Crystal Structure of an Arginase-like Protein from <i>Trypanosoma brucei</i> That Evolved without a Binuclear Manganese
Cluster
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Abstract
The X-ray crystal structure of an
arginase-like protein from the
parasitic protozoan <i>Trypanosoma brucei</i>, designated
TbARG, is reported at 1.80 and 2.38 Å resolution in its reduced
and oxidized forms, respectively. The oxidized form of TbARG is a
disulfide-linked hexamer that retains the overall architecture of
a dimer of trimers in the reduced form. Intriguingly, TbARG does not
contain metal ions in its putative active site, and amino acid sequence
comparisons indicate that all but one of the residues required for
coordination to the catalytically obligatory binuclear manganese cluster
in other arginases are substituted here with residues incapable of
metal ion coordination. Therefore, the structure of TbARG is the first
of a member of the arginase/deacetylase superfamily that is not a
metalloprotein. Although we show that metal binding activity is easily
reconstituted in TbARG by site-directed mutagenesis and confirmed
in X-ray crystal structures, it is curious that this protein and its
parasitic orthologues evolved away from metal binding function. Knockout
of the TbARG gene from the genome demonstrated that its function is
not essential to cultured bloodstream-form <i>T. brucei</i>, and metabolomics analysis confirmed that the enzyme has no role
in the conversion of l-arginine to l-ornithine in
these cells. While the molecular function of TbARG remains enigmatic,
the fact that the <i>T. brucei</i> genome encodes only this
protein and not a functional arginase indicates that the parasite
must import l-ornithine from its host to provide a source
of substrate for ornithine decarboxylase in the polyamine biosynthetic
pathway, an active target for the development of antiparasitic drugs