Biology Reference
In-Depth Information
regeneration of ATP in a cell, especially under starvation or stress conditions. Studies have
shown that starved
E. coli
cells that express proteorhodopsin and are illuminated by light
have more ATP than similar cells that are held in the dark.
11
Furthermore,
E. coli
cells
expressing proteorhodopsin under illumination conditions can help negate the effects of
respiration inhibition by azide.
12
Even a small boost to a nonphotosynthetic organism
s maximum metabolic capacity could
prove to be very important. This was recently tested by coexpressing both a hydrogenase
and proteorhodopsin in
E. coli
.
13
In theory, the presence of proteorhodopsin coupled with a
hydrogenase should offset some of the metabolic burden on the cell during hydrogen
production, and increase final H
2
yields by
E. coli
.
13
Addition of proteorhodopsin in that
system did improve the final H
2
yields, though the improvement was fairly small and it is
still not completely clear if the higher yield came from more protons being available for the
hydrogenase, or from a proteorhodopsin-generated boost to the available energy levels
inside the cell. If the increased hydrogen production came solely from more protons being
available for the hydrogenase, then the applications for this strategy are somewhat limited.
On the other hand, if proteorhodopsin is indeed able to provide a significant increase to the
energy available for metabolic expenditures in
E. coli
, many potential applications in
metabolically engineered
E. coli
can be envisioned.
'
An intriguing application is the coupling of proteorhodopsin to carbon fixation in a
nonphotosynthetic host producing biofuels. Fixing CO
2
would not only reduce feedstock
requirements of the host, but it would also help create a more carbon-neutral fuel product.
But is proteorhodopsin able to generate enough energy to facilitate carbon fixation? A recent
report shows that the presence of proteorhodopsin increases CO
2
fixation in the
nonphotosynthetic marine bacterium
Polaribacter
sp. MED152, though the bacterium does
require organic material in the medium for growth.
14
It is still unclear if expression of
proteorhodopsin in a nonphotosynthetic host will generate sufficient energy to drive central
metabolism, carbon fixation, or other potentially useful metabolic reactions. Several studies
have shown that bacteria benefit the most from the presence of proteorhodopsin if they are
under starvation, resource-limiting conditions, or alternatively, if the proton gradient across
the inner membrane has been disrupted.
15,16
There are several reasons why one could
expect limited help from the addition of proteorhodopsin to an energy-intensive metabolic
process. One reason is the fairly low membrane potential generated by proteorhodopsin
compared to the membrane potential maintained by
E. coli
under normal growth
conditions.
12
If proteorhodopsin could be engineered to generate a higher membrane
potential, it would allow for an increased generation of ATP under all conditions, and not
primarily when the host
305
'
s respiration is otherwise compromised. Another potential caveat to
using proteorhodopsin, as well as other rhodopsins, for engineered light-energy conversion,
is the narrow range of their light absorption spectra because of the single pigment molecule
involved. This could pose a problem if a system reliant on proteorhodopsin is scaled for
industrial applications as it may be difficult to obtain sufficient light penetration into the
culture medium, a problem that is currently faced when using cyanobacteria or algae for
industrial applications. One potential remedy for the narrow absorption wavelengths is the
use of several different rhodopsin molecules tuned to different wavelengths in the same cell.
It has been shown possible to tune the peak absorption wavelength by as much as 40 nm.
17
While this could broaden the useful light spectrum, the total spectrum used and the
efficiency of protons translocated across a membrane in this system still pales in
comparison with bacterial RC complexes.
Traditionally, metabolic engineering in microbes has been used in order to produce small
molecules that are difficult or expensive to synthesize through chemical methods. In recent
years, a major focus has been the production of biofuels or biopolymers to replace
petroleum-based plastics, as well as complex drugs that are economically impractical to
