Non-competitive inhibition is an irreversible inhibition of enzyme activity by the presence of a substance that has no structural similarity with the substrate. Cyanide inhibits the activity of cytochrome oxidase by combining with its metallic ions. It has no structural similarity with the substrate of the enzyme, namely cytochrome c. Cytochrome oxidase is a respiratory enzyme. In its inhibition, the animal is unable to perform the respiration properly and gets killed.