Crystallization and preliminary X-ray analysis of the hydroperoxidase I C-terminal domain from Escherichia coli

Hydroperoxidases (HP) are normally large haem-containing bifunctional enzymes capable of acting as both catalases and peroxidases. The C-terminal domain of HPI from Escherichia coli (KatG), extending from residue Tyr422 to Leu726, was found to be resistant to trypsin proteolysis. The segment of katG encoding this domain was cloned and overexpressed to produce a haemless protein that is soluble even at concentrations above 30 mg ml^-1. This protein shows a 25% sequence identity with cytochrome c peroxidase (CCP) from Saccharomyces cerevisae, despite lacking the characteristic catalytic and iron-binding residues. Crystals from this protein were grown in 0.6 M sodium citrate buffered to pH 7.5 with HEPES by the hanging-drop vapour-diffusion method at 293 K. These crystals diffracted beyond 2.0 Å resolution and belong to space group P2₁2₁2₁, with unit-cell parameters a = 84.2, b = 98.7, c = 302.8 Å. Three pseudo-origin peaks in the Patterson maps indicate an unusual packing compatible with the presence of three molecules in the crystal asymmetric unit and a solvent content of about 80% by volume.