Chemistry Reference
In-Depth Information
a polypeptide chain of amino acids residues joined together by peptide linkages,
which may also be crosslinked by disulfide bridges. Amino acids contain both a
weakly basic amino group, and a weakly acid carboxyl group both connected to a
hydrocarbon chain, which is unique to different amino acids. The three-dimensional
organization of proteins, or conformation, also involves secondary, tertiary, and
quaternary structures. The secondary structure refers to the spatial arrangement of
amino acid residues that are near one another in the linear sequence. The alpha-helix
and β-pleated sheet are examples of secondary structures arising from regular and
periodic steric relationships. The tertiary structure refers to the spatial arrangement
of amino acid residues that are far apart in the linear sequence, producing further
coiling and folding. If the protein is tightly coiled and folded into a somewhat spheri-
cal shape, it is called a globular protein. If the protein consists of long polypeptide
chains that are intermolecularly linked, they are called fibrous proteins. There are
some minor proteins that are associated with the FGM.
The concentration of proteins in milk is as follows:
Grams/liter
% of total Protein
Total protein
33
100
Total caseins
26
80
alpha s1
10
31
alpha s2
3
8.0
beta
9
28
kappa
3
10
Total whey proteins
6
19
alpha lactalbumin
1
4
beta lactoglobulin
3
10
BSA
0.4
1.2
Immunoglobulins
1
2.1
Proteose peptone
1
2.4
Caseins, as well as their structural form—casein micelles, whey proteins, and milk
enzymes are very important colloidal molecules in milk. The casein content of milk
represents about 80% of milk proteins. Caseins exhibit solubility at pH 4.6. The com-
mon compositional factor is that caseins are conjugated proteins, most with phos-
phate 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 pro-
portional to the phosphate content. The conformation of caseins is much like that of
denatured globular proteins. The high number of proline residues in caseins causes
particular bending of the protein chain and inhibits the formation of close-packed,
ordered secondary structures. Caseins contain no disulfide bonds. As well, the lack
of tertiary structure accounts for the stability of caseins against heat denaturation
because there is very little structure to unfold. Without a tertiary structure there is
considerable exposure of hydrophobic residues. This results in strong association
reactions of the caseins and renders them insoluble in water. Accordingly, strong
amphiphilic protein acts like a detergent molecule. Self-association is found to be
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