Casein is a proteolytic enzyme obtained from the stomachs of calves. It is extracted from the proteins of the milk; in the milk, casein is structured in voluminous globules.
CASEIN
Biological Source
Casein is a proteolytic enzyme obtained from the stomachs of
calves. It is extracted from the proteins of the milk; in the milk, casein is structured
in voluminous globules. These globules are mainly responsible for the white
colour of the milk. According to various species, the casein amount within the
total proteins of the milk varies.
The casein content of milk represents about 80% of milk
proteins. The principal casein fractions are alpha (s1) and alpha (s2)-caseins,
β-casein and κ-casein. The distinguishing
property of all casein is their low solubility at pH 4.6. The common
compositional factor is that caseins are conjugated proteins, most with
phosphate group(s) esterified to serine residues. These phosphate groups are
important to the structure of the casein micelle. Calcium binding by the
individual caseins is proportional to the phosphate content.
Within the group of caseins, there are several
distinguishing features based on their charge distribution and sensitivity to
calcium precipitation:
Alpha (s1)-casein: (molecular weight 23,000; 199
residues, 17 proline residues).
Two hydrophobia regions, containing all the proline
residues, separated by a polar region, which contains all but one of eight
phosphate groups. It can be precipitated at very low levels of calcium.
Alpha (s2)-casein: (molecular weight 25,000; 207
residues, 10 prolines).
Concentrated negative charges near N-terminus and positive
charges near C-terminus. It can also be precipitated at very low levels of
calcium.
β-casein: (molecular weight 24,000; 209
residues, 35 prolines).
Highly charged N-terminal region and a hydrophobia
C-terminal region. Very amphiphilic protein acts like a detergent molecule.
Self association is temperature-dependent; will form a large polymer at 20°C
but not at 4°C. Less sensitive to calcium precipitation.
κ-casein: (molecular weight 19,000; 169
residues, 20 prolines).
Very resistant to calcium precipitation, stabilizing other
caseins. Rennet cleavage at the Phe l05 – Met l06 bond eliminates the
stabilizing ability, leaving a hydrophobia portion, para- κ-casein and a hydrophilic portion called κ-casein glycomacropeptide (GMP), or more accurately, caseinomacropeptide (CMP).
Characteristics
The isoelectric point of casein is 4.6. The purified protein
is water insoluble. While it is also insoluble in neutral salt solutions, it is
readily dispersible in dilute alkalis and in salt solutions such as sodium
oxalate and sodium acetate. Casein does not coagulate on heating. It is
precipitated by acids and by a proteolytic enzyme (rennet).
Chemical Constituents
Milk consists of 80% of milk proteins (casein). The major
constituents of casein are alpha (s1) and alpha (s2)-caseins, β-casein and kappa-casein. These caseins are conjugated proteins with phosphate group(s) which are esterified into
serine residues they have a low solubility at pH 4.6.
Uses
It is used in the manufacture of binders, adhesives,
protective coatings, plastics (such as for knife handles and knitting
needles), fabrics, food additives, and many other products. It is commonly used
by bodybuilders as a slow-digesting source of amino acids. There is growing
evidence that casein may be addictive for some individuals, particularly those
on the autism spectrum or having schizophrenia.
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