DAIRY CHEMISTRY AND BIOCHEMISTRY OoH Figure 4.23 Schematic representation of the tertiary structure of bovine B-lactoglobulin, howing the binding of retinol; arrows indicate antiparallel B-sheet structures (from Papiz et al,1986) 4.7.6 Quaternary structure by Timasheff and co-workers that below pH 3.5, B-1g dissociates to mono- ners of 18kDa. Between pH 5.5 and 7.5, all bovine B-lg variants form dimers of molecular mass 36 kDa but they do not form mixed dimers, i.e.a dimer consisting of A and B monomers, possibly because B-lg A and b contain valine and alanine, respectively, at position 178. Since valine is Larger than alanine, it is suggested that the size difference is sufficient to prevent the proper fit for hydrophobic interaction, Porcine and other B-Igs that contain no free thiol do not form dimers; lack of a thiol group is probably not directly responsible for the failure to dimerize Between pH 3.5 and 5. 2, especially at pH 4.6, bovine B-lg forms octamers of molecular mass 144 kDa. B-Lg A associates more strongly than B-lg B, possibly because it contains an additional aspartic acid instead of glycine (in B)per monomer; the additional Asp is capable of forming additional hydrogen bonds in the pH region where it is undissociated. B-Lg from Droughtmaster cattle, which has the same amino acid composition as bovine B-lg a but is a glycoprotein, fails to octamerize, presumably due to stearic hinderance by the carbohydrate moiety190 DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 4.23 Schematic representation of the tertiary structure of bovine /?-lactoglobulin, showing the binding of retinol; arrows indicate antiparallel 8-sheet structures (from Papiz et al., 1986). 4.7.6 Quaternary structure p-Lg shows interesting association characteristics. Early work indicated that the monomeric molecular mass of bovine 8-lg was 36 kDa but it was shown by Timasheff and co-workers that below pH 3.5, p-lg dissociates to mono￾mers of 18kDa. Between pH 5.5 and 7.5, all bovine p-lg variants form dimers of molecular mass 36 kDa but they do not form mixed dimers, i.e. a dimer consisting of A and B monomers, possibly because p-lg A and B contain valine and alanine, respectively, at position 178. Since valine is larger than alanine, it is suggested that the size difference is sufficient to prevent the proper fit for hydrophobic interaction. Porcine and other p-lgs that contain no free thiol do not form dimers; lack of a thiol group is probably not directly responsible for the failure to dimerize. Between pH 3.5 and 5.2, especially at pH 4.6, bovine p-lg forms octamers of molecular mass 144kDa. p-Lg A associates more strongly than p-lg B, possibly because it contains an additional aspartic acid instead of glycine (in B) per monomer; the additional Asp is capable of forming additional hydrogen bonds in the pH region where it is undissociated. p-Lg from Droughtmaster cattle, which has the same amino acid composition as bovine p-lg A but is a glycoprotein, fails to octamerize, presumably due to stearic hinderance by the carbohydrate moiety