Electrochemical Analysis of Ion Channels and Transporters in Pore-Suspending Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 308

activation at room temperature. It is apparent that fusion of prote-
oliposomes to pore-suspending membranes needs to be accelerated
to overcome this problem.
V. ACTIVITY OF THE PROTON PUMP
BACTERIORHODOPSIN
Bacteriorhodopsin (bR) is an integral membrane protein that uses
light to translocate protons across membranes in Halobacterium
salinarium . The bacteria use the proton gradient in the absence of
light to generate ATP. In the lipid bilayers, bR forms trimers
packed in highly ordered two-dimensional hexagonal arrays,
which make up 75% of the membrane; hence, they are named pur-
ple membranes. 87 One monomer is comprised of seven transmem-
brane helices, connected by short extramembraneous loops, span-
ning the lipid bilayer. 88 In the inner core of the helix bundle, a reti-
nal moiety is covalently bound to the lysine residue Lys 216 via a
Schiff-base. The retinal is vital for the protein pump mechanism.
Upon illumination, it isomerizes from a trans to cis configuration,
which initiates the active transport of a proton. During one photo
cycle, a single proton is translocated across the bilayer. Eventually,
the retinal moiety is again adapting a trans configuration, allowing
for a new photo cycle to start. 89,90 In contrast to the above de-
scribed ion channel gramicidin, bR is an active transporter protein.
This class of proteins is characterized by small turnover numbers;
in the case of bR, 100 protons are translocated per second. 91 Com-
pared to ion channels, which conduct up to 10 7 species per second,
the number of transported molecules is by many orders of magni-
tude smaller. This fact hampers the investigation of transporters
with electrochemical methods due to an unfavourable signal-to-
noise ratio. In order to enhance signals from transporters, the den-
sity of molecules in the system needs to be sufficiently high. Here,
we employed bR to verify that we can establish high protein densi-
ties in pore-spanning membranes by fusing proteoliposomes on
porous alumina. The characteristic property of bR, the light-
induced translocation of protons, is well described, which makes it
a suitable model transporter for our research purposes.