on pH and temperature. Caseins precipitate when the pH is reduced to 4.6 and β-casein
self-assembles as small aggregates in a temperature-dependent manner.
2.7 Lactoferrin
Lactoferrin (Lf) is an iron-binding glycoprotein composed of 689 amino acids and
has a molar mass of about 80 kDa. It has approximately 41% α-helix and 24%
β-sheets and the polypeptide chain is folded in two homologous lobes (N- and
C-lobes) connected to each other by a three turns of an α-helix (residues 334–344)
[60]. Each lobe is composed by two domains (N 1 , N 2 and C 1 , C 2 ) forming a cleft in
between; the interdomain cleft forms a binding site for one iron ion or other metallic
ion concomitantly to a synergically bound carbonate anion [61, 62]. Lf tertiary
structure is stabilised by 17 disulfide bonds. Compared to the holo form, which is
conformationally rigid, the metal-free apo form of Lf is much more flexible and
more susceptible to denaturation. Lf is a basic protein with a pI of 8.6–8.9 and
consequently carries a positive net charge at neutral pH. The charge distribution on
Lf surface is uneven with some highly positive patches on the N-lobe and the interlobe region [62]. Additional negative charges on the Lf surface are also carried by
sialic moieties [60, 63, 64], which increase the hydration volume on the protein
surface and stabilise Lf [65]. Lf exhibits ionic strength-dependent aggregation
behaviour [66, 67]. These studies reported an Lf monomer–aggregate equilibrium
that is sensitive to variation in ionic strength. At low ionic strength (and neutral pH),
Lf molecules are mainly monomer and positively charged. When ionic strength
increased, Lf forms neutral [66] or negatively charged aggregates (pI ~6) [67].
2.8 Lysozyme
Lysozyme (C-type family without a specific metal binding site; LYS) is a globular
protein of 129 amino acids (14.3 kDa) and a strongly basic character (pI ¼ 10.7). It
is a protein consisting of two domains (domains α and β) linked together by a long
helix–loop–helix (residues 87–114). The secondary structure of the protein is
formed by 39% α-helix gathered mainly in the domain α and 11% β-sheet involved
in a three-strand antiparallel β-sheet that constitutes, with some helices, the
β-domain [68]. LYS has eight cysteine residues, all of which are involved in
intramolecular disulfide bonds making the protein compact and stable [69]. It was
noticed that under specific conditions of protein concentration, temperature and
ionic strength, LYS forms transient clusters [70, 71]. Clusters result from shortrange attractions, leading to surface energy reduction upon cluster formation, and
long-range repulsions that increase the Coulomb energy of the clusters and thus
limits their growth [72].
Spontaneous Assembly and Induced Aggregation of Food Proteins
75
self-assembles as small aggregates in a temperature-dependent manner.
2.7 Lactoferrin
Lactoferrin (Lf) is an iron-binding glycoprotein composed of 689 amino acids and
has a molar mass of about 80 kDa. It has approximately 41% α-helix and 24%
β-sheets and the polypeptide chain is folded in two homologous lobes (N- and
C-lobes) connected to each other by a three turns of an α-helix (residues 334–344)
[60]. Each lobe is composed by two domains (N 1 , N 2 and C 1 , C 2 ) forming a cleft in
between; the interdomain cleft forms a binding site for one iron ion or other metallic
ion concomitantly to a synergically bound carbonate anion [61, 62]. Lf tertiary
structure is stabilised by 17 disulfide bonds. Compared to the holo form, which is
conformationally rigid, the metal-free apo form of Lf is much more flexible and
more susceptible to denaturation. Lf is a basic protein with a pI of 8.6–8.9 and
consequently carries a positive net charge at neutral pH. The charge distribution on
Lf surface is uneven with some highly positive patches on the N-lobe and the interlobe region [62]. Additional negative charges on the Lf surface are also carried by
sialic moieties [60, 63, 64], which increase the hydration volume on the protein
surface and stabilise Lf [65]. Lf exhibits ionic strength-dependent aggregation
behaviour [66, 67]. These studies reported an Lf monomer–aggregate equilibrium
that is sensitive to variation in ionic strength. At low ionic strength (and neutral pH),
Lf molecules are mainly monomer and positively charged. When ionic strength
increased, Lf forms neutral [66] or negatively charged aggregates (pI ~6) [67].
2.8 Lysozyme
Lysozyme (C-type family without a specific metal binding site; LYS) is a globular
protein of 129 amino acids (14.3 kDa) and a strongly basic character (pI ¼ 10.7). It
is a protein consisting of two domains (domains α and β) linked together by a long
helix–loop–helix (residues 87–114). The secondary structure of the protein is
formed by 39% α-helix gathered mainly in the domain α and 11% β-sheet involved
in a three-strand antiparallel β-sheet that constitutes, with some helices, the
β-domain [68]. LYS has eight cysteine residues, all of which are involved in
intramolecular disulfide bonds making the protein compact and stable [69]. It was
noticed that under specific conditions of protein concentration, temperature and
ionic strength, LYS forms transient clusters [70, 71]. Clusters result from shortrange attractions, leading to surface energy reduction upon cluster formation, and
long-range repulsions that increase the Coulomb energy of the clusters and thus
limits their growth [72].
Spontaneous Assembly and Induced Aggregation of Food Proteins
75