Preliminary X-ray crystallographic studies of the catalytic subunit of Escherichia coli AHAS II with its cofactors.

Acetohydroxyacid synthase (AHAS) is the first common enzyme in the branched-chain amino-acid biosynthesis pathway and is the target of several classes of commercial herbicides. In this study, the Escherichia coli ilvG gene that encodes the catalytic subunit of AHAS II was cloned into the pET28a vector and expressed in soluble form at high levels in E. coli strain BL21 (DE3) cells. The protein was purified using Ni(2+)-chelating chromatography followed by size-exclusion chromatography. The catalytic subunit of E. coli AHAS II was cocrystallized with its cofactors Mg(2+), FAD and ThDP using the sitting-drop vapour-diffusion method and the crystals diffracted to 2.80 Å resolution.