Biology Reference
In-Depth Information
24.5 Allosteric regulation of dark
adaptation
The crucial question of how the power function obtained during rod
dark adaptation could be explained by underlying molecular processes
remained. At first, it was thought that the relationship had to be
determined by an enzyme, but no ordinary enzymatic reaction that
fitted the equation could be found. Soon, however, it was realized
that the power function could be explained by the classical model
of
allosteric
functions proposed by Monod
et al
. (
1965
) (see Perutz,
1989
; Stabell
et al
.,
1986a
,
b
,
1987
,
1989
,
1990
,
1992
,
1996
).
According to this model (see also Monod,
1970
), two states, an
active
R
and an inactive
T
state, are reversibly accessible to allosteric
proteins. Under conditions where a ligand has affinity exclusively for the
R
state, the fraction of the protein in this state is given by the equation:
R
= (1 + α)
n
/
L
+ (1 + α)
n
where
n
is the number of identical subunits in the protein molecule,
L
(the allosteric constant) represents the ratio of molecules in the
T
to
R
states in the absence of ligand, i.e.
L
=
T
0
/
R
0
, and α is simply
a normalization for the ligand concentration (ligand concentration
multiplied by ligand affinity for the
R
state).
When the allosteric constant (
L
) is very large (i.e. the intrinsic
equilibrium is grossly in favour of the
T
0
state), the equation can be
simplified to:
R
= cα
n
where c is a constant.
Presupposing that this simplified version of the model of Monod
et al
. (
1965
) underlies the equation
T
= c
B
n
found by Nordby
et al
.
(
1984
), it was suggested that the concentration of bleached photopig-
ment determined the displacement of the equilibrium between
the active and inactive state of an allosteric, positively cooperative
protein built as a tetramer (see Stabell
et al
.,
1986a
,
b
,
1992
,
1996
).
