Pyruvate Formate Lyase (PFL) and PFL Activating Enzyme in the Chytrid Fungus Neocallimastix frontalis : A Free‐Radical Enzyme System Conserved Across Divergent Eukaryotic Lineages 1

Fermentative formate production involves the activity of pyruvate formate lyase, an oxygen‐sensitive enzyme that employs a glycyl radical in its reaction mechanism. While common among anaerobic prokaryotes, this enzyme has so far been found in only two distantly related eukaryotic lineages, anaerobic chytridiomycetes and chlorophytes. Sequence comparisons of homologues from the chytridiomycetes Piromyces and Neocallimastix , the chlorophyte Chlamydomonas , and numerous prokaryotes suggest a single, eubacterial origin of eukaryotic pyruvate formate lyases. Pyruvate formate lyase activating enzyme introduces the glycyl radical into the pyruvate formate lyase protein chain. We discovered this enzyme, which had not previously been reported from eukaryotes, in the same two eukaryotic lineages and show that it shares a similar evolutionary history to pyruvate formate lyase. Sequences with high homology to pyruvate formate lyase activating enzyme were identified in the genomes of the anaerobic protozoan parasites Trichomonas vaginalis, Entamoeba histolytica , and Giardia intestinalis . While the occurrence of pyruvate formate lyase activating enzyme together with pyruvate formate lyase in fungi and chlorophytes was to be expected, the target protein of a glycyl radical enzyme‐activating enzyme in these protozoa remains to be identified.