Paramecium bursaria chlorella virus-1 encodes an unusual arginine decarboxylase that is a close homolog of eukaryotic ornithine decarboxylases

Paramecium bursaria chlorella virus (PBCV-1) is a large double-stranded DNA virus that infects chlorella-like green algae. The virus encodes a homolog of eukaryotic ornithine decarboxylase (ODC) that was previously demonstrated to be capable of decarboxylating L-ornithine. However, the active site of this enzyme contains a key amino acid substitution (Glu for Asp) of a residue that interacts with the δ-amino group of ornithine analogs in the x-ray structures of ODC. To determine whether this active-site change affects substrate specificity, kinetic analysis of the PBCV-1 decarboxylase (PBCV-1 DC) on three basic amino acids was undertaken. The k_cat/K_m for L-arginine is 550-fold higher than for either L-ornithine or L-lysine, which were decarboxylated with similar efficiency. In addition, α- difluoromethylarginine was a more potent inhibitor of the enzyme than α-difluoromethylornithine. Mass spectrometric analysis demonstrated that inactivation was consistent with the formation of a covalent adduct at Cys ³⁴⁷. These data demonstrate that PBCV-1 DC should be re-classified as an arginine decarboxylase. The eukaryotic ODCs, as well as PBCV-1 DC, are only distantly related to the bacterial and plant arginine decarboxylases from their common β/α-fold class; thus, the finding that PBCV-1 DC prefers L-arginine to L-ornithine was unexpected based on evolutionary analysis. Mutational analysis was carried out to determine whether the Asp-to-Glu substitution at position 296 (position 332 in Trypanosoma brucei ODC) conferred the change in substrate specificity. This residue was found to be an important determinant of substrate binding for both L-arginine and L-ornithine, but it is not sufficient to encode the change in substrate preference.