Mutational analysis of the active-site residues crucial for catalytic activity of adenosine kinase from Leishmania donovani.
Leishmania donovani adenosine kinase (LdAdK) plays a pivotal role in scavenging of purines from the host. Exploiting interspecies homology and structural co-ordinates of the enzyme from other sources, we generated a model of LdAdK that led us to target several amino acid residues (namely Gly-62, Arg-69, Arg-131 and Asp-299). Replacement of Gly-62 with aspartate caused a drastic reduction in catalytic activity, with decreased affinity for either substrate. Asp-299 was found to be catalytically indispensable. Mutation of either Arg-131 or Arg-69 caused a significant reduction in kcat. R69A (Arg-69-->Ala) and R131A mutants exhibited unaltered K(m) for either substrate, whereas ATP K(m) for R69K increased 6-fold. Importance of both of the arginine residues was reaffirmed by the R69K/R131A double mutant, which exhibited approx. 0.5% residual activity with a large increase in ATP K(m). Phenylglyoxal, which inhibits the wild-type enzyme, also inactivated the arginine mutants to different extents. Adenosine protected both of the Arg-69 mutants, but not the R131A variant, from inactivation. Binding experiments revealed that the AMP-binding property of R69K or R69A and D299A mutants remained largely unaltered, but R131A and R69K/R131A mutants lost their AMP binding ability significantly. The G62D mutant did not bind AMP at all. Free energy calculations indicated that Arg-69 and Arg-131 are functionally independent. Thus, apart from the mandatory requirement of flexibility around the diglycyl (Gly-61-Gly-62) motif, our results identified Asp-299 and Arg-131 as key catalytic residues, with the former functioning as the proton abstractor from the 5'-OH of adenosine, while the latter acts as a bidentate electrophile to stabilize the negative charge on the leaving group during the phosphate transfer. Moreover, the positive charge distribution of Arg-69 probably helps in maintaining the flexibility of the alpha-3 helix needed for proper domain movement. These findings provide the first comprehensive biochemical evidence implicating the mechanistic roles of the functionally important residues of this chemotherapeutically exploitable enzyme.
Datta
Rupak
R
Division of Infectious Diseases, Leishmania Group, Indian Institute of Chemical Biology, 4, Raja S.C. Mullick Road, Kolkata 700 032, India.
Das
Ishita
I
Sen
Banibrata
B
Chakraborty
Anutosh
A
Adak
Subrata
S
Mandal
Chhabinath
C
Datta
Alok K
AK
D000248:Adenosine Kinase / Q000737:chemistry* / Q000378:metabolism; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D001665:Binding Sites; D002942:Circular Dichroism; D007700:Kinetics; D007893:Leishmania donovani / Q000201:enzymology*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D011487:Protein Conformation; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
D000248:Adenosine Kinase