Dairy Chemistry And Biochemistry

190 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.