| Abstract: | Ovine whole casein contains 2 multiphosphorylated beta-casein components designated as beta 1 and beta 2-caseins. The complete sequence of beta 1-casein and the partial sequence of beta 2-casein have been determined from cyanogen bromide and tryptic digests. The ovine beta 1 and beta 2-caseins have the same polypeptide chain and appear to differ only in that they contain 6 and 5 phosphates respectively. The amino acid composition of ovine beta 1-casein can be written as: Asp4, Asn4, Thr10, ThrP1, Ser9, SerP5, Glu19, Gn21, Pro34, Gly5, Ala4, Val21, Met6, Ile9, Leu22, Tyr3, Phe9, Trp1, Lys12, His5, Arg3. Compared to bovine beta-casein A2, which is made up of 209 residues, ovine beta 1-casein has a deletion of 2 residues (either Pro-179--Tyr-180 or Tyr-180--Pro-181) and 20 largely conservative amino acid substitutions. Although 20% of the substitutions involve proline residues, the proline contents of ovine beta 1 and bovine beta A2-caseins are very similar, around 16%. The average hydrophobicity, calculated according to Bigelow, is 5.51 kJ/residue, which is similar to that calculated for bovine beta-casein A2. The cluster of 4 phosphorylated serine residues and the highly charged nature of the amino terminal region observed for bovine beta-casein are conserved in the ovine beta-caseins. The substitution from Ile-12 (bovine) to Thr-12 (ovine) results in a new phosphorylation site, according to the phosphorylation code proposed for caseins. This site is only partially phosphorylated hence the occurrence of both beta 1 and beta 2-caseins in ovine milk. |
