Inorganic nanomaterials-based artificial enzymes (nanozymes) have received considerable attention over the past years. However, the substrates studied for nanozymes have so far been limited to small organic molecules. The catalytic oxidation of biomacromolecules, such as proteins, by nanozymes has not yet been reported to date. In this study, we report that cuprous oxide nanoparticles (Cu2O NPs) possess cytochrome c oxidase (CcO)-like activity and catalyze the oxidation of cytochrome c (Cyt c), converting it, from the ferrous state to the ferric state under atmospheric oxygen conditions. Furthermore, the CcO-like activity of Cu2O NPs is pH- and size-dependent. The lower the solution pH and the smaller the particle size, the higher the CcO-like activity. The artificial Cyt c-Cu2O NPs system closely mimics the native Cyt c-CcO enzyme system, which opens new vistas in enzyme construction and potential applications.