Plasmodium falciparum is the causative agent of malaria, a disease where new 
drug targets are required due to increasing resistance to current 
anti-malarials. TMPK (thymidylate kinase) is a good candidate as it is essential 
for the synthesis of dTTP, a critical precursor of DNA and has been much studied 
due to its role in prodrug activation and as a drug target. Type I TMPKs, such 
as the human enzyme, phosphorylate the substrate AZT 
(3'-azido-3'-deoxythymidine)-MP (monophosphate) inefficiently compared with type 
II TMPKs (e.g. Escherichia coli TMPK). In the present paper we report that 
eukaryotic PfTMPK (P. falciparum TMPK) presents sequence features of a type I 
enzyme yet the kinetic parameters for AZT-MP phosphorylation are similar to 
those of the highly efficient E. coli enzyme. Structural information shows that 
this is explained by a different juxtaposition of the P-loop and the azide of 
AZT-MP. Subsequent formation of the transition state requires no further 
movement of the PfTMPK P-loop, with no steric conflicts for the azide moiety, 
allowing efficient phosphate transfer. Likewise, we present results that confirm 
the ability of the enzyme to uniquely accept dGMP as a substrate and shed light 
on the basis for its wider substrate specificity. Information resulting from two 
ternary complexes (dTMP-ADP and AZT-MP-ADP) and a binary complex with the 
transition state analogue AP5dT [P1-(5'-adenosyl)-P5-(5'-thymidyl) 
pentaphosphate] all reveal significant differences with the human enzyme, 
notably in the lid region and in the P-loop which may be exploited in the 
rational design of Plasmodium-specific TMPK inhibitors with therapeutic 
potential.