prosthetic group
A non-protein component tightly bound to a protein and essential for its biological activity; a technical term in biochemistry.
The heme in hemoglobin is an example of a prosthetic group.
From 補欠 (supplementary, substitute) + 分子 (molecule) + 族 (group, family). The term is a calque or translation of the English 'prosthetic group' in biochemistry.