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

## TL;DR

This paper reports the crystal structure of CalO6, an O-methyltransferase involved in calicheamicin biosynthesis, despite challenges in structure determination.

## Contribution

The study successfully determined the structure of CalO6 using a combination of molecular replacement and anomalous dispersion methods.

## Key findings

- CalO6 adopts a methyltransferase fold and dimeric organization typical of bacterial and plant O-methyltransferases.
- The central helical lid-like region near the active site is disordered in the absence of cofactors.
- The missing region is proposed to regulate cofactor entry and exit and shape the active site for catalysis.

## Abstract

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.

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).

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.

The online version of this article (doi:10.1186/s12900-015-0040-6) contains supplementary material, which is available to authorized users.