Tertiary structure of thiopurine methyltransferase from Pseudomonas syringae, a bacterial orthologue of a polymorphic, drug-metabolizing enzyme.
In humans, the enzyme thiopurine methyltransferase (TPMT) metabolizes 6-thiopurine (6-TP) medications, including 6-thioguanine, 6-mercaptopurine and azathioprine, commonly used for immune suppression and for the treatment of hematopoietic malignancies. S-Methylation by TPMT prevents the intracellular conversion of these drugs into active 6-thioguanine nucleotides (6-TGNs). Genetic polymorphisms in the TPMT protein sequence have been associated with decreased tissue enzymatic activities and an increased risk of life-threatening myelo-suppression from standard doses of 6-TP medications. Biochemical studies have demonstrated that TPMT deficiency is primarily associated with increased degradation of the polymorphic proteins through an ubiquitylation and proteasomal-dependent pathway. We have now determined the tertiary structure of the bacterial orthologue of TPMT from Pseudomonas syringae using NMR spectroscopy. Bacterial TPMT similarly catalyzes the S-adenosylmethionine (SAM)-dependent transmethylation of 6-TPs and shares 45% similarity (33% identity) with the human enzyme. Initial studies revealed an unstructured N terminus, which was removed for structural studies and subsequently determined to be required for enzymatic activity. Despite lacking sequence similarity to any protein of known three-dimensional structure, the tertiary structure of bacterial TPMT reveals a classical SAM-dependent methyltransferase topology, consisting of a seven-stranded beta-sheet flanked by alpha-helices on both sides. However, some deviations from the consensus topology, along with multiple insertions of structural elements, are evident. A review of the many experimentally determined tertiary structures of SAM-dependent methyltransferases demonstrates that such structural deviations from the consensus topology are common and often functionally important.
Scheuermann
Thomas H
TH
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520-8114, USA.
Lolis
Elias
E
Hodsdon
Michael E
ME
D000595:Amino Acid Sequence; D001665:Binding Sites; D001711:Biotransformation; D006801:Humans; D008780:Methyltransferases / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D020836:Protein Structure, Quaternary; D011549:Pseudomonas / Q000201:enzymology*; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship
D008780:Methyltransferases; C022745:thiopurine methyltransferase