Biomedical Engineering Reference
In-Depth Information
Periplasm
Membrane
Cytoplasm
•
Cyt c
3+
Cyt
2+
SP
+
BCh
−
Q
A
−
BPh
½Q
B
H
3
O
+
h
ν
H
2
O
Cyt c
2+
Cyt
3+
BPh
−
SP
BCh
½Q
B
H
2
Q
A
Cyt c
2+
Cyt b/c
3+
½Q
B
H
2
Cyt c
3+
Cyt b/c
2+
½Q
B
H
2
O
H
+
H
3
O
+
(A)
Cytoplasmic
space
Membrane
phase
Extracellular
space
H
3
O
+
A
1
−
A
2
H
3
−
A
5
−
H
3
O
+
A
4
H
h
A
1
HA
2
−
A
4
−
H
2
O
A
3
H
A
5
H
2
O
(B)
FIGURE 15.17
Coupled consecutive charge transfer reactions. (A) The scheme shows coupled consecutive electron-transfer
reactions with reverse electron-proton cotransport in the photosynthetic reaction center of
Rhodopseudomonas
viridis
. SP is the “special pair”, BChl is voyeur bacteriochlorophyll, BPhe is bacteriopheophytin, Q
A
is the tightly
bound quinone, and Q
B
is the loosely bound quinone. The electron-transfer reactions in the cytochrome b
-
c
1
com-
plex are simplified to become a single step, so are those in the four bound cytochromes on the periplasmic sur-
face. Cyt c
2
is the peripheral protein cytochrome c
2
. The dotted arrows indicate the diffusion of Q
B
and Cyt c
2
.
The reverse reactions are not shown in the schematic. (B) The scheme shows coupled consecutive proton-trans-
fer reactions across the purple membrane of
Halobacterium salinarum
. The actual number of binding sites is not
known. For simplicity, only five binding sites (
A
1
through
A
5
) are shown. Site
A
3
is the proton-binding site at the
Schiff's base linkage. Its two adjacent sites,
A
2
and
A
4
, are the residue 96 aspartate and the residue 85 aspartate,
respectively. It is possible that additional sites exist between site
A
1
and site
A
2
, as well as between site
A
4
and
A
5
.
It is understood that the Schiff's base is neutral when unprotonated and is positively charged when protonated.
The reverse reactions are not shown. (From Hong, F. T. (1994). Retinal proteins in photovoltaic devices. In: Birge,
R. R. (Ed.).
Molecular and Biomolecular Electronics
(Advances in Chemistry Series No. 240). Washington, DC:
American Chemical Society, pp. 527-559. (A) and Hong, F. T. (1990). Bacteriorhodopsin as an intelligent material.
In: Wang, C.-Y., Chen, C.-T., Cheng, C.-K., Huang, Y.-Y., Lin, F.-H. (Eds.).
Biomedical Engineering in the 21st
Century
. Taipei, Taiwan: National Taiwan University Center for Biomedical Engineering. pp. 85-95. (B))
membrane thickness, but the OD model is readily modified to accommodate this variation
(Figure 15.8 in [53]). Evidence for the presence of the AC photoelectric signal in the reac-
tion center can be found in the data of Hara et al. [82] and those of Govorunova et al. [83]
(Figure 15.18). The reported signals elicited from immobilized bacterial reaction center of
R. viridis
and those elicited from that of
Chlamydomonas rheinhardtii
exhibit a biphasic
waveform and satisfies the zero time-integral condition, indicating that it is indeed a
capacitative photocurrent, most likely generated by an OD mechanism.
