Molecular characterization of Plasmodium falciparum S-adenosylmethionine synthetase

S-Adenosylmethionine (AdoMet) synthetase (SAMS : EC 2.5.1.6) catalyses the formation of AdoMet from methionine and ATP. We have cloned a gene for Plas modium falciparum AdoMet synthetase (PfSAMS) (GenBank accession no. AF09792 3), consisting of 1209 base pairs with no introns, The gene encodes a polyp eptide (PRAMS) of 402 amino acids with a molecular mass of 44844 Da, and ha s an overall base composition of 67% A + T. PfSAMS is probably a single cop y gene, and was mapped to chromosome 9, The PfSAMS protein is highly homolo gous to all other SAMS, including a conserved motif for the phosphate-bindi ng P-loop, HGGGAFSGKD, and the signature hexapeptide, GAGDQG. All the activ e-site amino acids for the binding of ADP, P-i and metal ions are similarly preserved, matching entirely those of human hepatic SAMS and Escherichia c oli SAMS. Molecular modelling of PfSAMS guided by the Xray crystal structur e of E. coli SAMS indicates that PRAMS binds ATP/Mg2+ in a manner similar t o that seen in the E, coli SAMS structure. However, the PfSAMS model shows that it can not form tetramers as does E. coli SAMS, and is probably a dime r instead. There was a differential sensitivity towards the inhibition by c ycloleucine between the expressed PfSAMS and the human hepatic SAMS with K- i values of 17 and 10 mM, respectively. Based on phylogenetic analysis usin g protein parsimony and neighbour-joining algorithms, the malarial PfSAMS i s closely related to SAMS of other protozoans and plants.