Light Harvesting and Light Activatable Protein Maquettes Designed from Scratch

Abstract

Electron transport chain (ETC) modularity inspires framework for renewable energy harvesting. In order to design maquettes capable of light-harvesting and light activated electron transfer using natural and synthetic Zn porphyrins, chlorins and bacteriochlorins, we have explored cofactor structural requirements for binding to the maquettes. Binding of several synthetic Zn porphyrins with different substituents to the hydrophilic single chain maquettes were studied. Based on this binding data we have hypothesized an amphiphillic character of a tetrapyrrole as an essential requirement for efficient and fast binding (with in few seconds) at room temperature. Having a nonpolar side compatible with the hydrophobic interior of the maquette and a polar side compatible with polar amino acids and solvent on the outside of the maquette are the simple requirements which will allow for hydrophobic partitioning, hence facilitating the ligation of the metal to specifically tailored histidines to stabilize binding. using this approach we can have a control over orientation of the molecule in the maquette, which is an important requirement for efficient energy transfer or electron transfer in the maquettes. This simple amphiphillic model will enable us to design new synthetic tetrapyrrole cofactors that bind to maquettes, which will allow for engineering and assembly of maquettes capable of light-harvesting as well as photochemistry. We have also engineered a covalently expressible c-type cytochrome maquette and successfully replaced the central iron with Zn creating a light activatable covalently attached Zn porphyrin containing maquette. We also present preliminary studies showing that billins, which are involved in harvesting of yellow-red light of solar spectrum can be attached in-vitro to cysteines in designed protein maquettes.