lysozyme, and the egg-white gradually liquefies.
Like lysozyme, other egg-white components,
which belong functionally or structurally to other
protein families, are discussed in various sections
of this book. For example, conalbumin (ovotransferrin) of the chicken egg differs from serum
transferrin in the blood plasma only in its carbohydrate fraction, and not in the amino acid
sequence, and it has already been introduced in
Chapter 5. The protease inhibitors of egg-white,
ovomucoid, ovoinhibitor and ovostatin were
mentioned in Chapter 3. Therefore, what follows
is mainly concerned with the ovalbumin family
and with avidin.
Ovalbumin is the predominant protein of the
egg-white. It is easy to isolate and is, therefore,
well investigated, but its biological role is not
clear. Chicken ovalbumin is a polypeptide of
383 amino acids with a mass of 42.7 kDa. The
polypeptide chain is post-translationally modified
at four sites: the N-terminal glycine is acetylated,
the residues Ser-68 and Ser-344 are phosphorylated, and Asn-292 is N-glycosylated. More than
ten different oligosaccharides have been identified in the ovalbumin of the chicken, and other
avian species also show marked heterogeneity in
the carbohydrate fraction. The ovalbumin gene
was the first gene in which interruptions of the
coding sequence by introns were described in
1978. The promoter activity is normally suppressed and this inhibition is relieved by steroids [39].
Two further homologous genes, the X and the
Y gene, occupy the same 40-kb DNA segment on
which the ovalbumin gene is located; they show
the same divisions into eight exons and are also
expressed in the oviduct in an oestrogendependent manner. However, their protein products have not yet been identified. The sequence of
388 amino acids encoded by the Y gene is 58 %
similar to that of ovalbumin, and the sites of posttranslational modification, with the exception of
the substitution 69-Ser to Thr, are also the same
[51]. In the egg-white of the penguins, e.g. Pygoscelis adeliae, about half the ovalbumin is replaced
by a protein, penalbumin, which is found in trace
amounts in other avian eggs (Table 11.3). This is a
glycoprotein of 61 kDa which, with a carbohydrate fraction of 15 %, is twice as highly glycosylated as ovalbumin; the sequence is not yet known
[33].
Avidin is a basic glycoprotein (pI = 10) with
128 amino acids and 10 % carbohydrate, and it
binds biotin with an extremely high affinity. Avidin molecules are homotetramers of 66-kDa subunits, each of which carries a biotin-binding site.
11.6.1 Caseins
389
This protein is synthesized in the oviducts of
amphibians, reptiles and birds, but not in mammals. Its concentration in the egg-white varies
more than 100-fold between different avian species, with especially large amounts occurring in
the egg of the turkey Meleagris gallopava. Avidin
synthesis in the oviduct cells is regulated by steroid hormones, and in some circumstances other
cells of both males and females can synthesize the
protein, e.g. following wounding or during
inflammation, bacterial infection or retroviral
transformation; avidin apparently has an overall
protective function [42]. In addition to avidin,
other vitamin-binding proteins are found in
chicken egg-white; for example, there is a
cobalamin-binding protein and a riboflavinbinding protein which is also present in the yolk
[147, 149]. The yolk also contains a biotin-binding
protein which has no immunological relationship
to avidin [86]. The albumen of the eggs of the crocodile Crocodylus porosus contains significantly
less dry matter than that of chicken eggs (3.6 %
compared with 12 % ). About 80 % of the total
protein is comprised of a glycoprotein which
resembles ovomucin of avian eggs in its amino
acid spectrum and other properties. There is also
a large amount of a further protein with the characteristics of ovomacroglobulin. Proteins that
are comparable with ovalbumin and ovotransferrin are not found in the crocodile; instead, a
novel 59-kDa protein with the name crocalbumin
has been demonstrated [15].
11.6 Milk Proteins
11.6.1 Caseins
Caseins are the major proteins of mammalian
milk. They provide the young animal not only
with amino acids but also with calcium phosphate
for skeleton growth; these are bound in colloidal
particles referred to as micelles. Bovine micelles
are formed by calcium-dependent aggregation of
molecules of type asb a s2 and ~ caseins, and are
stabilized and protected from premature precipitation by casein x. Interactions between nonpolar regions of the polypeptide chains are
responsible for the aggregation of casein molecules. Negative charges concentrated in particular
parts of the sequence bring about interaction
between Ca 2 + and casein x. Caseins as and ~ can
be phosphorylated on serines which lie mainly in
Like lysozyme, other egg-white components,
which belong functionally or structurally to other
protein families, are discussed in various sections
of this book. For example, conalbumin (ovotransferrin) of the chicken egg differs from serum
transferrin in the blood plasma only in its carbohydrate fraction, and not in the amino acid
sequence, and it has already been introduced in
Chapter 5. The protease inhibitors of egg-white,
ovomucoid, ovoinhibitor and ovostatin were
mentioned in Chapter 3. Therefore, what follows
is mainly concerned with the ovalbumin family
and with avidin.
Ovalbumin is the predominant protein of the
egg-white. It is easy to isolate and is, therefore,
well investigated, but its biological role is not
clear. Chicken ovalbumin is a polypeptide of
383 amino acids with a mass of 42.7 kDa. The
polypeptide chain is post-translationally modified
at four sites: the N-terminal glycine is acetylated,
the residues Ser-68 and Ser-344 are phosphorylated, and Asn-292 is N-glycosylated. More than
ten different oligosaccharides have been identified in the ovalbumin of the chicken, and other
avian species also show marked heterogeneity in
the carbohydrate fraction. The ovalbumin gene
was the first gene in which interruptions of the
coding sequence by introns were described in
1978. The promoter activity is normally suppressed and this inhibition is relieved by steroids [39].
Two further homologous genes, the X and the
Y gene, occupy the same 40-kb DNA segment on
which the ovalbumin gene is located; they show
the same divisions into eight exons and are also
expressed in the oviduct in an oestrogendependent manner. However, their protein products have not yet been identified. The sequence of
388 amino acids encoded by the Y gene is 58 %
similar to that of ovalbumin, and the sites of posttranslational modification, with the exception of
the substitution 69-Ser to Thr, are also the same
[51]. In the egg-white of the penguins, e.g. Pygoscelis adeliae, about half the ovalbumin is replaced
by a protein, penalbumin, which is found in trace
amounts in other avian eggs (Table 11.3). This is a
glycoprotein of 61 kDa which, with a carbohydrate fraction of 15 %, is twice as highly glycosylated as ovalbumin; the sequence is not yet known
[33].
Avidin is a basic glycoprotein (pI = 10) with
128 amino acids and 10 % carbohydrate, and it
binds biotin with an extremely high affinity. Avidin molecules are homotetramers of 66-kDa subunits, each of which carries a biotin-binding site.
11.6.1 Caseins
389
This protein is synthesized in the oviducts of
amphibians, reptiles and birds, but not in mammals. Its concentration in the egg-white varies
more than 100-fold between different avian species, with especially large amounts occurring in
the egg of the turkey Meleagris gallopava. Avidin
synthesis in the oviduct cells is regulated by steroid hormones, and in some circumstances other
cells of both males and females can synthesize the
protein, e.g. following wounding or during
inflammation, bacterial infection or retroviral
transformation; avidin apparently has an overall
protective function [42]. In addition to avidin,
other vitamin-binding proteins are found in
chicken egg-white; for example, there is a
cobalamin-binding protein and a riboflavinbinding protein which is also present in the yolk
[147, 149]. The yolk also contains a biotin-binding
protein which has no immunological relationship
to avidin [86]. The albumen of the eggs of the crocodile Crocodylus porosus contains significantly
less dry matter than that of chicken eggs (3.6 %
compared with 12 % ). About 80 % of the total
protein is comprised of a glycoprotein which
resembles ovomucin of avian eggs in its amino
acid spectrum and other properties. There is also
a large amount of a further protein with the characteristics of ovomacroglobulin. Proteins that
are comparable with ovalbumin and ovotransferrin are not found in the crocodile; instead, a
novel 59-kDa protein with the name crocalbumin
has been demonstrated [15].
11.6 Milk Proteins
11.6.1 Caseins
Caseins are the major proteins of mammalian
milk. They provide the young animal not only
with amino acids but also with calcium phosphate
for skeleton growth; these are bound in colloidal
particles referred to as micelles. Bovine micelles
are formed by calcium-dependent aggregation of
molecules of type asb a s2 and ~ caseins, and are
stabilized and protected from premature precipitation by casein x. Interactions between nonpolar regions of the polypeptide chains are
responsible for the aggregation of casein molecules. Negative charges concentrated in particular
parts of the sequence bring about interaction
between Ca 2 + and casein x. Caseins as and ~ can
be phosphorylated on serines which lie mainly in