The transâsialidase from the african trypanosome Trypanosoma brucei
2002
Montagna, Georgina | Cremona, M Laura | Paris, Gastón | Amaya, M Fernanda | Buschiazzo, Alejandro | Alzari, Pedro M. | Frasch, Alberto C. C.
Trypanosoma brucei is the cause of the diseases known as sleeping sickness in humans (T.âbrucei ssp. gambiense and ssp. rhodesiense) and ngana in domestic animals (T.âbrucei brucei) in Africa. Procyclic trypomastigotes, the tsetse vector stage, express a surfaceâbound transâsialidase that transfers sialic acid to the glycosylphosphatidylinositol anchor of procyclin, a surface glycoprotein covering the parasite surface. Transâsialidase is a unique enzyme expressed by a few trypanosomatids that allows them to scavenge sialic acid from sialylated compounds present in the infected host. The only enzyme extensively characterized is that of the American trypanosome T.âcruzi (TcTS). In this work we identified and characterized the gene encoding the transâsialidase from T.âbrucei brucei (TbTS). TbTS genes are present at a small copy number, at variance with American trypanosomes where a large gene family is present. The recombinant TbTS protein has both sialidase and transâsialidase activity, but it is about 10 times more efficient in transferring than in hydrolysing sialic acid. Its Nâterminus contains a region of 372 amino acids that is 45% identical to the catalytic domain of TcTS and contains the relevant residues required for catalysis. The enzymatic activity of mutants at key positions involved in the transfer reaction revealed that the catalytic sites of TcTS and TbTS are likely to be similar, but are not identical. As in the case of TcTS and TrSA, the substitution of a conserved tryptophanyl residue changed the substrate specificity rendering a mutant protein capable of hydrolysing both αâ(2,3) and αâ(2,6)âlinked sialoconjugates.
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