Investigation of non-native metals bound to reaction centers from Rhodobacter sphaeroides: Manganese binding and zinc-substituted cofactors

Rhodobacter (R.) sphaeroides reaction centers have been studied for many years as important analogues to understanding photosystem II. In this dissertation, two aspects of the reaction center are investigated. First, the cofactors that make up the electron transport chain are examined via a magnesium chelatase mutant with identical electron transport cofactors. This mutant contains zinc bacteriochlorophyll and shows electron transport times similar to wild type proteins even though the solution redox potentials for all cofactors are identical. The second aspect investigated is the manganese cluster found in photosystem II. This cluster is conspicuously absent from the wild-type reaction centers of R. sphaeroides. Previous work has introduced a metal binding site into the reaction center in a position analogous to that in photosystem II with the necessary redox potential to oxidize any bound manganese ions. In this work, a new mutant reaction center is presented that exhibits an enhanced binding constant for manganese. Both optical and electron paramagnetic resonance spectroscopy confirm binding and electron donation from manganese to the protein. These observations hold true for binding of manganese chloride as well as a synthesized dinuclear manganese compound. The information gained from the creation and testing of new binding sites for metals and metal complexes may lead to a greater understanding of the assembly process for the manganese cluster in photosystem II.