Dairy Chemistry And Biochemistry

ffs YcnB

8-cnA^2

I I I I I I I I I I I

MILK PROTEINS 169

+155

-185

K-cnB

An interesting feature of the primary structures of all caseins is that polar
and apolar residues are not uniformly distributed but occur in clusters,
giving hydrophobic and hydrophilic regions (Figures 4.1 3-4.15). This fea-
ture makes the caseins good emulsifiers. The organic phosphates, which are
attached to serines, occur in clusters due to the mechanism by which
phosphorylation occurs (see below and section 4.14.4). The phosphate
clusters bind Ca2 + strongly. The proline residues are fairly uniformly
distributed, giving the caseins a type of poly-proline helix. 0-Casein is the
most hydrophobic of the caseins and a,,-casein is the most hydrophilic. The
C-terminal region of K-casein is strongly hydrophilic due to a high content
of sugars (in some cases), few apolar residues and no aromatic residues,
while the N terminus is strongly hydrophobic; this detergent-like structure
is probably important in micelle stabilization. The hydrophilic segment of
K--casein is cleaved off during rennet action, rendering the residual caseins
coagulable by Ca2+ (Chapter 10).
The caseins are one of the most evolutionarly divergent families of
mammalian proteins. Since their function is nutritional, minor amino acid
substitutions or deletions are not critical. Holt and Sawyer (1993), who
aligned the published sequences of ciSl-, 0- and K--caseins from various