This denaturation may lead to protein aggregation. One other consequence for
globular proteins is a loss of solubility close to the pI.
2.2 β-Lactoglobulin
β-Lg is a globular protein of 162 amino acids and a molar mass of 18.3 kDa. It
belongs to the lipocalin superfamily, sharing the common β-barrel calyx structural
feature as an ideal binding site for hydrophobic ligands [36–38]. Its molecular
structure is well established [36]: basically, β-Lg has 10–15% α-helix, 43% β-sheet
and 47% unordered structures, including β-turn. Its structure contains nine β-strands
(labelled A–I) that are organised into two β-sheets facing each other and a C-terminal
α-helix, as determined by X-ray crystallography [39]. β-Lg has two disulfide bonds,
which play an important role in the reversibility of β-Lg denaturation [40]. β-Lg also
contains one free sulfhydryl group, which is buried within the protein structure on the
β-strand H and plays an important role in stabilising the protein structure [41]. Its pI is
about 5.2. At neutral pH (5.5–7.5) and room temperature, native β-Lg exists as a
stable non-covalent dimer but its oligomerisation sate is dependent on the medium
conditions. At pH below 3.5 and above 7.5, β-Lg is mainly monomer; between pH
3.5–5.5, it is mainly associated as octamer. These pH ranges also vary according to
the ionic strength, temperature and the presence of hydrophobic ligands in the central
cavity of the protein. Changes that occur in protein structure when heated have been
described [42].
2.3 Serum Albumin
BSA consists of a polypeptide chain of 582 amino acids and a molar mass of
66.4 kDa. It is mainly a helical protein having a pI of 4.9 [43]. BSA is a monomer
containing one sulfhydryl group and 17 disulfide bonds, which stabilise the structure
of the protein. All the disulfide bonds are relatively close to each other in the
polypeptide chain, which is therefore organised in a series of short loops. BSA
exhibits several binding sites for hydrophobic ligands on its surface.
2.4 α-Lactalbumin
α-La consists of 123 amino acids and has a molecular weight of 14.2 kDa. Its
isoelectric point is between 4.2 and 4.5. α-La has close homology in sequence with
hen egg-white lysozyme [44]. Among the 123 amino acid residues, 54 are identical to
corresponding residues in lysozyme and a further 23 residues are structurally similar.
α-La consists of 26% α-helix, 14% β-sheet and 60% unordered structure. α-La
Spontaneous Assembly and Induced Aggregation of Food Proteins
73
globular proteins is a loss of solubility close to the pI.
2.2 β-Lactoglobulin
β-Lg is a globular protein of 162 amino acids and a molar mass of 18.3 kDa. It
belongs to the lipocalin superfamily, sharing the common β-barrel calyx structural
feature as an ideal binding site for hydrophobic ligands [36–38]. Its molecular
structure is well established [36]: basically, β-Lg has 10–15% α-helix, 43% β-sheet
and 47% unordered structures, including β-turn. Its structure contains nine β-strands
(labelled A–I) that are organised into two β-sheets facing each other and a C-terminal
α-helix, as determined by X-ray crystallography [39]. β-Lg has two disulfide bonds,
which play an important role in the reversibility of β-Lg denaturation [40]. β-Lg also
contains one free sulfhydryl group, which is buried within the protein structure on the
β-strand H and plays an important role in stabilising the protein structure [41]. Its pI is
about 5.2. At neutral pH (5.5–7.5) and room temperature, native β-Lg exists as a
stable non-covalent dimer but its oligomerisation sate is dependent on the medium
conditions. At pH below 3.5 and above 7.5, β-Lg is mainly monomer; between pH
3.5–5.5, it is mainly associated as octamer. These pH ranges also vary according to
the ionic strength, temperature and the presence of hydrophobic ligands in the central
cavity of the protein. Changes that occur in protein structure when heated have been
described [42].
2.3 Serum Albumin
BSA consists of a polypeptide chain of 582 amino acids and a molar mass of
66.4 kDa. It is mainly a helical protein having a pI of 4.9 [43]. BSA is a monomer
containing one sulfhydryl group and 17 disulfide bonds, which stabilise the structure
of the protein. All the disulfide bonds are relatively close to each other in the
polypeptide chain, which is therefore organised in a series of short loops. BSA
exhibits several binding sites for hydrophobic ligands on its surface.
2.4 α-Lactalbumin
α-La consists of 123 amino acids and has a molecular weight of 14.2 kDa. Its
isoelectric point is between 4.2 and 4.5. α-La has close homology in sequence with
hen egg-white lysozyme [44]. Among the 123 amino acid residues, 54 are identical to
corresponding residues in lysozyme and a further 23 residues are structurally similar.
α-La consists of 26% α-helix, 14% β-sheet and 60% unordered structure. α-La
Spontaneous Assembly and Induced Aggregation of Food Proteins
73