Tertiary structure of thiopurine methyltransferase from Pseudomonas syringae, a bacterial orthologue of a polymorphic, drug-metabolizing enzyme

J Mol Biol. 2003 Oct 24;333(3):573-85. doi: 10.1016/j.jmb.2003.08.039.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Binding Sites
  • Biotransformation
  • Humans
  • Methyltransferases / chemistry*
  • Methyltransferases / metabolism*
  • Models, Molecular
  • Molecular Sequence Data
  • Nuclear Magnetic Resonance, Biomolecular
  • Protein Structure, Quaternary
  • Pseudomonas / enzymology*
  • Sequence Homology, Amino Acid
  • Structure-Activity Relationship

Substances

  • Methyltransferases
  • thiopurine methyltransferase

Associated data

  • PDB/1PJZ