Functional implications of structural differences between variants A and B of bovine β-lactoglobulin

The structure of the trigonal crystal form of bovine β-lactoglobulin variant B at pH 7.1 has been determined by X-ray diffraction methods at a resolution of 2.22 Å and refined to values for R and Rfree of 0.239 and 0.286, respectively. By comparison with the structure of the trigonal crystal form of bovine β-lactoglobulin variant A at pH 7.1, which was determined previously [Qin BY et al., 1998, Biochemistry 37:14014–14023], the structural consequences of the sequence differences D64G and V118A of variants A and B, respectively, have been investigated. Only minor differences in the core calyx structure occur. In the vicinity of the mutation site D64G on loop CD (residues 61–67), there are small changes in main-chain conformation, whereas the substitution V118A on β-strand H is unaccompanied by changes in the surrounding structure, thereby creating a void volume and weakened hydrophobic interactions with a consequent loss of thermal stability relative to variant A. A conformational difference is found for the loop EF, implicated in the pH-dependent conformational change known as the Tanford transition, but it is not clear whether this reflects differences intrinsic to the variants in solution or differences in crystallization.