Consequences of binding an S-adenosylmethionine analogue on the structure and dynamics of the thiopurine methyltransferase protein backbone

In humans, the enzyme thiopurine methyltransferase (TPMT) metabolizes 6-thiopurine (6-TP) medications, commonly used for immune suppression and for the treatment of hematopoietic malignancies. Genetic polymorphisms in the TPMT protein sequence accelerate intracellular degradation of the enzyme through an ubiquitylation and proteasomal-dependent pathway. Research has led to the hypothesis that these polymorphisms destabilize the native structure of TPMT, resulting in the formation of misfolded or partially unfolded states, which are subsequently recognized for intracellular degradation. Addition of the cosubstrate, S-adenosylmethionine (SAM), prevents degradation of the TPMT polymorphs in experimental assays, presumably by stabilizing the native structure. Using a bacterial orthologue of TPMT from Pseudomonas syringae, we have used NMR spectroscopy to describe the consequences of binding sinefungin, a SAM analogue, on the structure and dynamics of the TPMT protein backbone. NMR chemical shift mapping experiments localize sinefungin to a highly conserved site in classical methyltransferases. Distal chemical shift changes involving the presumed active site cover imply indirect conformational changes induced by sinefungin, which may play a role in substrate recognition or the catalytic mechanism. Analysis of protein backbone dynamics based on NMR relaxation reveals a combination of complementary effects. Whereas the peripheral, inserted structural elements of the TPMT topology are conformationally stabilized by the presence of sinefungin, a consistent increase in backbone mobility is observed for the central, conserved structural elements. The potential implications for the structural and dynamic effects of binding sinefungin for the catalytic mechanism of the enzyme and the stabilization of the degradation-susceptible TPMT polymorphs are discussed.