A new crystal form of carboxypeptidase G₂ from Pseudomonas sp. strain RS-16 which is more amenable to structure determination

Carboxypeptidase G₂ is a zinc-dependent exopeptidase which has applications in cancer therapy. Crystallization of carboxypeptidase G₂, first achieved more than a decade ago, yields large crystals; however, problems with non-isomorphism between native crystals as well as failure to obtain any useful heavy-atom derivatives have precluded structure solution. A modification of the crystallization protocol leading to a promising new crystal form which diffracts beyond 3.0 Å resolution on a rotating-anode source is now reported. These crystals are readily indexed on an apparent C-centred orthorhombic lattice with a = 81.35, b = 230.9 and c = 105.5 Å, but the correct crystal system is monoclinic. The crystals have space group P2₁, with a = 81.35, b = 105.5, c = 122.4 Å and β = 109.3°. There are two possible non-equivalent monoclinic indexings with these lattice constants. A partial native data set collected at the SRS, Daresbury, indicates that 1.9 Å diffraction is attainable. Structure determination using MIR methods is in progress.