Coproporphyrinogen oxidase. II. Reaction mechanism and role of tyrosine residues on the activity.

Purified coproporphyrinogen oxidase catalyzed conversion of 2-beta-hydroxypropionic acid-4-propionic acid deuteroporphyrinogen IX, 2-propionic acid-4-beta-hydroxypropionic acid deuteroporphyrinogen IX, 2,4-bis (beta-hydroxypropionic acid) deuteroporphyrinogen IX, harderoporphyrinogen, and isoharderoporphyrinogen to protoporphyrinogen IX. This result suggests that the enzymatic conversion of propionate groups of coproporphyrinogen III to vinyl groups of protoporphyrinogen IX occurs stepwise starting from position 2 to 4 through beta-hydroxypropionate porphyrinogen as an intermediate. When coproporphyrinogen oxidase was treated with tetranitromethane, an initial modification of 1 tyrosine residue per molecule did not affect the enzyme activity, whereas modification of a second tyrosine residue resulted in a substantial inactivation of the enzyme. Conversion of 2,4-bis-(beta-hydroxypropionic acid) deuteroporphyrinogen XI into protoporphyrinogen IX was not affected by the tyrosine residue modification. Both modification and kinetic studies led to a conclusion that at least one tyrosine residue is involved in the active site of the enzyme, presumably participating in the initial reaction of the oxidation step of a propionate group to beta-hydroxypropionate.