STRUCTURAL BASIS OF THE TANFORD TRANSITION OF BOVINE BETA-LACTOGLOBULIN

The structures of the trigonal crystal form of bovine beta-lactoglobul in variant A at pH 6.2, 7.1, and 8.2 have been determined by X-ray dif fraction methods at a resolution of 2.56, 2.24, and 2.49 Angstrom resp ectively. The corresponding values for R (R-free) are 0.192 (0.240), 0 .234 (0.279), and 0.232 (0.277). The C and N termini as well as two di sulfide bonds are clearly defined in these models. The glutamate side chain of residue 89 is buried at pH 6.2 and becomes exposed at pH 7.1 and 8.2. This conformational change, involving the loop 85-90, provide s a structural basis for a variety of pH-dependent chemical, physical, and spectroscopic phenomena, collectively known as the Tanford transi tion.