Biology Reference
In-Depth Information
16
CHAPTER
Towards Engineered
Light
Energy Conversion
in Nonphotosynthetic
Microorganisms
Ilya Tikh and Claudia Schmidt-Dannert
University of Minnesota, St. Paul, MN, USA
INTRODUCTION
Photosynthesis has been essential for the development of complex life on earth and is
generally defined as the ability to convert light into chemical energy coupled to carbon
fixation. Depending on their needs, organisms have developed different strategies for
capturing and utilizing energy from the sun. The diversity of light-capturing machinery
ranges from single protein-driven proton pumping via simple proteorhodopsin, to the
incredibly complex multiprotein assemblies of photosystems I and II found in plants and
cyanobacteria.
1
303
The way that light energy is transferred and stored as chemical energy during photosynthesis
varies greatly between organisms. The primary mechanism by which chemical energy is
stored is through the generation of a proton gradient across a membrane. In most bacteria,
such a proton gradient is created across the inner membrane. In more complex
photosynthetic systems, a special organelle, such as a thylakoid in cyanobacteria, houses all
of the phototsynthetic machinery and is used to generate a proton gradient.
2
After a proton
gradient is created, its energy can be converted to chemical energy in the form of ATP or
NAD(P)H which are utilized by the organism for growth. Ultimately, some of the captured
energy ends up as C
C bonds during CO
2
fixation. Unlike truly photosynthetic organisms,
phototrophic organisms generally use a simpler method for light capture and are not able to
utilize CO
2
as their sole carbon source. Instead of CO
2
fixation, these organisms use the
additional energy from light capture to help drive other metabolic pathways (reviewed in
1
).
It is important to note that the core photosystem components responsible for light capture
and conversion do not exist in a vacuum. Light-energy conversion in a photosynthetic
organism requires a number of accessory proteins.
1
Complex pigments like chlorophylls and
carotenoids are also required for the proper function of bacterial photosynthetic reaction
centers (RCs) and photosystems I and II (PSI, PSII) from plants and algae. The enzymes
responsible for the production of these pigments are encoded in large operons, and their
importance will be discussed later.
3
