2009_Book_FoodChemistry

10.1 Milk 507
Table 10.10. Chymosin specificity: relative rate of hydrolysis of peptides from the κ-casein amino acid sequence
Substrate
105 106
Phe-Met 0.00
104 108
Ser-Phe-Met-Ala-Ile 0.04
109
Ser-Phe-Met-Ala-Ile-Pro 0.11
103
Leu-Ser-Phe-Met-Ala-Ile 21.6
102
His-Leu-Ser-Phe-Met-Ala-Ile 31
110
Leu-Ser-Phe-Met-Ala-Ile-Pro-Pro 100
101
Pro-His-Leu-Ser-Phe-Met-Ala-Ile 100
98 112
His-Pro-His-Pro-His-Leu-Ser-Phe-Met-Ala-Ile-Pro-Pro-Lys-Lys 2500
a Relative rate: kcat/K m .
V a rel
In the C variant of κ-casein, Arg 97 is replaced
with His 97 (Table 10.9), which has a weaker
positive charge. As a result, chymosin is not as
strongly bound as in the case of the B variant;
the rate of catalysis decreases. Therefore, C variant
milk is less suitable for the production of
sweet-milk cheese than B variant milk.
γ-Caseins. These proteins are degradation products
of the β-caseins, formed by milk proteases,
e. g., γ 1 -casein is obtained by cleavage of the residues
1–28. The peptide released is identical to
the proteose-peptone PP8F which has been found
in milk. Correspondingly, γ 2 -andγ 3 -caseins are
formed by hydrolysis of the amino acid residues
1–105 and 1–107 respectively. According to more
recent nomenclature recommendations, β-casein
fragments should be described by the position
numbers. Thus, γ 1 -casein from any β-casein variant
X is called, e. g., β-casein X (f29–209) and the
corresponding proteose peptone PF8F β-casein X
(f1–28).
λ-Caseins.Theλ-casein fraction consists mainly
of fragments of the α s1 -caseins. In vitro the
α-caseins are formed by incubation of the
α s1 -caseins with bovine plasmin.
The molar ratio of the main components α s1 /β +
γ/κ/α s2 is on an average 8/8/3/2. All casein
forms contain phosphoric acid, which always occurs
in a tripeptide sequence pattern (Pse = phosphoserine):
Pse-X-Glu or Pse-X-Pse (10.5)
in which X is any amino acid, including phosphoserine
and glutamic acid. Examples are:
α s1 -Casein :
Pse-Glu-Pse
Pse-Ile-Pse-Pse-Pse-Glu
Pse-Val-Glu
Pse-Ala-Glu
β-Casein : Pse-Leu-Pse-Pse-Pse-Glu
Pse-Glu-Glu
κ-Casein : Pse-Pro-Glu (10.6)
Most probably this regular pattern originates from
the action of a specific protein kinase. The various
distribution of polar and apolar groups of the individual
proteins outlined above are summarized
in Table 10.11. The hydrophobicity values listed
are average hydrophobicity values Hoftheamino
acid side chains present in the sequence of the
given segments, and are calculated as follows:
A measure of the hydrophobicity of a compound
is the free energy, F t , needed to transfer the compound
from water into an organic solvent, and is
given as the ratio of the compound’s solubility in
water (N w , as mole fraction) and in the organic
solvent (N org , as mole fraction), involving the ac-