contains eight cysteine residues, all engaged in disulfide bonds that stabilize the
tertiary structure of the protein. In addition, α-La binds one calcium ion per molecule
(K A ¼ 2.9 Â 10
8 M
À1 [45]) in a pocket containing four aspartate residues [46].
When the pH is decreased below 5, the aspartate residues are progressively
protonated and α-La ability to bind calcium decreases. The calcium-free apo form
of α-La is highly heat-sensitive and it denatures rapidly when the temperature
increases. The denaturation temperature of apo α-La is around 30
C [47] whereas
holo α-La is stable up to around 60
C [48, 49] (see Table 1). The apo α-La selfassociates at pH close to 5.0 at about 50
C to give microscopic aggregates.
2.5 Ovalbumin
Ovalbumin (Ova) is a phosphoglycoprotein of 385 amino acids and a molar mass
of about 45 kDa [50]. Its amino acid sequence contains about 50% hydrophobic
residues and about 33% charged residues, mostly acidic, giving the protein a pI of
4.5 [51]. The sequence includes six cysteine residues, two of which are involved in
a disulfide bond, and the N-terminal residue of the protein is an acetylated glycine
[52]. Ova has one glycosylation site and two phosphorylation sites [53]. Ova
contains 32% β-sheets and 30% α-helix as determined by X-ray crystallography
[54, 55]. Some evidences indicate that Ova molecules self-assemble depending
on protein concentration and pH. Ova is monomer at concentrations lower than
about 0.1% and forms oligomers (dimer, trimer and tetramer) at higher protein
concentrations; the association behaviour is favoured when pH decreases from
a neutral value to the protein pI [56].
2.6 Caseins
Caseins (α S1 , α S2 , β, κ) are a group of flexible proteins of about 20–24 kDa (from 169
to 209 amino acids) sharing some common features such as the presence of esterbound phosphate (organic phosphate) in their structure and rather high number of
charged (glutamic acid, lysine) and uncharged (leucine, isoleucine, proline) residues,
but low number of sulphur-containing amino acids. They are often considered as
intrinsically unstructured proteins [57] or rheomorphic proteins [58] as the structure
of caseins is sensitive to environmental variations. In addition, caseins consist
of well-separated hydrophobic and hydrophilic domains, especially β-casein and
κ-casein. The caseins also exhibit microheterogeneity regarding their degree of
phosphorylation and glycosylation [59]. α S - and β-caseins are extensively
phosphorylated whereas κ-casein has only one or two ester-bound phosphates but is
glycosylated. Due to the presence of a high amount of phosphoseryl groups, α S - and
β-casein precipitate in the presence of calcium. In contrast, κ-casein is still soluble
even in the presence of high amount of calcium. Casein aggregation state also depends
74
S. Bouhallab and T. Croguennec
tertiary structure of the protein. In addition, α-La binds one calcium ion per molecule
(K A ¼ 2.9 Â 10
8 M
À1 [45]) in a pocket containing four aspartate residues [46].
When the pH is decreased below 5, the aspartate residues are progressively
protonated and α-La ability to bind calcium decreases. The calcium-free apo form
of α-La is highly heat-sensitive and it denatures rapidly when the temperature
increases. The denaturation temperature of apo α-La is around 30
C [47] whereas
holo α-La is stable up to around 60
C [48, 49] (see Table 1). The apo α-La selfassociates at pH close to 5.0 at about 50
C to give microscopic aggregates.
2.5 Ovalbumin
Ovalbumin (Ova) is a phosphoglycoprotein of 385 amino acids and a molar mass
of about 45 kDa [50]. Its amino acid sequence contains about 50% hydrophobic
residues and about 33% charged residues, mostly acidic, giving the protein a pI of
4.5 [51]. The sequence includes six cysteine residues, two of which are involved in
a disulfide bond, and the N-terminal residue of the protein is an acetylated glycine
[52]. Ova has one glycosylation site and two phosphorylation sites [53]. Ova
contains 32% β-sheets and 30% α-helix as determined by X-ray crystallography
[54, 55]. Some evidences indicate that Ova molecules self-assemble depending
on protein concentration and pH. Ova is monomer at concentrations lower than
about 0.1% and forms oligomers (dimer, trimer and tetramer) at higher protein
concentrations; the association behaviour is favoured when pH decreases from
a neutral value to the protein pI [56].
2.6 Caseins
Caseins (α S1 , α S2 , β, κ) are a group of flexible proteins of about 20–24 kDa (from 169
to 209 amino acids) sharing some common features such as the presence of esterbound phosphate (organic phosphate) in their structure and rather high number of
charged (glutamic acid, lysine) and uncharged (leucine, isoleucine, proline) residues,
but low number of sulphur-containing amino acids. They are often considered as
intrinsically unstructured proteins [57] or rheomorphic proteins [58] as the structure
of caseins is sensitive to environmental variations. In addition, caseins consist
of well-separated hydrophobic and hydrophilic domains, especially β-casein and
κ-casein. The caseins also exhibit microheterogeneity regarding their degree of
phosphorylation and glycosylation [59]. α S - and β-caseins are extensively
phosphorylated whereas κ-casein has only one or two ester-bound phosphates but is
glycosylated. Due to the presence of a high amount of phosphoseryl groups, α S - and
β-casein precipitate in the presence of calcium. In contrast, κ-casein is still soluble
even in the presence of high amount of calcium. Casein aggregation state also depends
74
S. Bouhallab and T. Croguennec