Structure of two crystal forms of sheep β-lactoglobulin with EF-loop in closed conformation.

Ovine β-lactoglobulin has been isolated from whey fraction of sheep milk and crystallized. The high-resolution structures of two crystal forms (triclinic and trigonal) obtained at pH 7.0 have been determined revealing that ovine protein, similarly to its bovine analog, is dimeric. Access to the binding site located in the eight-stranded antiparallel β-barrel in both structures is blocked by the EF loop that has been found in closed conformation. Similarly to bovine lactoglobulin (BLG), conformation of the EF loop is stabilized by hydrogen bond between Glu89 and Ser116 indicating that Tanford transition might occur with the same mechanism. The substitution at six positions in relation to the most abundant isoform B of BLG also affects the distribution of electrostatic potentials and the total charge.