Biomedical Engineering Reference
In-Depth Information
energy, including light quality and quantity, duration, and direction—in essence, this
group of proteins enables plants to respond to varying light conditions (for a recent
review, see Montgomery and Lagarias [121]). They also facilitate light-mediated control at
a genetic or transcriptional level, and have complex cofactor and prosthetic group associ-
ations. Two optically interconvertible forms exist, Pr (red-light absorbing) and Pfr (far red-
light absorbing)—conversion between the two forms is accompanied by isomerization of
a bilin chromophore and the concomitant structural rearrangement of the protein, which
modulates phytochrome's biochemical activity. The bilin chromophore is a derivatized
tetrapyrrole, and the light-initiated isomerization causes the fourth pyrrole ring to rotate
180
. The exceedingly complex nature of both the protein's structure and its light-medi-
ated response give phytochrome potential in sensor architectures; the large number of
interactions provides many avenues for chemical antigens to intercalate and disrupt.
However, that same complexity may prove to be a liability, in that stabilizing the protein
might be problematic, and its response might be difficult to interpret. Nonetheless, phy-
tochrome's potential needs to be investigated, if only due to its already powerful physio-
logical role as a biosensor. Photoactive yellow protein, in contrast, is a much simpler
sensor protein that mediates negative phototaxis for its host organism,
Ectothiorhodospira
halophila
. Just as for the rhodopsins and phytochrome, PYP's biological activity is initiated
by a blue-light-induced chromophore isomerization, which in this case is
para
-hydroxy
cinnamic acid. A photocycle follows light absorption, which like BR, encompasses a time
scale ranging from picoseconds to milliseconds (for a review, see Hellingwerf, 2000 [122]).
The extent to which the PYP's biophysical properties can be exploited in sensor architec-
tures remains to be determined.
°
14.4
Future Directions
Biological molecules hold great promise for introducing novel function to conventional
technologies, and as the researcher's ability to integrate these molecules becomes more
advanced, the potential for more sophisticated sensor platforms can be realized.
Bacteriorhodopsin is one of a broad class of light- and or signal-transducing proteins that
has potential in biosensor architectures. Sensor platforms for light modulation, voltage,
and chemical detection and identification are conceivably possible. More advanced appli-
cations include conferring advanced sensing capabilities to preexisting devices (micro-
electronic or otherwise), and construction of an artificial retina. Central to these efforts is
the ability to modify the protein to better suit the needs of a given application—the tech-
niques of site-directed and semirandom mutagenesis provide the researcher with the abil-
ity to introduce novel attributes at the genetic level, enabling the production of
custom-designed proteins for specific device applications. Given this ability, the potential
exists for an entire new class of sensors that brings new functionalities to conventional
designs.
The data presented above clearly demonstrate proof of principle for the BR-based chem-
ical sensor. A manuscript containing a more detailed account of the latter research effort is
currently being prepared [115]. The range of experiments must be expanded to encompass
a wider variety of both BR variants and toxic chemicals that are of potential hazard to
human health, including industrial pollutants and chemical warfare agents. Characterizing
and integrating a larger number of mutants into the sensor platform will result in a higher
probability of positively identifying the toxin. Furthermore, as demonstrated by Bryl et al.
[88], the BR photoelectric effect is also a valuable tool for distinguishing between toxins,
