Normal and abnormal heme biosynthesis. 3. Synthesis and metabolism of tripropionate analogues of coproporphyrinogen-III: Novel probes for the active site of coproporphyrinogen oxidase

Coproporphyrinogen oxidase (copro'gen oxidase) catalyses the oxidative deca rboxylation of two propionate side chains on coproporphyrinogen-III to prod uce protoporphyrinogen-IX. This process is very poorly understood at a mole cular level, and copro'gen oxidase remains one of the least well-characteri zed enzymes in the heme biosynthetic pathway. To provide a rigorous test fo r a proposed model for substrate recognition and binding by this enzyme, tw o tripropionate analogues of copro'gen-III were prepared where an ethyl gro up replaced one of the usual propionate residues on positions 13 or 17. Alt hough the required substrate probes are porphyrinogens (hexahydroporphyrins ), the corresponding porphyrin methyl esters were initially synthesized via tripyrrene and a,c-biladiene intermediates. These were hydrolyzed and redu ced with 3% sodium-amalgam to give the unstable porphyrinogens needed for t he biochemical investigations. The modified structure with a 13-ethyl moiet y was metabolized by avian preparations of copro'gen oxidase to give a mono vinylic product, but the isomeric 17-ethylporphyrinogen afforded a divinyli c product, albeit with poorer overall conversion. These results strongly su pport the proposed model for substrate binding at the active site of copro' gen oxidase.