Coproporphyrinogen oxidase (copro'gen oxidase), which catalyses the conversion of coproporphyrinogen-III via a monovinylic intermediate to protoporphyrinogen-IX, is one of the least well understood enzymes in the heme biosynthetic pathway. To develop a model for the substrate recognition and binding recognition for this enzyme, a series of substrate analogues were prepared with two alkyl substituents on positions 13 and 17 in place of the usual propionate residues. Although the required substrate probes are porphyrinogens (hexahydroporphyrins), the corresponding porphyrin methyl esters were initialy synthesized via a,c-biladiene intermediates. These were hydrolyzed and reduced with 3% sodium amalgam to give the unstable porphyrinogens needed for the biochemical investigations. These modified structures were metabolized by avian preparations of copro'gen oxidase to give monovinylic products, but the second propionate residue was not further metabolized. In three cases, the metabolites were isolated and further characterized by proton NMR spectroscopy and mass spectrometry. When methyl or ethyl groups were placed at the 13 and 17 positions, the resulting porphyrinogens were very good substrates (although the ethyl version, mesoporphyrinogen-VI, gave slightly better results), but when propyl units were introduced metabolism was significantly inhibited and the butyl-substituted structure was only slightly transformed after long incubation periods. These results suggest the presence of an active-site lipophobic region near the catalytic site for copro'gen oxidase. The observation that the related 3-vinyl- and 3-ethylporphyrinogens with 13,17-diethyl substituents were not substrates for this enzyme confirmed the need for a second propionate residue to hold the substrate in place at the catalytic site.

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