depending on the conditions,
the rate limiting steps of the photocycle could be the first or second
electron transfer/proton uptake, the exchange rates of the quinone/quinol
at the cytoplasmic site or the cyctochrome at the periplasmic site of the
RC or the light intensity [8]. Wide variety of mutations [2,4,5,10] leading
to surprising changes in the electron and proton transfer [2,3,6,9] characteristics
of the bacterial RC will be discussed. We will argue that significant part
of the observed effects can be attributed to changes in the electrostatic
environment caused by mutation [2,4,5]. It will be demonstrated that the
electrostatic potential near QB- is finely tuned to allow efficient electron
and proton transfer to QB [2,7,10].
- P. Marуti: Flash-induced proton transfer in photosynthetic bacteria
(minireview). Photosynthesis Research 37, 1-17 (1993)
- P. Marуti, D. K. Hanson, L. Baciou, M. Schiffer and P. Sebban: Proton
conduction within the reaction centers of Rhodobacter capsulatus: The electrostatic
role of the protein. Proc. Natl. Acad. Sci. USA, 91, 5617-5621 (1994).
- L. Kбlmбn and P. Marуti: Stabilization of reduced primary quinone by
proton uptake in reaction centers of Rhodobacter sphaeroides. Biochemistry,
33, 9237-9244 (1994).
- P. Sebban, P. Marуti, M. Schiffer and D.K. Hanson: Electrostatic dominoes:
Long distance propagation of mutational effects in photosynthetic reaction
centers of Rhodobacter capsulatus. Biochemistry, 34, 8390-8397 (1995).
- P. Marуti, D.K. Hanson, M. Schiffer and P. Sebban: Long-range electrostatic
interaction in the bacterial photosynthetic reaction centre. Nature - Structural
Biology 2 (12), 1057-1059, (1995).
- L. Kбlmбn, T. Gajda, P. Sebban and P. Marуti: pH-metric study of reaction
centers from photosynthetic bacteria in micellular solutions: protonatable
groups equilibrate with the aqueous bulk phase. Biochemistry 36
(15) 4489-4496 (1997).
- P. Marуti and C.A. Wraight: Kinetics of H+-ion binding by the P+QA-
state of the bacterial photosynthetic reaction centers: Rate limitation
within the protein. Biophysical Journal 73, 367-381 (1997)
- Sz. Osvбth and P. Marуti: Coupling of cytochrome and quinone turnovers
in photocycle of reaction center from photosynthetic bacteria Rhodobacter
sphaeroides. Biophysical Journal 73, 972-982 (1997).
- L. Kбlmбn and P. Marуti: Conformation-activated protonation in reaction
centers of the photosynthetic bacterium Rhodobacter sphaeroides. Biochemistry
36, 15269-15276 (1997).
- J. Miksovska, L. Kбlmбn, M. Schiffer, P. Marуti, P. Sebban and D. K.
Hanson: In bacterial reaction centers rapid delivery of the second proton
to QB can be achieved in the absence of L212 Glu. Biochemistry 36,
12216-12226 (1997).