Biomedical Engineering Reference
In-Depth Information
14.2
Bacteriorhodopsin as a Biophotonic Material
Historically, bacteriorhodopsin (BR) has received more attention than any other protein
with respect to device applications. Early interest was for the use of the protein as the
active element in reusable holographic materials [6-16]. Later came architectures for ran-
dom access memories, nonvolatile computer memories, photodiodes, and applications as
active elements in microelectronic circuits [9,17-34]. A great deal of effort has been put into
the integration of BR onto semiconductor surfaces for the purposes of signal transduction;
upon exposure to yellow-green light (570
60 nm), the protein responds in a cyclic man-
ner, thereby modulating its visible absorption spectrum over a wide spectral range
(410-640 nm) and exhibiting a well-defined voltage response in the pico- to millisecond
range. The utilization of BR in microelectronics and a new class of chemical sensors will
be discussed below.
14.2.1
Basic Properties
Bacteriorhodopsin is a 26-kDa integral membrane protein that is isolated from
Halobacterium salinarum
, an archaea that thrives in salt marshes where the temperature
ranges between 40 and 60°C, and the average concentration of salt is approximately 4 M,
roughly six times that of sea water (Figure 14.1). BR has 248 amino acids arranged into
seven interconnected
-helices are arranged in a
seven-helix bundle, a structural motif characterized by many G-protein-coupled receptors.
An all-
trans
retinal chromophore is bound covalently to Lys-216 via a protonated Schiff
base bond; this prosthetic group gives BR its characteristic purple color, and is responsible
for actinic light absorption by the protein. Bacteriorhodopsin is the sole protein compo-
nent of the purple membrane, a term that refers to discrete membrane patches in
H. sali-
narum
consisting of approximately 75% BR and 25% lipid. These patches are rugged (as
compared with the nonprotein-bearing membrane component) and can be isolated by
osmotically lysing the cell followed by differential centrifugation. Within the purple mem-
brane, BR monomers are organized in a hexagonal arrangement of trimers. The retinal
chromophores mirror the trimer arrangement, oriented with an angular separation
between monomers of about 60°. This arrangement ensures that the purple membrane
captures light of all polarizations with equal efficiency. Owing to the enhanced stability
imparted to the protein by this arrangement, BR is most often utilized as the purple mem-
brane—solubilizing the protein reduces its stability from years to days.
α
-helices that span the membrane. The
α
14.2.2
The Bacteriorhodopsin Photocycle
Absorption of light by the retinal chromophore initiates a series of events in BR that ulti-
mately results in the translocation of a proton across the cell membrane, thereby acidifying
the external medium—the protein acts as a light-driven proton pump that drives the pro-
duction of ATP via a standard F
0
F
1
ATPase. Proton translocation occurs as the protein goes
through a cyclic response to light, typically referred to as a photocycle (Figure 14.1). Upon
absorption of light, the all-
trans
retinal chromophore isomerizes to 13-
cis
, along with a con-
comitant shift in electron density toward the protonated Schiff base. This change in the elec-
trostatic nature of the binding site is the driving force for a series of thermally produced
intermediates, each defined by several characteristics, including absorption maximum,
protonation state of key amino acids, chromophore isomerization state, lifetime, and ther-
mal stability. Prior to light absorption, the bacteriorhodopsin resting state is referred to as bR
