Structure of bovine β-lactoglobulin (variant A) at very low ionic strength

Bovine β-lactoglobulin (BLG) is a globular protein of uncertain physiological function and a member of the lipocalin superfamily of proteins. Here, we present the X-ray structure at 3.0 Å of BLG (variant A) from an orthorhombic (P212121) pseudo-tetragonal crystal form that suffers from pseudo-merohedral twinning (final Rworking = 0.224, Rfree = 0.265). Crystals were grown by dialysis against ultra-purified water (i.e., at very low ionic strength), at pH ∼5.2 (≈pI), conditions vastly different from all other BLG structures determined previously. This allows critical assessment of the BLG structure and of the influence that pH, ionic strength, and crystal packing may have on the molecular structure of BLG. The pH-sensitive EF loop is found in the closed conformation characteristic of BLG at pH less than 7 and moderate to high ionic strength. Although the hydrophobic pocket appears to be empty, the presence of highly disordered water molecules cannot be excluded. The dimer interface and the hydrophobic pocket (calyx) are preserved. However, the orientation of the subunits in the dimer varies considerably with crystal form. Structure is deposited with PDB ID 2akq.