Nucleoside analogue prodrugs are dependent on efficient intracellular stepwise phosphorylation to their triphosphate form to become therapeutically active. In many cases it is this activation pathway that largely determines the efficacy of the drug. To gain further understanding of the determinants for efficient conversion by the enzyme thymidylate kinase (TMPK) of clinically important thymidine monophosphate analogues to the corresponding diphosphates, we solved the crystal structures of the enzyme, with either ADP or the ATP analogue AppNHp at the phosphoryl donor site, in complex with TMP, AZTMP (previous work), NH2TMP, d4TMP, ddTMP, and FLTMP (this work) at the phosphoryl acceptor site. In conjunction with steady-state kinetic data, our structures shed light on the effect of 3'-substitutions in the nucleoside monophosphate (NMP) sugar moiety on the catalytic rate. We observe a direct correlation between the rate of phosphorylation of an NMP and its ability to induce a closing of the enzyme's phosphate-binding loop (P-loop). Our results show the drastic effects that slight modifications of the substrates exert on the enzyme's conformation and, hence, activity and suggest the type of substitutions that are compatible with efficient phosphorylation by TMPK.

The nucleotide binding and release cycle of the molecular chaperone DnaK is regulated by the accessory proteins GrpE and DnaJ, also called co-chaperones. The concerted action of the nucleotide exchange factor GrpE and the ATPase-stimulating factor DnaJ determines the ratio of the two nucleotide states of DnaK, which differ in their mode of interaction with unfolded proteins. In the Escherichia coli system, the stimulation by these two antagonists is comparable in magnitude, resulting in a balance of the two nucleotide states of DnaK(Eco) in the absence and the presence of co-chaperones. The regulation of the DnaK chaperone system from Thermus thermophilus is apparently substantially different. Here, DnaJ does not stimulate the DnaK-mediated ATP hydrolysis and thus does not appear to act as an antagonist of the nucleotide exchange factor GrpE(Tth). This raises the question of whether T. thermophilus GrpE stimulates nucleotide exchange to a smaller degree as compared to the E. coli system and how the corresponding rates relate to intrinsic ATPase and ATP binding as well as luciferase refolding kinetics of T. thermophilus DnaK. We determined dissociation constants as well as kinetic constants that describe the interactions between the T. thermophilus molecular chaperone DnaK, its nucleotide exchange factor GrpE and the fluorescent ADP analogue N8-(4-N'-methylanthraniloylaminobutyl)-8-aminoadenosine-5'-diphosphate by isothermal equilibrium titration calorimetry and stopped-flow kinetic experiments and investigated the influence of T. thermophilus DnaJ on the DnaK nucleotide cycle. The interaction of GrpE with the DnaK.ADP complex versus nucleotide-free DnaK can be described by a simple equilibrium system, where GrpE reduces the affinity of DnaK for ADP by a factor of about 10. Kinetic experiments indicate that the maximal acceleration of nucleotide release by GrpE is 80,000-fold at a saturating GrpE concentration. Our experiments show that in T. thermophilus, although the thermophilic DnaK system displays no stimulation of the DnaK-ATPase activity by DnaJ, nucleotide exchange is still efficiently stimulated by GrpE. This indicates that two counteracting factors are not absolutely necessary to maintain a functional and regulated chaperone cycle. This conclusion is corroborated by data that show that the slower ATPase cycle of the DnaK system as well as of heterologous T. thermophilus DnaK/E. coli DnaK systems is directly reflected in altered refolding kinetics of firefly luciferase but not necessarily in refolding yields.

The 60-fold reduced phosphorylation rate of azidothymidine (AZT) monophosphate (AZTMP), the partially activated AZT metabolite, by human thymidylate kinase (TMPK) severely limits the efficacy of this anti-HIV prodrug. Crystal structures of different TMPK nucleotide complexes indicate that steric hindrance by the azido group of AZTMP prevents formation of the catalytically active closed conformation of the P-loop of TMPK. The F105Y mutant and a chimeric mutant that contains sequences of the human and Escherichia coli enzyme phosphorylate AZTMP 20-fold faster than the wild-type enzyme. The structural basis of the increased activity is assigned to stabilization of the closed P-loop conformation.

Background: Thymidylate kinase (TMPK) is a nucleoside monophosphate kinase that catalyzes the reversible phosphoryltransfer between ATP and TMP to yield ADP and TDP. In addition to its vital role in supplying precursors for DNA synthesis, human TMPK has an important medical role participating in the activation of a number of anti-HIV prodrugs.Results: Crystal structures of human TMPK in complex with TMP and ADP, TMP and the AIP analog AppNHp, TMP with ADP and the phosphoryl analog AIF(3), TDP and ADP, and the bisubstrate analog TP,A were determined. The conformations of the P-loop, the LID region, and the adenine-binding loop vary according to the nature of the complex. Substitution of ADP by AppNHp results in partial closure of the P-loop and the rotation of the TMP phosphate group to a catalytically unfavorable position, which rotates back in the AIF3 complex to a position suitable for in-line attack. In the fully closed state observed in the TP(5)A and the TDP-ADP complexes, Asp15 interacts strongly with the 3'-hydroxyl group of TMP.Conclusions: The observed changes of nucleotide state and conformation and the corresponding protein structural changes are correlated with intermediates occurring along the reaction coordinate and show the sequence of events occurring during phosphate transfer. The low catalytic activity of human TMPK appears to be determined by structural changes required to achieve catalytic competence and it is suggested that a mechanism might exist to accelerate the activity.

Hsp90 is an abundant molecular chaperone that functions in an ATP-dependent manner in vivo. The ATP-binding site is located in the N-terminal domain of Hsp90. Here, we dissect the ATPase cycle of Hsp90 kinetically. We find that Hsp90 binds ATP with a two-step mechanism. The rate-limiting step of the ATPase cycle is the hydrolysis of ATP. Importantly, ATP becomes trapped and committed to hydrolyze during the cycle. In the isolated ATP-binding domain of Hsp90, however, the bound ATP was not committed and the turnover numbers were markedly reduced. Analysis of a series of truncation mutants of Hsp90 showed that C-terminal regions far apart in sequence from the ATP-binding domain are essential for trapping the bound ATP and for maximum hydrolysis rates. Our results suggest that ATP binding and hydrolysis drive conformational changes that involve the entire molecule and lead to repositioning of the N and C-terminal domains of Hsp90.

Based on the knowledge of the crystal structures of yeast and Escherichia coli thymidylate kinases (TmpKs) and the observation that TmpK from E. coli can phosphorylate azidothymidine monophosphate (AZT-MP) much more efficiently than either the yeast or the highly homologous human enzyme, we have engineered yeast and human TmpKs to obtain enzymes that have dramatically improved AZT-MP phosphorylation properties. These modified enzymes have properties that make them attractive candidates for gene therapeutic approaches to potentiating the action of AZT as an inhibitor of human immunodeficiency virus (HIV) replication. In particular, insertion of the lid domain of the bacterial TmpK into the human enzyme results in a pronounced change of the acceptance of AZT-MP such that it is now phosphorylated even faster than TMP.