Biomedical Engineering Reference
In-Depth Information
proteins tested. Despite the fact that several of the chemicals elicit similar trends in the
wild-type protein as their concentrations are increased, introduction of mutant proteins
enabled a wider variety of responses. As evident in Figure 14.10, all proteins examined
exhibited distinct and specific responses to a given chemical, and the protein's individual
responses varied strongly from chemical to chemical. Furthermore, the responses exhib-
ited by genetically engineered BR variants varied from wild type, and from mutant to
mutant. The sensitivity is defined in Figure 14.10 as the ratio of percent change in the time
constant to that of the change in the micromolar concentration of the added chemical.
Defining sensitivity in this way provides a unit-less, quantitative method that facilitates
comparisons. The best overall sensitivities were found when combining the responses of
both the M- and O-state data to the given chemical. At this point the detection limit seems
to be approximately the same as the concentration of the protein.
14.3.2.2.2 Other Photoactive Proteins
Given that a sensor based on BR has proven potential for detection of wide variety chem-
icals, it stands to reason that other proteins might be capable of the similar behavior and
activity. Most examples of protein-based sensors in the literature refer to proteins that play
a biological sensing role by detecting the levels of various cellular constituents (sugars,
ions, and phosphates). However, there are a large number of light-activated proteins that
may have use in architectures similar to those described above, including the recently
described proteorhodopsins (PRs), PRCs, phytochromes, and PYP. Proteorhodopsin refers
to a wide variety of related proteins originating from marine organisms (phytoplankton)
worldwide. This group was discovered only recently and shares many attributes with BR,
including a common structural motif, an all-
trans
retinal chromophore, and proton-pump
activity. Unlike BR, however, there are a large number of naturally occurring variants, and
the native organisms have yet to be identified (the first protein was produced after search-
ing an RNA library isolated from the marine waters of Monterey Bay in California; see the
references by Beja et al. [116,117]). As such, all PR reported thus far has been the result of
heterologous expression in
E. coli
, and no information exists about the protein's physio-
logical arrangement in the native cell membrane. The PR expressed in
E. coli
has some
other significant differences from BR, but perhaps the most significant from the standpoint
of device design is that the solubilized protein is remarkably stable. Solubilized BR has a
lifetime of only a few days and needs to be used in purple membrane form for device
applications. The purple membrane imposes a number of limitations on device architec-
tures, especially for optical memory and holography. PR's enhanced stability in the solu-
bilized state may give it an advantage over BR in several aspects, especially with respect
to integrating it into a semiconductor environment. PR's potential in sensor architectures
(optical, chemical, or otherwise) remains to be evaluated.
Photosynthetic reaction centers (PRCs) are redox proteins that transport electrons as
part of Photosystem I; the physiological role of this widespread group of proteins is energy
transduction in photosynthesis. This class of proteins is isolated from plant tissues with
relative ease, and remains stable for prolonged periods in dried preparations (e.g., see Lee
et al. [118]). Several chemical sensor platforms have been proposed in the literature, some
utilizing the isolated reaction center [119,120] while others immobilized tissues containing
PRCs [1,114]. However, while this group of proteins has the potential for highly sensitive
detection schemes, it remains to be seen whether PRCs can be used in chemical sensors
capable of both specificity and selectivity.
Phytochrome and PYP are further examples of photoactive proteins that produce quasi-
stable thermal intermediates in photocycles analogous to BR and PR. Phytochromes are
photo-regulatory proteins found in plants that are responsible for a host of signal trans-
duction and regulatory pathway mechanisms; they respond to modulations in solar
