[en] Some key electron-transfer (ET) proteins have domains containing redox cofactors connected by a flexible tether. The relative motion of the domains is essential for reaction because of the strong ET rate dependence on distance. The constrained conformational flexibility produces a kinetic regime intermediate between 'unimolecular' and 'bimolecular'. We used a simple model for ET coupled to conformational diffusion to explore the structure dependence of the ET kinetics. The model describes the evolution of an initially prepared reduced donor state, and recovers the diffusion and electron tunneling-limited regimes. The restriction of the conformational space imposed by the tether introduces an entropic component to the effective donor-acceptor interaction potential. As such, the tether length may control the transition between the electron tunneling and diffusion-limited regimes