Enzymes targeting purine dependent

Structure-Based Libraries Targeting Kinases and Other Purine-Dependent Enzymes... 

T. brucei is unable to synthesize purines de novo and, as such, is dependent upon salvage mechanisms from the host. A number of transporters and enzymes are used by T. brucei to accomplish this task, and inhibition of these targets offers promise for development of trypanocides [39]. This strategy has been validated by demonstration that cordycepin (34), a substrate for T. brucei adenosine kinase (TbAK), which terminates RNA synthesis and parasite growth, can cure stage 2 HAT infections in mice when coadministered with deoxycoformycin (35), an adenosine deaminase inhibitor [40]. 

Dihydrofolate Reductase. The reduced form of folate (tetrahydrofolate) acts as a one-carbon donor in a wide variety of biosynthetic transformations. This includes essential steps in the synthesis of purine nucleotides and of thymidylate, essential precursors to EHA and I A. For this reason, folate-dependent enzymes have been useful targets for the development of anticancer and anti-inflammatory drugs Ce.g., methotrexate) and anti-infectives (trimethoprim, pyrimethamine). During the reaction catalyzed by thymidylate synthase (TS), tetrahydrofolate also acts as a reducltant and is converted stoichiometrically to dihydrofolate. The regeneration of tetrahydrofolate, required for the continuous fimc-tioning of this cofactor, is catalyzed by dihydrofolate reductase (DHFR). 

This enzyme is of considerable biological, medicinal and commercial interest. It plays a fundamental role in purine metabolism, it degrades anticancer drugs being targeted to specific cancers and its absence is associated with severe T-cell deficiency. Crystals of the enzyme on a conventional source diffractometer did not diffract beyond 4 A resolution, a resolution which is not adequate to study the structural interactions with various substrates and inhibitors. With intense synchrotron X-radiation at Daresbury, data could be collected to between 3.2 A and 2.8 A resolution the data to 3.2 A (table 10.3) were sufficient to solve the structure. This is another example of time-dependent radiation damage. Very fast data collection methods at the SRS involved collection of 100 reflections per second, where one crystal was used for four minutes of exposure time. The structure has been described in Ealick et al (1990) (figures 10.5 and 2.1). 

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