During heme biosynthesis the oxygen-independent coproporphyrinogen III oxidase HemN catalyzes the oxidative decarboxylation of the two propionate side chains on rings A and B of coproporphyrinogen III to the corresponding vinyl groups to yield protoporphyrinogen IX. Here, the sequence of the two decarboxylation steps during HemN catalysis was investigated. A reaction intermediate of HemN activity was isolated by HPLC analysis and identified as monovinyltripropionic acid porphyrin by mass spectrometry. This monovinylic reaction intermediate exhibited identical chromatographic behavior during HPLC analysis as harderoporphyrin (3-vinyl-8,13,17-tripropionic acid-2,7,12,18-tetramethylporphyrin). Furthermore, HemN was able to utilize chemically synthesized harderoporphyrinogen as substrate and converted it to protoporphyrinogen IX. These results suggest that during HemN catalysis the propionate side chain of ring A of coproporphyrinogen III is decarboxylated prior to that of ring B.

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