One example of oxidation is rust. Rusting is the oxidation of iron or its alloys when exposed to oxygen and moisture. Iron reacts with oxygen in the presence of water to form iron oxide (rust). Another example is the brown color on a cut apple. When an apple is cut and exposed to air, the polyphenol oxidase enzyme in the apple catalyzes the oxidation of phenolic compounds to quinones. These quinones then react with amino acids and other compounds to produce brown-colored melanins.