Plasmodium falciparum glutaredoxin-like proteins

Glutaredoxin-like proteins form a new subgroup of glutaredoxins with a serine replacing the second cysteine in the CxxC-motif of the active site. Yeast Grx5 is the only glutaredoxin-like protein studied biochemically so far. We identified and cloned three genes encoding glutaredoxin-like proteins from the malaria parasite Plasmodium falciparum (PfGlp1, PfGlp2, and PfGlp3) containing a conserved cysteine in the CGFS-, CKFS-, and CKYS-motif, respectively. Here, we describe biochemical properties of PfGlp1 and PfGlp2. Cys(99), the only cysteine residue in PfGlp1, has a pK(a) value as low as 5.5 and is able to mediate covalent homodimerization. Monomeric and dimeric PfGlp1 react with GSSG and GSH, respectively. PfGlp2 is monomeric and both of its cysteine residues can be glutathionylated. Molecular models reveal a thioredoxin fold for the putative C-terminal domain of PfGlp1, PfGlp2, and PfGlp3, as well as conserved residues presumably required for glutathione binding. However, PfGlp1 and PfGlp2 neither possess activity in a classical glutaredoxin assay nor display activity as glutathione peroxidase or glutathione S-transferase. Mutation of Ser(102) in the CGFS-motif of PfGlp1 to cysteine did not generate glutaredoxin activity either. We conclude that, despite their ability to react with glutathione, glutaredoxin-like proteins are a mechanistically and functionally heterogeneous group with only little similarities to canonical glutaredoxins.