Module 4, Research Proposal
Mike Lee and Mike Oh's research proposal for Module 4 of 20.109 Spring 2007.
We intend to explore the research done by Prof. Ed DeLong with proteorhodopsin in E. coli. The ultimate aim is to be able to harvest sunlight with proteorhodopsins for solar electrical purposes. We may later decide to focus on only one of the following aspects of our proposal, but there are a couple goals that we find to be essential for accomplishing our final objective. First, we would like to build a library of strains that are activated by different wavelengths of light, much in the way that Shaner, et al created strains of GFP that would fluoresce different colors. This would maximize the amount of sunlight harvested by the proteorhodopsins. The second goal is to create a mechanism that would transform the proton gradient created by the proteorhodopsins into an electrical current that could be hooked into a circuit.
Proteorhodopsin is a light-activated protein that can produce a H+ gradient across a cell membrane. This gradient could be used as a source of energy, as it is in the oxidative phosphorylation of ATP. Proteorhodopsin is found in some marine microorganisms, and it has been successfully introduced into E. coli. Jan Liphardt at UC Berkeley observed E. coli with flagellar motors activated by green light.
Cytochrome C reduces O2 to H2O at the end of the oxidative phosphorylation pathway. However, with a strong enough proton gradient and certain concentrations of cytochrome c, the protein's catalytic activity can be reversed to act as an oxidase. This frees electrons from H2O as it is oxidized to O2, and these electrons can be directed into a current (Zink).
Nakajima's research creates an electrochemical cell using a hydrated ruthenium solution. At certain concentrations, a proton gradient can trigger redox reactions in this cell, creating a current.
Source: Technology Review 
Original Paper at PubMed 
Shaner Paper on GFP Variants 
Zink Cytochrome C Oxidase Coupling 
Nakajime Ruthenium-Aqua Peper [www.ims.ac.jp/publications/ann_rev_99/ar199976.pdf]
We would like to a solar cell system using proteorhodopsin to capture light energy. This involves 1) creating variants of proteorhodopsin to capture a wider spectrum of visible light wavelengths, and 2) converting the resulting proton gradient into electrical current to charge solar batteries.
Project Details and Methods
We will use directed evolution to create mutants and select for sensitivity to different wavelengths of light (Shaner, et al). The electrical current will be created by either ruthenium-aqua complex (Nakajima, et al) or cytochrome c oxidase coupling (Zink). The former has a publication, and thus is the most likely candidate for our proposal. However, we will continue to research the possibilities of Zink's work.