Mercury induced modifications in the stereochemistry of the active site through Cys-73 in a serine protease - Crystal structure of the complex of a partially modified proteinase K with mercury at 1.8 Å resolution

Abstract

Proteinese K(PK) isolated from <i>Tritirachium album </i>Limber was crystallized with HgCl₂ in excess. under microgravity conditions. The intensity data were collected at 4°C to 1.8 Ǻ resolution and the final R-factor after refinement for all the reflections was 0. 164. Mercury has been found at two sites with partial occupancies (0.4 and 0.6) which are at distances of 2.48 Ǻ and 2.58 Ǻ respectively from Cys-73 Sγ. The Cys-73 in the enzyme structure is located close to the active site residue. His-69. This region is completely buri ed and is not accessible to the solvent. It is rather tightly packed. Therefore. the binding of mercury distorts the stereochemistry of the neighbouring residues including those be longing to the catalytic tri ad. As a result of this. the Oγ of Ser-224 is displaced by 0.6 Ǻ which causes the inactivation of proteinase K by increasing the H-bond distance to 3.7 Ǻ between Ser-224 Oγ and His-69 Nε2.