Crystallization and structure determination of goat lactoferrin at 4.0 Å resolution: A new form of packing in lactoferrins with a high solvent content in crystals

Abstract

Lactoferrin was purified from fresh samples of goat colostrums, saturated with Fe³⁺ and CO₃^2- ions and crystallized by microdialysis method. The crystals belong to orthorhombic space group P2₁2₁2₁ with a=104.6 Å, b=153.8Å, c= 155.1 Å and Z=4. The quality of crystals was poor, thus the intensity data were restricted to 4.0 Å resolution only. The structure was determined by molecular replacement method using diferric buffalo lactoferrin as a model. The solution clearly indicated the presence of one molecule in the asymmetric unit, which corresponds to a <i>V</i>m value of 7.1 A³/Da. The structure was refined with stringent constraints to an R-factor of 0.246 using all the reflections 15,870 to 4.0 Å resolution. The overall structure of goat lactoferrin is essentially similar to those of buffalo and bovine lactoferrins. However, the iron-binding environment in goat lactoferrin is somewhat different, in which 2 CO₃^2-ions have low occupancies. The solvent content of approximately 84% was very high in the present case which explains the fragility of the crystals of goat lactoferrin. In a way, it is very surprising that the crystals grow at all, although crystals with solvent as high as 89% have been reported.