Long-range electron transfer (LRET) is a core elementary step in a wealth of processes central to chemistry and biology including photosynthesis, respiration, and catalysis. In nature, biological catalysis is performed by enzymes. However, enzymes are structurally fragile and have limited stability in vitro. Development of robust biomimetic nanostructures is therefore highly desirable. Here, with Prussian Blue nanoparticles (PBNPs) as an example we have demonstrated preparation of highly stable and water-soluble mixed-valence nanoparticles under mild conditions. We have mapped their enzyme-mimicking catalytic properties and controlled LRET to single-nanoparticle resolution. PBNPs show high substrate binding affinity and tunable electrocatalytic efficiency towards hydrogen peroxide reduction, resembling the patterns for similar size redox metalloenzymes. We have further disclosed a correlation between electrocatalytic efficiency and distance dependent interfacial ET kinetics. Given their high stability and low cost, such enzyme-mimicking nanoparticles could offer new perspectives in the fields of catalysis, sensors, and electrochemical energy conversion.