Design, Structure, And Action Of An Artificial Photosynthetic Reaction Center
At the heart of photosystem II is the reaction center, where solar energy is used to separate charge. Set within a large and highly complex protein system, the handful of redox cofactors that make up the reaction center form an electron transport chain that converts the energy of a central, light-activated pigment into a reductant at one end and an oxidant at the other. The central aim of this thesis is to reproduce the charge separating function of photosystem II in a comparatively simple de novo designed protein maquette. The maquette effectively eliminates the complexity contributed by parts of photosystem II that are not directly involved in charge separation and facilitates a streamlined investigation of fundamental factors essential to this function. Previous work has produced light-activatable maquettes that are competent for electron transfer between tetrapyrroles but are unable to trap a charge separated state that is necessary for fuel generation. This thesis details the design, structure, and action of MZH3, a multi-cofactor maquette that stabilizes a long-lived charge separated state. X-ray crystallographic structures of MZH3 are solved in complex with heme B, a synthetic zinc porphyrin, and metal ion cofactors. Despite sharing low sequence identity with natural proteins, MZH3 exhibits significant structural similarity to cytochrome b at the heme site and bacterioferritin at the metal site. Transient absorption spectroscopy shows that the reduced state of heme B is stabilized in the presence of a tyrosine residue for 150 ms after light absorption at pH 9.5. The binding of ferrous iron extends the charge separated state lifetime to 300 ms at pH 7.5. Providing the tyrosine with a hydrogen bond to histidine increases the yield of the charge separated state but decreases its lifetime. Mutation of a heme-ligating histidine to alanine gives rise to an unexpected oxygen binding function with an oxyferrous lifetime of 38 hours, comparable to natural oxygen transport proteins. These results show that MZH3 is a uniquely structured, functional reaction center maquette that is readily adapted to new functions. Continuing development will be directed toward multinuclear metal cluster assembly and in vivo generation of solar fuel from water.
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