MILK COMPOSITION: PROTEINS

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MILK COMPOSITIONPROTEINS

• Leo Timms
• Iowa State University

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MILK PROTEINS

• Synthesized in the mammary gland from blood
  amino acids ( except serum albumin and most
  immunoglobulins)
• CASEIN major class of milk proteins
• whey proteins
• non protein nitrogen / others
• Species variation 1 (primates) - gt15
• Limited variation within species/ animal

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Protein Composition
Protein cow human
sow Total 3.2-4.2
.88-1 6 Casein
79 35 58 Whey
protein 21 65
42 Alpha-lactalbumin 3.5 17
15 Beta lactoglobulin 9
0 2 Lactoferrin
lt1 17 1
Immunoglobulin 3 11
7 Serum albumin 1
6 2
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CASEINS

• Major milk proteins ( 80)
• Excellent amino acid composition
• Highly digestible
• Secreted as a micelle (colloid P Ca)
• Acid precipitation at pH 4.6
• (normal milk pH 6.6-6.9)
• Micelle hydrolyzed by rennin (curd)
• Variants alpha, beta, gamma, kappa

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CASEINS
Casein alpha beta gamma
kappa grams/ liter 13.7 6.2
1.2 3.7 phos. (P) 1.05
.6 .15 .3 mol. wt.
24K 24K 20K
19K Pi/ molecule 8-10 5
1 1 Carbohydrate
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NON - CASEIN PROTEINS
Protein function
Conc. mg/l alpha lactalbumin lactose syn.
700 beta - lactoglobulin ???
3000 albumin
like blood 300 Immunoglobulin
immune prot. 600? Lactoferrin
iron binding 18 Ceruloplasmin
Cu carrier low Protein
hormones Prl / PL
(blood) Other enzymes / growth factors/ etc.
low not in humans mainly
blood derived
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NON - PROTEIN NITROGEN
Compound mg / l Total
protein 30,000 Total NPN
300 Urea - N
150 Mimics blood urea - excellent diagnosti
c tool esp. for ruminants
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The value of milk protein
Nomenclature and relative amounts
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AMINO ACIDS

• What do the blood amino acids look like that get
  to the mammary gland?

• diet mammary gland (non-ruminants)
• diet is different than mammary gland in
  ruminants
• Rumen degradeable protein - Ammonia - bacterial
  protein (CHO) or blood urea
• Rumen by-pass protein/AA mammary gland!

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LIPIDS OR FATS
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AMINO ACID (nutrient)UPTAKE

• Arterial concentration of amino acids
• Rate of mammary blood flow
• Extraction rate of amino acids by gland
• Measure venous concentration of AA
• Arterio venous difference (AVD)
• AVD X blood flow rate total nutrient
• 
  extracted

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AMINO ACID (nutrient)UPTAKE
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PROTEIN SYNTHESIS

• DNA
• 
  transcription
• RNA
• messenger ribosomal transfer
• 
  translation
• PROTEINS

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PROTEIN SYNTHESIS
ATP AMP P P
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PROTEIN SYNTHESIS
Attach move AA on chain 2 GTP 2 GDP
2 P
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PROTEIN SYNTHESIS
GOLGI APPARATUS casein ATP
casein-P ADP (casein kinase) Assembled into
micelles Casein-P Ca 140 nanometers secretory
vesicles
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CASEIN MICELLES
90 of Ca in micelle
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MILK COMPOSITION
Casein micelles
Apical membrane
Basement membrane
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The value of milk protein
Amino acids

• 20 possible amino acids Connected via peptide
  bond to form protein

• Casein is a phospho protein (phosphorus is linked
  to a serine amino acid)

P
P
P
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Casein micelle structure calcium phosphate
interactions

• Casein phosphate centers act as nucleation sights
  and several peptide chains inhibit the growth of
  calcium phosphate microgranules
• Proposed that 66 calcium and 66 phosphates (1/3
  organic) ions along with water form calcium
  phosphate microgranules

Adapted from Heat Induced Changes in Milk 1995
Holt 105-133
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Casein micelle structure proposed model

• 10,000 polypeptide chains of the four caseins
• micro-granules of calcium phosphate
• glyco-macro peptide portion on k-casein is
  concentrated on the surface
• rennet coagulation - remove hairs

Adapted from Adv. Prot. Chem 1992, Holt
4363-151
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The value of milk protein
Casein vs whey protein
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The value of milk protein
Casein vs whey protein

• Separated during cheese making

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Cheese Structure at set
Adapted from Kiely et al 1992

• The protein matrix contains embedded fat and
  moisture
• Protein matrix is cross-linked by calcium and
  phosphate

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Cutting and removal of whey
10 lbs milk
1 lb cheese
9 lb whey
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The value of milk protein
Milk components

• Protein is considered to be the most valuable
• World demand for dairy protein currently exceeds
  the world supply will continue for the next
  decade

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Fractionation of milk protein
Whey utilization
1970
Today