Biomedical Engineering Reference
In-Depth Information
h
Photodiode
Rectification (
G
f
>
G
r
)
h
G
f
Light on
−
+
G
r
Light off
−
+
Bacteriorhodopsin
No rectification
h
G
p
> 0
Light on
−
+
−
Light off
+
G
p
= 0
FIGURE 15.14
Schematic diagram comparing rectification in a conventional photodiode and the “step-function” photoswitch-
ing in bR.
G
f
is the forward conductance, and
G
r
is the reverse conductance of the photodiode. Photoactivation
causes a photocurrent to flow in the forward direction as indicated. As a consequence, the photodiode becomes
back-biased. Back-biasing drives a current in the reverse direction during illumination, but the reverse current
encounters a much greater resistance because of rectification (
G
r
G
f
). In the dark, charge recombination is min-
imized because the conductance
G
r
is much lower than
G
f
. In bR, there is no rectification: the forward photocur-
rent and the reverse current (driven by back-biasing) encounter the same resistance (1/
G
p
). Thus, the observed
photocurrent is actually the difference of these two currents. The constancy of the measured true photoemf
(Figure 15.13B) indicates that illuminated bR's ability to maintain a net “driving force” is not affected by the
extent of back-biasing. In other words, the performance of bR during illumination has not been seriously com-
promised by the lack of rectification but it would be seriously compromised if the proton channel remained open
in the dark. “Step-function” photoswitching averts this potential problem:
G
p
= 0 in the dark. The plus and the
minus signs indicate the direction of biasing. In the case of bR, the signs also indicate the polarity of the photo-
voltage. (From Hong F. T. (1999). Interfacial photochemistry of retinal proteins.
Prog. Surf. Sci.
62:1-237.)
As for enhancing the efficiency of photoconversion, bR apparently relies on a different
strategy than rectification. We argue here that “step-function” photoswitching allows bR
to prevent wasteful dissipation of converted energy
in the dark
. The generation of a trans-
membrane proton gradient is the intermediate step between photon energy conversion
and bioenergetic synthesis of ATP (the universal energy currency in living cells). The for-
mation of the transmembrane electrical gradient (i.e., the photovoltage) constitutes a situ-
ation similar to back-biasing a photodiode. Back-biasing provides the driving force to
drive protons back into the cell (strictly speaking, the proton backflow is driven by both
the electrical component and the chemical component of the electrochemical potential). In
principle, there are several ways the electrochemical potential can drive protons back to
the cytoplasm: (a) protons reentering the cell via a reverse proton flow through the
same
proton channel in bR, (b) protons or other small ions reentering the cell via leakage
through the phospholipid portion of the purple membrane, and thus dissipating the elec-
trical portion of converted energy, (c) protons reentering the cell via the proton channel of
ATP synthase (residing in the red membrane), thus synthesizing ATP, and (d) protons
