The human
malaria parasite Plasmodium falciparum possesses a single gene with high similarity to the
metalloproteins arginase and
agmatinase. The
recombinant protein reveals strict specificity for
arginine, and it has been proposed that its function in
ornithine production is as a precursor for
polyamine biosynthesis. The specific activity of the plasmodial
arginase was found to be 31 micromol min(-1) mg(-1)
protein and the k(cat) was calculated as 96 (s-1) . The Km value for
arginine and Ki value for
ornithine were determined as 13 mM and 19 mM, respectively. The active
arginase is a homotrimer of ca. 160 kDa. Dialysis of the
arginase against
EDTA results in monomers of approximately 48 kDa; however, the quaternary structure can be restored by addition of Mn 2+ . Mutagenic analyses of all the
amino acid residues proposed to be involved in
metal binding led to complex dissociation, except for the His-193-Ala mutant, which was also inactive but retained the trimeric structure. Substitution of His-233, which has been suggested to be in charge of
proton shuttling within the active site, disrupted the trimeric structure and thereby the activity of the Pf
arginase. Northern blot analysis identified a stage-specific expression pattern of the plasmodial
arginase in the ring/young trophozoite stage, which guarantees the provision of
ornithine for essential
polyamine biosynthesis.