7.7 Relaxation Times of the Electrical and Buffer Reservoirs
119
tonation times of about 2–3 μs in I MM preparation, what is of the same order as
the relaxation time of the P MF fluctuations we describe above.
The access resistance best estimated to match the common relaxation time of
the membrane and buffer “compartments” was found to be 9 × 10 9 , what is about
100 times smaller than the proton channel resistance. This value is high as compared
with proton resistance of an aqueous pathway having equivalent geometry. We took
as an assumption that the limiting factor in proton translocation was the channel
crossing step.
In effect, DeCoursey and Cherny, 1996 [12] concluded that H + diffusion is not a
rate limiting step in the overall proton translocation across channels.
Additional reason for adjusting the access resistance value is that the mechanisms
of proton translocation near or at biological interface are far from being adequately
understood. Values of reported, calculated or predicted proton conductance vary
considerably (Nagle and Morowitz [13], Heberle et al. [14]). The general consensus,
however, is that proton translocation across microdomains is significantly faster as
compared to bulk water. Since proton conductivities in or near surfaces are subject
to great controversy (see Kasianowicz and Bezrukov [7]) it is reasonable to make
the access resistance an adjustable parameter, rather than estimate its value from the
proton mobility in bulk water and geometric parameters.
Considering that typical time averaged P MF s across I MM are about 140 mV
(Läuger [4]), the values of mean P MF fluctuation here reported can be considered
as relevant to the proton dynamics in the μs time scale. The effects of this P MF
fluctuations should be interpreted along the following lines:
(a) Coupling of P MF fluctuations with pump/synthase conformational states,
different pump states and their corresponding dwell times (Stein and Läuger
[6]).
(b) Coupling of the fluctuations with protonation/deprotonation of sites: part of the
influence of the fluctuating P MF could be felt at the protonation/deprotonation
dynamics and part involved in the modulation of the channel conductance for
other ions (see Kazianowicz and Bezrukov [7]).
The above calculated fluctuations of P MF across the I MM should be interpreted in connection with the molecular machines under their influence. Two
general assumptions about the pump/synthase reacting time serve as the basis of
our analysis:
(1) The assumption of a “slow” pump mechanism translate into a system reacting
to only time averaged P MF , calculated from the known values of pH and voltage
across the I MM. (2) If the reacting time of the synthase machine is “fast”, say in
the 10 μs range, then the fluctuational changes of P MF may importantly interfere
with its function.
We conclude that the 100 mV fluctuations in P MF having characteristic time of
1 μs may be relevant for the processes of proton translocation inside the F 0 sector
of the AT P ase. Lesser amplitude P MF fluctuations with corresponding longer
relaxation times might influence conformational changes in the AT P ase molecule.
119
tonation times of about 2–3 μs in I MM preparation, what is of the same order as
the relaxation time of the P MF fluctuations we describe above.
The access resistance best estimated to match the common relaxation time of
the membrane and buffer “compartments” was found to be 9 × 10 9 , what is about
100 times smaller than the proton channel resistance. This value is high as compared
with proton resistance of an aqueous pathway having equivalent geometry. We took
as an assumption that the limiting factor in proton translocation was the channel
crossing step.
In effect, DeCoursey and Cherny, 1996 [12] concluded that H + diffusion is not a
rate limiting step in the overall proton translocation across channels.
Additional reason for adjusting the access resistance value is that the mechanisms
of proton translocation near or at biological interface are far from being adequately
understood. Values of reported, calculated or predicted proton conductance vary
considerably (Nagle and Morowitz [13], Heberle et al. [14]). The general consensus,
however, is that proton translocation across microdomains is significantly faster as
compared to bulk water. Since proton conductivities in or near surfaces are subject
to great controversy (see Kasianowicz and Bezrukov [7]) it is reasonable to make
the access resistance an adjustable parameter, rather than estimate its value from the
proton mobility in bulk water and geometric parameters.
Considering that typical time averaged P MF s across I MM are about 140 mV
(Läuger [4]), the values of mean P MF fluctuation here reported can be considered
as relevant to the proton dynamics in the μs time scale. The effects of this P MF
fluctuations should be interpreted along the following lines:
(a) Coupling of P MF fluctuations with pump/synthase conformational states,
different pump states and their corresponding dwell times (Stein and Läuger
[6]).
(b) Coupling of the fluctuations with protonation/deprotonation of sites: part of the
influence of the fluctuating P MF could be felt at the protonation/deprotonation
dynamics and part involved in the modulation of the channel conductance for
other ions (see Kazianowicz and Bezrukov [7]).
The above calculated fluctuations of P MF across the I MM should be interpreted in connection with the molecular machines under their influence. Two
general assumptions about the pump/synthase reacting time serve as the basis of
our analysis:
(1) The assumption of a “slow” pump mechanism translate into a system reacting
to only time averaged P MF , calculated from the known values of pH and voltage
across the I MM. (2) If the reacting time of the synthase machine is “fast”, say in
the 10 μs range, then the fluctuational changes of P MF may importantly interfere
with its function.
We conclude that the 100 mV fluctuations in P MF having characteristic time of
1 μs may be relevant for the processes of proton translocation inside the F 0 sector
of the AT P ase. Lesser amplitude P MF fluctuations with corresponding longer
relaxation times might influence conformational changes in the AT P ase molecule.