Molecular cloning and expression of adenosine kinase from Leishmania donovani: identification of unconventional P-loop motif

The unique catalytic characteristics of adenosine kinase (Adk) and its stag e-specific differential activity pattern have made this enzyme a prospectiv e target for chemotherapeutic manipulation in the purine-auxotrophic parasi tic protozoan Leishmania donovani. However, nothing is known about the stru cture of the parasite Adk. We report here the cloning of its gene and the c haracterization of the gene product. The encoded protein, consisting of 345 amino acid residues with a calculated molecular mass of 37173 Da, shares l imited but significant similarity with sugar kinases and inosine-guanosine kinase of microbial origin, supporting the notion that these enzymes might have the same ancestral origin. The identity of the parasite enzyme with th e corresponding enzyme from two other sources so far described was only 40 %. Furthermore, 5' RNA mapping studies indicated that the Adk gene transcri pt is matured post-transcriptionally with the trans-splicing of the mini-ex on (spliced leader) occurring at nt -160 from the predicted translation ini tiation site. The biochemical properties of the recombinant enzyme were sim ilar to those of the enzyme isolated from leishmanial cells. The intrinsic tryptophan fluorescence of the enzyme was substrate-sensitive. On the basis of a multiple protein-alignment sequence comparison and ATP-induced fluore scence quenching in the presence or the absence of KI and acrylamide, the d ocking site for ATP has been provisionally identified and shown to have mar ked divergence from the consensus P-loop motif reported for ATP- or GTP-bin ding proteins from other sources.