Interprotein electron transfer (ET) reactions play an important role in biological energy conversion processes. One of these reactions, the ET between cytochrome c2 (cyt) and reaction center (RC) from photosynthetic bacteria, is the focus of this theoretical study. Electrostatic interactions strongly enhance the ET reaction between cyt and RC. The proposed mechanism involves an encounter complex (EC) stabilized by electrostatic interactions, followed by a transition state (TS), leading to the bound complex (BC) active in ET. The present work is a computational analysis to determine the ensemble configurations of the TS and EC and the molecular detail of the interprotein ET reactions. The TS ensemble was obtained by comparing changes in the TS energies due to different mutations with the differences in the electrostatic energies calculated for a wide range of cyt/RC configurations. The resulting TS ensemble is close to the average position of the EC ensemble, with strong electrostatic interactions between cyt and the RC surface. The similarity between the structures of the EC, TS, and BC can account for the rapid association. The changes in the ET rate constant during the association process were calculated as the cyt moved from the EC to the BC. The ET rate increased smoothly as the cyt approached from the EC to the BC, with a tunneling decay factor of 1.1 Å-1. This relatively efficient coupling between redox centers is due to the ability of interfacial water molecules to form multiple strong hydrogen bonding pathways connecting tunneling pathways between the two proteins.