Optimization of crystals of an inhibitory antibody of urokinase plasminogen activator receptor (uPAR) with hydrogen peroxide and low protein concentration

Optimization of protein crystal formation is often a necessary step leading to diffraction-quality crystals to enable collection of a full X-ray data set. Typical protein crystal optimization involves screening different components, e.g., pH, precipitants, and additives of the precipitant solution. Here we present an example using an inhibitory antibody of urokinase plasminogen activator receptor (uPAR) where such procedures did not yield diffracting crystals. In contrast, it was the treatment of the protein with hydrogen peroxide incubation and the protein concentration reduction that were found to be key factors in obtaining diffracting crystals. Final crystals diffracted to 1.75 Å, and belong to orthorhombic P2₁2₁2 ₁ space group with unit cell parameters a=37.162 Å, b=84.474 Å, c=134.030 Å, and contain one molecule of Fab fragment of anti-uro kinase receptor antibody in the asymmetric unit.