Dairy Chemistry And Biochemistry

MILK PROTEINS 203

amino acids for which uptake from blood is inadequate (Group 111), their

carbon skeletons are oxidized to CO,. Considered as a whole, total uptake

and output of amino acids from blood are the major, or sole, precursors of

the milk-specific proteins (i.e. the caseins, P-lactoglobulin and a-lactal-

bumin).

0 Group I amino acids: methionine, phenylalanine, tyrosine, histidine and

tryptophan.

0 Group I1 amino acids: valine, leucine, isoleucine, lysine, arginine and

threonine.

0 Group I11 amino acids: aspartic acid, glutamic acid, glycine, alanine,

serine, cysteine/cystine, proline.

The interrelationships between the carbon and nitrogen of amino acids

are summarized in Figure 4.31.

4.14.2 Amino acid transport into the mammary cell

Since the cell membranes are composed predominantly of lipids, amino

acids (which are hydrophilic) cannot enter by diffusion and are transported

by special carrier systems. In the case of mammary cells, the carrier

system(s) has not yet been elucidated.

4.14.3 Synthesis of milk proteins
Synthesis of the major milk proteins occurs in the mammary gland; the
principal exceptions are serum albumin and some of the immunoglobulins,
which are transferred from the blood. Polymerization of the amino acids
occurs on ribosomes fixed on the rough endoplasmic reticulum of the
secretory cells, apparently by a method common to all cells.
The primary blueprint for the amino acid sequence of proteins is
contained in deoxyribonucleic acid (DNA) within the cell nucleus. The
requisite information is transcribed in the nucleus to ribonucleic acid (RNA)
of which there are three types: messenger RNA (mRNA), transfer RNA
(tRNA) and ribosomal RNA (rRNA). These are transferred to the cyto-
plasm where each plays a specific role in protein synthesis.
Protein synthesis actually takes place in the ribosomes of the rough
endoplasmic reticulum (RER) which contain rRNA. There is a specific
tRNA for each amino acid, with which it forms an acyl complex:

Amino acid + tRNA + ATP Mg2: amino acyl-tRNA + AMP + PPi

amino acyl-tRNA synthetase
There is a specific amino acyl-tRNA synthetase for each amino acid; these
enzymes have two specific binding sites, one for the amino acid and the