Directed Modification of Reaction Centers from Purple Bacteria

Reaction centers from purple bacteria form a superb test system for the manipulation of electron transfer parameters. The wealth of cofactors and electron transfer reactions provides opportunities for directed modification of specific properties. In particular, the energies of each cofactor can be selectively changed by mutations of neighboring amino acid residues. The starting point for the initial electron transfer, the bacteriochlorophyll dimer, has proven to be exceptionally malleable, allowing large changes in energetics and rates. Most of the other cofactors can be exchanged or eliminated entirely, permitting considerable alteration of pathways. By orchestrating multiple changes in the reaction center, the light-initiated electron transfer pathway can be directed towards alternate ends, for example down the B branch of cofactors rather than the naturally preferred A branch. Extensive modeling of features of electron transfer such as the energetics, the coupling, and the protein dynamics has been corroborated by observed changes in the characteristics of the reactions after modification of the cofactor properties. For example, the maximum rates for several electron transfer reactions, determined by application of Marcus theory to the rates of reactions in a range of mutants, show a correlation with the distance between the cofactors. Other measurements revealing the intimate interaction of the protein and cofactors show that protein motion controls the rate of the initial electron transfer. Thus the reaction center provides a natural and modifiable template for understanding the factors governing electron transfer.