Crystal structure of O-methyltransferase CalO6 from the calicheamicin biosynthetic pathway: A case of challenging structure determination at low resolution

Oleg V. Tsodikov, Caixia Hou, Christopher T. Walsh, Sylvie Garneau-Tsodikova

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8 Scopus citations


Background: Calicheamicins (CAL) are enedyine natural products with potent antibiotic and cytotoxic activity, used in anticancer therapy. The O-methyltransferase CalO6 is proposed to catalyze methylation of the hydroxyl moiety at the C2 position of the orsellinic acid group of CAL. Results: Crystals of CalO6 diffracted non-isotropically, with the usable data extending to 3.4 Å. While no single method of crystal structure determination yielded a structure of CalO6, we were able to determine its structure by using molecular replacement-guided single wavelength anomalous dispersion by using diffraction data from native crystals of CalO6 and a highly non-isomorphous mercury derivative. The structure of CalO6 reveals the methyltransferase fold and dimeric organization characteristic of small molecule O-methyltransferases involved in secondary metabolism in bacteria and plants. Uncommonly, CalO6 was crystallized in the absence of S-adenosylmethionine (SAM; the methyl donor) or S-adenosylhomocysteine (SAH; its product). Conclusions: Likely as a consequence of the dynamic nature of CalO6 in the absence of its cofactor, the central region of CalO6, which forms a helical lid-like structure near the active site in CalO6 and similar enzymes, is not observed in the electron density. We propose that this region controls the entry of SAM into and the exit of SAH from the active site of CalO6 and shapes the active site for substrate binding and catalysis.

Original languageEnglish
Article number13
JournalBMC Structural Biology
Issue number1
StatePublished - Jul 15 2015

Bibliographical note

Funding Information:
This work was supported by a National Science Foundation grant CAREER MCB-1149427 (to S.G.-T.), by startup funds from the College of Pharmacy at the University of Kentucky (to O.V.T. and S.G.-T.), and by a National Institutes of Health (NIH) grant GM20011 (to C.T.W.). We thank the staff of sector X-12 of the National Synchrotron Light Source of the Brookhaven National Laboratory for assistance with data collection. We thank Prof. Jon S. Thorson (University of Kentucky) for providing the N-acetylcysteamine orsellinic acid used in some crystallization trials. We thank Dr. Tapan Biswas for help with preliminary data collection on native CalO6 crystals.

Publisher Copyright:
© 2015 Tsodikov et al.


  • Anticancer drug
  • Enediyne
  • Low-resolution refinement
  • Methylation
  • Natural product biosynthesis

ASJC Scopus subject areas

  • Structural Biology


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