Use of a novel microtitration protocol to obtain diffraction-quality crystals of 4-hydroxy-2-oxoglutarate aldolase from Bos taurus

The enzyme 4-hydroxy-2-oxoglutarate aldolase (HOGA) catalyses the retro-aldol degradation of 4-hydroxy-2-oxoglutarate to pyruvate and glyoxylate as part of the hydroxyproline catabolic pathway in mammals. Mutations in the coding region of the human HOGA gene are associated with primary hyperoxaluria type 3, a disease characterized by excessive oxalate production and ultimately stone deposition. Native HOGA was purified from bovine kidney using an improved and streamlined purification protocol from which two crystal forms were obtained using two different approaches. Vapour diffusion using PEG 3350 as a precipitant produced monoclinic crystals that belonged to space group C2 and diffracted to 3.5 Å resolution. By comparison, orthorhombic crystals belonging to space group I222 or I2₁2₁2₁ and diffracting to beyond 2.25 Å resolution were obtained using a novel microtitration protocol with ammonium sulfate. The latter crystal form displayed superior diffraction quality and was suitable for structural determination by X-ray crystallography.