Biology Reference
In-Depth Information
Plasma
membrane
photon
T
c
Guanylate
cyclase
Open channel
R
*
PDE
c
T
*
c
PDE
*
c
Closed channel
G
Fig. 24.2 Phototransduction. The enzymatic cascade that leads from
the photon absorption of rhodopsin molecules in rods to the decrease in
cGMP concentration is shown. Abbreviations: R, rhodopsin molecule; R*,
activated form of rhodopsin molecule; T, transducin; T*, transducin in
an activated state; PDE, phosphodiesterase; PDE*, activated phosphodi-
esterase; c, cyclic guanosine monophosphate; G, hydrolyzed c.
In the dark-adapted state, these molecules keep the membrane
channels of the receptor open. Light, therefore, tends to close the
channels. This light-induced closure reduces the inward cation current
(mainly Na+) that flows through the channels and, consequently,
tends to hyperpolarize the receptor membrane (see
Fig. 24.3
).
Since a significant amount of Ca
2+
ions also flow into the
receptor cell through the membrane channels when the eye is in
a dark-adapted state, the closure reduces the influx of Ca
2+
. This
reduction in influx leads to a decrease in the intracellular Ca
2+
concentration, since the extrusion of Ca
2+
continues. Because Ca
2+
tends to inhibit guanylate cyclase activity (the enzyme that synthe-
sizes cGMP from GTP), the decrease tends to relieve the inhibi-
tory effect of Ca
2+
and, hence, to increase guanylate cyclase activity
leading to an increase of cGMP (see Fain
et al
.,
1989
) and, thereby, to
a reopening of the membrane channels.
From this description of the phototransduction process the
question arose as to whether
the membrane channels
, in fact,
subserved the allosteric enzymatic function found for the dark-
adaptation process. Biochemical evidence supported this suggestion,
indicating that the opening and closing of the ion channels of the
plasma membrane of the outer segment of the photoreceptor were,
