ILK PROTEINS amino acids for which uptake from blood is inadequate( Group Ill), 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, B-lactoglobulin and a-lactal Group I amino acids: methionine, phenylalanine, tyrosine, histidine and tryptophan. Group II amino acids: valine, leucine, isoleucine, lysine, arginine and threonine Group Ill 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 cids(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 RNA. There is a speci tRNA for each amino acid, with which it forms an acyl complex Amino acid trna+ atP amino acyl-tRNA AMP +P amino acyl-trna synthetase There is a specific amino acyl-tRNA synthetase for each amino acid; the enzymes have two specific binding sites, one for the amino acid and the
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-lactalbumin). 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 cytoplasm 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
DAIRY CHEMISTRY AND BIOCHEMISTRY Channe ing of the gr。 wing polypeptide polypeptides and a proposed mechanism for cotranslational crossing of the RER growing Figure 4.32 Schematic representation of ribosomes attached to mRNa showin (from Mercier and Gaye, 1983 second for the appropriate tRNA. The specificity of the tRNAs is deter mined by the sequence of the anticodon which recognizes and hydrogen bonds with the complementary codon of the mRNA. Interaction between the tRNa and the appropriate amino acid occurs in the cytoplasm but the remaining reactions in protein synthesis occur in the ribosomes, which are complex structures of rRNA and a number of proteins(including enzymes, tiators and controlling factors). The ribosomes of animal cells hay diameters of about 22nm and a sedimentation coefficient of 80S; they consist of two principal subunits: 60S and 40S. mRNa passes through a groove or tunnel between the 60S and 40S subunits; while in the groove mRNA is protected from the action of ribonuclease( Figure 4.32) The information for the amino acid sequence is contained in the mRNA Synthesis commences at the correct codon of the mrna because a specia amino acid derivative, N-formyl methionine: H NH H, CSCH, CH, C-COOH H bound to a specific special codon and it forms the temporary N-terminal residue of the protein; N-formyl methionine is later hydrolysed off, together
204 DAIRY CHEMISTRY AND BIOCHEMISTRY (30-40 Res ) Binding Of the ribophorins growing polypeptide sianal to a of Removal of the signal putatrve receptor a transient proteinaceous tunnel Figure 4.32 Schematic representation of ribosomes attached to mRNA showing the growing polypeptides and a proposed mechanism for cotranslational crossing of the RER membrane (from Mercier and Gaye, 1983). second for the appropriate tRNA. The specificity of the tRNAs is determined by the sequence of the anticodon which recognizes and hydrogen bonds with the complementary codon of the mRNA. Interaction between the tRNA and the appropriate amino acid occurs in the cytoplasm but the remaining reactions in protein synthesis occur in the ribosomes, which are complex structures of rRNA and a number of proteins (including enzymes, initiators and controlling factors). The ribosomes of animal cells have diameters of about 22nm and a sedimentation coefficient of 80s; they consist of two principal subunits: 60s and 40s. mRNA passes through a groove or tunnel between the 60s and 40s subunits; while in the groove, mRNA is protected from the action of ribonuclease (Figure 4.32). The information for the amino acid sequence is contained in the mRNA. Synthesis commences at the correct codon of the mRNA because a special amino acid derivative, N-formyl methionine: H c=o NH I I I I H,CSCH,CH$-COOH H is bound to a specific special codon and it forms the temporary N-terminal residue of the protein; N-formyl methionine is later hydrolysed off, together