The prosthetic group of milk lactoperoxidase is protoheme IX.

The prosthetic group of milk lactoperoxidase has been isolated from a Pronase hydrolysate of the enzyme and identified spectroscopically and chromatographically as protoheme IX. Partial degradation of the heme occurred during the proteolysis, possibly as a result of coupled oxidation in the presence of glycine and oxygen. The heme is assumed to be buried in the protein molecule in a crevice, which allows ligands to bind to the iron on one side only. The pyridine hemochrome of lactoperoxidase with alpha-band at 563 nm is interpreted as a mixed ligand complex with pyridine on one side of the heme and a ligand originating from the protein on the other. No experimental evidence supports the view that the heme is covalently bound to the protein through an ester linkage.