Thermodynamic control of electron transfer rates in multicentre redox proteins

In the analysis of kinetic data from multicentre redox proteins, it is essential to distinguish between the observable macroscopic rate constants and the structurally relevant microscopic properties. This ' distinction is complicated by the existence of interactions between centres. The problem is illustrated by the case of two interacting redox centres and generalised for the analysis of stopped-flow kinetic data for the reduction of cytochrome c₃, in which four redox centres and at least one proteolytic centre are mutually interacting. It is shown that fast intramolecular electron transfer, which is typical of many multicentre redox proteins, and, where present, fast proton exchange, ensure that only N rate constants can be measured for a protein with N redox centres. The equations that relate the observable macroscopic rate constants to the microscopic rate constants of individual centres depend on a set of parameters that can be approximated by using the Marcus theory of electron transfer together with a set of reasonable assumptions. The results are tested by fitting experimental data for the reduction of cytochrome c₃ by sodium dithionite, including its pH dependence.