Normal and abnormal heme biosynthesis. 3.(1)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 decarboxylation of two propionate side chains on coproporphyrinogen-III to produce protoporphyrinogen-IX. This process is very poorly understood at a molecular level, and copro'gen oxidase remains one of the least well-characterized enzymes in the heme biosynthetic pathway. To provide a rigorous test for a proposed model for substrate recognition and binding by this enzyme, two tripropionate analogues of copro'gen-III were prepared where an ethyl group replaced one of the usual propionate residues on positions 13 or 17. Although 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 reduced with 3% sodium-amalgam to give the unstable porphyrinogens needed for the biochemical investigations. The modified structure with a 13-ethyl moiety was metabolized by avian preparations of copro'gen oxidase to give a monovinylic product, but the isomeric 17-ethylporphyrinogen afforded a divinylic product, albeit with poorer overall conversion. These results strongly support the proposed model for substrate binding at the active site of copro'gen oxidase.