Effect of covalent modification on coproporphyrinogen oxidase from chicken red blood cells

The biosynthesis of heme is a complex multi-step pathway requiring the efforts of eight enzymes. The initial enzymes in the heme biosynthetic pathway have been well characterized in relation to their mechanisms. Coproporphyrinogen oxidase (Copro'gen oxidase) is one of the last three enzymes in the pathway and is one of the least well understood. Copro'gen oxidase converts coproporphyrinogen III to protoporphyrinogen IX via oxidative decarboxylation of the 3-and 8-propionic side chain moieties. To further our understanding of the recognition and binding of substrate, Copro'gen oxidase was partially purified from chicken red blood cell hemolysates then incubated with covalent modifiers of specific amino acids. Incubation with tetranitromethane, p-hydroxyphenylglyoxal, N-acetylimidazole, or trinitrobenzenesulfonic acid resulted in substantial reduction of Copro'gen oxidase activity implying the presence of critical tyrosine, arginine and lysine residues. We conclude that these amino acids play important roles in the enzymic mechanism (for both binding and catalysis) of Copro'gen oxidase