  Experimental studies of systems containing active proteins that undergo
conformational changes driven by catalytic chemical reactions have shown that
the diffusion coefficients of passive tracer particles and active molecules are
larger than the corresponding values when chemical activity is absent. Various
mechanisms have been proposed for such behavior, including, among others, force
dipole interactions of molecular motors moving on filaments and collective
hydrodynamic effects arising from active proteins. Simulations of a
multi-component system containing active dumbbell molecules that cycle between
open and closed states, a passive tracer particle and solvent molecules are
carried out. Consistent with experiments, it is shown that the diffusion
coefficients of both passive particles and the dumbbells themselves are
enhanced when the dumbbells are active. The dependence of the diffusion
enhancement on the volume fraction of dumbbells is determined, and the effects
of crowding by active dumbbell molecules are shown to differ from those due to
inactive molecules.
