Biomedical Engineering Reference
In-Depth Information
base that links the chromophore to the apo-protein moiety of bR, whereas we interpret
B
1
as electron-hole pair production, which is similar to what happens in a chlorophyll-based
photosynthetic reaction center. The relaxation of the electric dipole residing in the chro-
mophore then provides the energy to deprotonate the Schiff's base. The detail arguments
leading to our alternative interpretation can be found in [80]. Thus, the molecular
processes underlying
B
1
,
R
1
, and
H
1
may constitute the common pathway for photon
energy conversion in these retinal proteins. The subsequent conformational changes then
diverge in their functional purposes, leading to visual transduction (rhodopsin), proton
and chloride ion transport (bR and hR, respectively). For bR and rhodopsin, the common
pathway may be further extended to the step of the generation of
B
2
and
R
2
, but not the
B
2
generation. What then is the possible role of the
R
2
signal in visual transduction?
Historically, the ERP was regarded as an epiphenomenon—an evolutionary vestige
serving no known physiological function. However, just because no known function is
known does not mean a heretofore-unsuspected function does not exist. While direct evi-
dence is lacking, circumstantial evidence abounds, which points to a possibility of the
R
2
component being a mechanistic trigger for visual transduction [75,89,90]. Our interpreta-
tion of the mechanistic origin of
R
2
predicted that the molecular process responsible for the
R
2
signal also generate a surface potential [52]. The prediction was subsequently con-
firmed by Cafiso and Hubbel [91], using a hydrophobic spin-label to monitor surface
potential changes. Such a surface potential might serve as an electrostatic trigger to initi-
ate the cyclic GMP cascade. How a light-induced surface potential works as a mechanistic
trigger is best illustrated with an experiment performed by Drain et al. [92] (Figure 15.19).
These investigators studied the voltage-driven transport of a lipophilic anion,
tetraphenylborate, across an artificial BLM. At the same time, they configured the BLM such
that light illumination would generate a symmetric positive surface potential on both sur-
faces of the membrane. These surface potentials are generated by a light-induced electron
transfer from the membrane-bound magnesium octaethylporphyrin molecule to an aqueous
electron acceptor with equal concentrations in the two aqueous phases (Figure 15.19A). No
macroscopic photovoltaic electric signal can be detected because the photocurrents gener-
ated by interfacial electron transfers at both surfaces are equal in magnitude but opposite in
polarity. However, the positive surface potential induced by light at the two membrane sur-
faces increases the surface concentration of the negatively charged tetraphenylborate ions
suddenly. As a consequence, the ionic current carried by tetraphenylborate ions is also
increased (Figure 15.19B). Thus, illumination triggers an increase of ionic current.
If, however, tetraphenylborate ions are replaced by the lipophilic cation
tetraphenylphosphonium ions, illumination then causes a sudden decrease of the ionic
current (Figure 15.19C). This surface potential-induced change of ionic concentration
would have been even more pronounced if the ionic species being transported were mul-
tivalent. This prototype device illustrates that light-induced surface potentials can be con-
figured to implement a field-effect transistor (FET) or phototransistor. Thus it is not
inconceivable that visual transduction process may depend on a surface potential-based
FET to couple the photochemical reaction of rhodopsin to the biochemical processes of the
cyclic GMP cascade.
The actual mechanistic event during visual transduction is of course more complex.
Although electrostatic interactions feature prominently, other more subtle interactions are
also present. In the crucial step of initiation of the cyclic GMP cascade, the binding surface
of transducin (a G-protein) appears to have a patch of negative charge that can conceiv-
ably steer towards rhodopsin's exposed cytoplasmic surface via electrostatic attraction.
There is no quantitative estimate of how much the
R
2
-generated surface potential con-
tributes to the binding process. It is nevertheless quite certain that transducin binding is
not a random process of diffusion and collision. It is also reasonably certain that binding
