Expression, Purification, Characterization, and X-Ray Analysis of Selenomethionine 215 Variant of Leukocyte Collagenase

Matrix metalloproteinases belong to the superfamily of metzincins containing, besides a similar topology and a strictly conserved zinc environment, a 1,4-tight turn with a strictly conserved methionine residue at position three (the so called Met-turn Bode et al. (1993) FEBS 331, 134–140; Stöcker et al. (1995) Protein Sci. 4, 823–840]. The distal S–CH₃ moiety of this methionine residue forms the hydrophobic basement of the three His residues liganding the catalytic zinc ion. To assess the importance of this methionine, we have expressed the catalytic domain of neutrophil collagenase (rHNC, residues Met80–Gly242) in the methionine auxotrophic Escherichia coli strain B834DE3](hsd metB), with the two methionine residues replaced by Selenomethionine. Complete replacement was confirmed by amino acid analysis and electrospray mass spectrometry. The folded and purified enzyme retained its catalytic activity, but showed modifications which are reflected in changed kinetic parameters. The Met215SeMet substitution caused a decrease in conformational stability upon urea denaturation. The X-ray crystal structure of this Selenomethionine rHNC was virtually identical to that of the wild-type catalytic domain except for a very faint local disturbance around the sulfur-seleno substitution site.