Heme groups ascorbate peroxidase

The third covalent bond present in MPO is also thought to form by an autocatalytic process, although it has not been clearly demonstrated to do so. However, incubation of an ascorbate peroxidase mutant into which a methionine has been introduced by mutagenesis has been shown to result in covalent attachment of the Met sulfur atom to a heme vinyl group [54]. Although the link is not identical to that in MPO, this finding provides strong circumstantial evidence that the bond in MPO is also formed by an autocatalytic mechanism. 

FIGURE 19. Active site of ascorbate peroxidase (lOAF) with the heme anchored to the protein via Hisl63, containing a bound ascorbic acid substrate (pink molecule in Plate XLIII). Ascorbate is H-bonded to the 6-propionate group of the heme and to Lys30 and Argl72 (orange)... 

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