The redox potential sets the energy yield possible in metabolism, and is also a key determinant of the rate at which the redox reaction proceeds. Understanding the factors that govern redox reactivity represents a current frontier in structure-function relationships for electron transfer proteins. The heme protein cytochrome bw2 is used as a paradigm to study the co-evolution of redox potential and redox reactivity within a systematic library of variants. Function evolves by single or pair wise mutations of residues in the vicinity of the heme. Both thermodynamic potential (EO) and redox reactivity (self exchange and cross exchange rates) are measured. All possible variants (all 20 amino acids) at each position are cloned, and a simple color screen allows the mutants that have retained both structure and affinity for the b-type heme to be determined. The extrema are used as progenitors of the next generation of variants. Comparing the variation of redox potential with that of the kinetic reactivity (within the context of Marcus theory) in these mutants will give insights into the degree of evolutionary coupling between these two properties.