Prosthetic group: A tightly bound, specific non-polypeptide unit required for the biological function of some proteins.

Heme group: A prosthetic group consisting of a protoporphyrin ring and a central iron (Fe) atom <>. A protoporphyrin ring is made up of four pyrrole rings linked by methene bridges. Four methyl, two vinyl, and two propionate side chains are attached. The iron can either be in the ferrous (Fe++) or the ferric (Fe+++) oxidation state.
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