MAD on threonine synthase: the phasing power of oxidized selenomethionine

The use of selenomethionine and anomalous dispersion has become the most wi dely used way of solving the phase problem for de novo protein structure de termination. In this paper, MAD data collected from oxidized and reduced se lenomethionine-containing protein are described, and it is shown that oxidi zed selenomethionine has a very strong phasing power and can be efficiently used if the oxidation is uniform. The comparison was performed on threonin e synthase crystals. For example, the phasing power of the oxidized data is doubled for the dispersive signal and is 20% stronger for the anomalous si gnal at the peak wavelength. The strength of the anomalous signal can be us ed to improve the signal when a protein contains few methionines or for sin gle anomalous dispersion. The oxidation of some selenomethionines shows in the electron-density map through the presence of water molecules within hyd rogen-bonding distance of the putative O atom.