Biology Reference
In-Depth Information
This new model of dark adaptation presupposed that the
cooperative effect of the bleached rhodopsin molecules giving rise
to the power function was mediated by a conformational alteration
(allosteric transition) induced in the enzyme molecule when it bound
a ligand. The allosteric effect was thus assumed to be mediated
entirely by the enzyme molecule and to depend upon its specific
structure (Stabell
et al
.,
1989
).
This intramolecular model of dark adaptation was opposed to
previous photochemical dark-adaptation theories where the exponential
effect of bleached rhodopsin was explained by assuming that bleaching a
single molecule of rhodopsin changed the conditions of other rhodopsin
molecules in its vicinity (e.g. Wald,
1954
; Barlow,
1964
).
24.6 A search for the allosteric adaptation
mechanisms
The close relationship between threshold elevation and amount of
bleached photopigment obtained during long-term dark adaptation
indicated that the middle straight-line segment of the long-term
dark-adaptation curves was determined by photochemical processes.
The question, therefore, arose as to whether there were allosteric,
cooperative processes within the phototransduction cascade that
might explain the relation found, i.e. whether a site could be
found where signals from the bleached photopigment might affect
sensitivity in an allosteric, positively cooperative manner.
By the late 1980s the phototransduction process had become
fairly well understood (see for example, Fain
et al
.,
1989
). Thus, it
had been found that the absorption of light by the visual pigments
in rods and cones triggers a cascade of biochemical events (
Fig. 24.2
).
Firstly, the activated photopigment molecule diffuses within the
disk membrane and activates molecules of the regulatory protein
transducin. Each of these activated molecules, then, activates a
great number of phosphodiesterase (PDE) molecules, which in turn,
hydrolyzes a large number of cGMP (cyclic guanosine monophos-
phate) molecules.
