Sequence Divergence in the Arginase Domain of Ornithine Decarboxylase/Arginase in Leads to Loss of Function in Oral Associated Species.

A number of species within the family of Gram-negative bacteria uniquely encode for an ornithine decarboxylase/arginase (ODA) that ostensibly channels l-ornithine generated by hydrolysis of l-arginine to putrescine formation. However, two aspartate residues required for coordination to a catalytically obligatory manganese cluster of arginases are substituted for a serine and an asparagine. Curiously, these natural substitutions occur only in a clade of Fusobacterium species that inhabit the oral cavity. Herein, we expressed and isolated full-length ODA from the opportunistic oral pathogen along with the individual arginase and ornithine decarboxylase components. The crystal structure of the arginase domain reveals that it adopts the classical α/β arginase-fold, but metal ions are absent in the active site. As expected, the ureohydrolase activity with l-arginine was not detected for wild-type ODA or the isolated arginase domain. However, engineering of the complete metal coordination environment through site-directed mutagenesis restored Mn binding capacity and arginase activity, although the catalytic efficiency for l-arginine was low (60-100 M s). Full-length ODA and the isolated ODC component were able to decarboxylate both l-ornithine and l-arginine to form putrescine and agmatine, respectively, but / of l-ornithine was ∼20-fold higher compared to l-arginine. We discuss environmental conditions that may have led to the natural selection of an inactive arginase in the oral associated species of .