Biomedical Engineering Reference
In-Depth Information
We further suspect that the “step-function” photoswitching mechanism of a light-
dependent proton channel may also be present in chlorophyll-based photosynthetic
membranes. The asymmetrical transient current spikes that appear upon the onset and the
cessation of illumination of a giant chloroplast from
Peperomia metallica
[73] suggest that the
charge conduction pathway in the photosynthetic reaction center may also be activated
only by light (Figure 15.15A). The same waveform persisted in a BLM reconstituted with
the bacterial reaction center of
Rhosobacter spheroides
[74] (Figure 15.15B), thus reflecting an
intrinsic property of the membrane-bound pigment. Evidence from direct electrical meas-
urement of
G
p
(as compared with
G
m
) is presently lacking. But if this interpretation is
correct, then it appears that Nature has implemented the same design principle by using
completely different molecular constructs (see discussion in [75-77]).
15.5
Generalization to Other Photoelectric Systems
The interpretation of the asymmetric capacitative transients in chloroplasts presented
in the previous section tacitly assumes that similar AC photoelectric events take place
in chlorophyll-based photosynthetic membranes. The OD and the ICT models are gen-
eral in the sense that they capture most, if not all, of the
mechanistic
features of light-
induced charge movements in various photobiological membranes. The analysis of
charge separation and recombination of either OD or ICT type also suggests that the
charged species being moved in the membrane need not be limited to electrons or pro-
tons and may include small ions other than protons. The inherent generality of the
analysis thus allows us to make additional predictions. For example, the ICT model
predicts that a halorhodopsin (hR) membrane, which transports chloride ions across
the membrane upon illumination, will exhibit a photoelectric signal by means of inter-
facial Cl
transfer (hR is another retinal-containing protein found in the red membrane
fraction of
H. salinarum
, and is a light-driven Cl
pump). This component should be
sensitive to the aqueous Cl
concentration rather than the proton concentration. This
prediction was confirmed experimentally: the photoelectric signal has a
B
1
-like compo-
nent (
H
1
) and a
B
2
-like component (
H
2
). The
H
2
component, but not
H
1
, is sensitive to
Cl
concentration [78] (Figure 15.16).
Can the scheme of analysis for bR be extended to cover more complex light-driven pro-
ton pumps (photosynthetic reaction centers)? Superficially, the structure of photosynthetic
reaction centers, such as that of
Rhodopseudomonas viridis
[79], appears to be considerably
more complex than that of bR. At a mechanistic level, they are rather similar. Both systems
can be characterized as systems of
coupled ICT reactions
[80,81]. The two interfacial proton-
transfer reactions are coupled by transmembrane charge (electrons or protons) transport.
Such transmembrane charge transport is accomplished by breaking up the charge move-
ment into several small steps of consecutive charge transfers with charge donors and
acceptors arranged in a chainlike spatial organization (electron transport chain). The reac-
tants of a given reaction are the products of the preceding reaction whereas its products
become the reactants of the subsequent reaction. Schematically, these coupled consecutive
charge transfer reactions are shown in Figure 15.17A, for bacterial photosynthetic reaction
center, and in Figure 15.17B, for bR. The mechanistic similarity is apparent.
Although not shown in Figure 15.17A, reverse charge transfer reactions are quite preva-
lent in photosynthetic membranes. For each forward charge transfer reaction (charge sep-
aration) there is a reverse charge transfer (charge recombination) including the two
interfacial reactions. The imbalance between the forward charge transfer and reverse
