A prosthetic group is the non-amino acid component that is part of the structure of the heteroproteins or conjugated proteins, being tightly linked to the apoprotein.
[1] The prosthetic group may be organic (such as a vitamin, sugar, RNA, phosphate or lipid) or inorganic (such as a metal ion).
Thus, the term "prosthetic group" is a very general one and its main emphasis is on the tight character of its binding to the apoprotein.
Loosely bound metal ions and coenzymes are still cofactors, but are generally not called prosthetic groups.
Further examples of organic prosthetic groups are vitamin derivatives: thiamine pyrophosphate, pyridoxal-phosphate and biotin.