In organic chemistry, a heteroatom (from Ancient Greek heteros, different, + atomos) is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure. Typical heteroatoms are nitrogen, oxygen, sulfur, phosphorus, chlorine, bromine, and iodine.[1][2]
In the description of protein structure, in particular in the Protein Data Bank file format, a heteroatom record (HETATM) describes an atom as belonging to a small molecule cofactor rather than to part of a biopolymer chain.[3]