Biomedical Engineering Reference
In-Depth Information
80
200 pA
40
100 ms
0
−
40
−
80
−
2
0
2
4
6
8
10
(A)
(B)
Time (
µ
s)
FIGURE 15.18
Pulsed-light-induced photoelectric responses in reconstituted photosynthetic membranes. (A) Purified photosyn-
thetic reaction centers of
Rhodopseudomonas viridis
were immobilized, by means of avidin-biotin coupling tech-
nique, on transparent metal electrode. (B) Purified photosynthetic reaction centers of
Chlamydomonas reinhardtii
were oriented and immobilized in gel. (From Hara, M., Majima, T., Miyake, J., Ajiki, S., Sugino, H., Toyotama, H.,
Kawamura, S. (1990). Oriented immobilization of bacterial photosynthetic membrane.
Appl. Microbiol. Biotechnol.
32:544-549. (A) and Govorunova, E., Dér, A., Tóth-Boconádi, R., Keszthelyi, L. (1995). Photosynthetic charge sep-
aration in oriented membrane fragments immobilized in gel.
Bioelectrochem. Bioenerg.
38:53-56. (B))
The striking structural and chemical similarities between bR and rhodopsin suggest
that there may be something in common between the two pigments [77]. Functionally speak-
ing, the two systems serve diametrically opposite purposes. Efficiency of photon energy
conversion is crucial to photosynthetic membranes such as the purple membrane and the
chloroplast membrane, which store the converted energy as a transmembrane proton gradi-
ent. In contrast, rhodopsin is a photon sensor of which the efficiency of photon energy conver-
sion is not crucial, but the sensitivity and the dynamic range of the sensory response are.
It is well known that under optimal conditions, a single photon, if absorbed by a single
molecule of rhodopsin, can be detected psychophysically [84]. What transpires during
visual transduction is the amplification of the initial photon energy by 100,000 fold. This
amplification is achieved by means of a biochemical scheme known as the cyclic GMP cas-
cade [85]. Apparently, a transmembrane proton gradient serves no useful purpose in
vision. Ostrovsky and coworkers [86,58] demonstrated the presence of interfacial proton
uptakes at the cytoplasmic surface of visual membranes and the absence of interfacial pro-
ton releases at the opposite side. The absence of interfacial proton releases implies that
there is no transmembrane proton transport in visual membranes. The
B
2
and the
R
2
com-
ponents are manifestation of interfacial proton uptake (and rerelease) at the cytoplasmic
surface of the membrane. In contrast,
B
2
reflects extracellular proton release in bR but its
counterpart
R
2
is not expected to exist in rhodopsin, in view of the finding of Ostrovsky
and coworkers.
The presence of similar AC photoelectric signals suggests a common scheme for various
types of retinal proteins. The fast signals
B
1
,
R
1
, and
H
1
are characterized by ultrafast rise-
time, which is estimated to be shorter than 5 ps [87,88]. Such fast signals are unlikely to be
caused by charge motion accompanying conformational changes or proton motion during
the pumping process. Rather they are most likely the manifestation of rapid electron
movements that accompany photoisomerization of the chromophore. Apparently, photon
energy is temporarily stored as an electric dipole, which subsequently vanishes, and the
stored energy is transferred to the apoprotein moiety of rhodopsin or bR. In other words,
the initial energy is stored in a
-like mechanism, which subsequently drives the
protein conformational change when the
spring
recoils. This mechanism is similar to a
“slingshot” model previously proposed by Keszthelyi [41] for bR. But there is an impor-
tant difference. Keszthelyi interpreted the
B
1
component as deprotonation of the Schiff's
spring
