Download 11. Milk Proteins.pptx

11.10.12
Milk Composition
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Dairy Proteins
Cows Milk, a “3, 4, 5 Food” (approx.)
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86% water
Milk contains three phases:
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Milk Proteins (Cows’ Milk)
Total Protein
Total Caseins
αs1
αs2
β
κ (kappa)
Total Whey Proteins
α-lactalbumin
β-lactoglobulin
Serum Albumin
Immunoglobulins
Proteose peptone
g/L
33
26
10
2.6
9.3
3.3
6.3
1.2
3.2
0.4
0.7
0.8
% of total protein
100
79.5
30.6
8.0
28.4
10.1
19.3
3.7
9.8
1.2
2.1
2.4
3% protein
4% fat
5% lactose
Liquid, colloid & emulsion
proteins are present in every phase
Emulsion phase - the milk fat
globule
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95% of milk lipid is present in globules, mostly
triglycerides
Ensheathed in the membrane of the secreting
cell, including its phospholipids and
membrane proteins
Serum proteins may bind, especially after
homogenization (which decreases the size of
the mfg, ∴increases surface area)
€
MFG Proteome
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~120 proteins found in Cows’ milk, seem
different than those of human milk.
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membrane/protein trafficking (23%)
cell signaling (23%) for secretion?
unknown functions (21%)
fat transport/metabolism (11%)
transport (9%)
protein synthesis/folding (7%)
immune proteins (4%)
milk (serum) proteins (2%)
Milk Protein Fractions-Traditional
Skim Milk
Filtrate
pH 4.6
precipitate
Whey or serum
Casein
20% of protein
80% of protein
50%sat
ammonium sulfate
precipitate
Lactoglobulins
12% of proteins
Filtrate
Lactalbumins
+ proteose peptone
+NPN
8% of protein
In blood: albumins are soluble in water &
dilute salt; globulins precipitate in DI water
1
11.10.12
Milk Protein Commercial Fractions
Skim Milk
pH 4.6
Casein Curd
Chymosin
Heat, Ca++
&/or acid
Casein Curd
Co-precipitate
Acid Casein
Caseinate
Heat aggregation
Lactalbumin
Aqueous Phase (Whey) Proteins
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β-Lactoglobulin
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α-Lactalbumin
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Bovine Serum Albumin (BSA)
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Immunoglobulins, esp in colostrum
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Proteose Peptones
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Rennet Casein
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Whey
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ultrafiltration
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Ion exchange
Whey Prot Conc
Whey Prot Isol
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Most abundant (absent in human milk)
Ca++ binding
Nutritional effects?
Passive immunity transfer to calf
Smallish glycosylated miniproteins
Comparison - Whey Proteins
β-Lactoglobulin
Property
α‑La‑B
Molecular Weight 14,176
Amino acids
123
Proline
2
Cys
8
‑S‑S‑
4
Phospho‑
0
Carbohydrate
Hydrophobicity (kJ/res) 4.7
Charge
mol % residues
28
Net charge/residue
‑0.02
Charge distribution
even
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β-Lactoglobulin
β‑LG‑B
18,363
162
8
5
2
0
5.1
30
‑0.04
even
BSA 66,267
582
34
35
17 0
0
4.3
34
‑0.02
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α-Lactalbumin
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• β-barrel, binding
site inside
• Hydrophobic on
the inside
• Small amount of
helix
• 5 Cys (1 -SH left
over)
Nearly half of total whey proteins
Dominates T behavior
β-barrel protein (“Lipocalin”)
Exists as dimers (octamers @ pH ~4)
Stable @ pH 2, where it is a monomer
Binds lipids, retinol, but not in milk
8 genetic variants, mostly -A, -B
Not present in human milk - immunogenic?
Less functional than suggested by composition
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Very similar to lysozyme structure, except pI
Two major genetic variants, -A and -B
Binds Ca++
8 Cys (4 S-S bonds), 4 Trp
Relatively heat stable, releases Ca ion at pH <4
Modifier subunit of Lactose Synthase
2
11.10.12
α-Lactalbumin
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Very similar to
lysozyme structure,
except pI
Two major genetic
variants, -A and -B
Binds Ca++
8 Cys (4 S-S bonds),
4 Trp
Proteose Peptones
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BSA & IG
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Bovine Serum Albumin
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binds lipids & metal ions
In blood, may aid lipid transport
Immunoglobulins - passive immunity
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Lease heat-stable proteins in milk
IGG
IGA
IGM
Properties of the αs-Caseins
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αS-casein
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ppts from micelle in 0.4M CaCl2, from
solution in 10mM Ca++
αS1-casein - major component
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2 hydrophobic, one polar region
αS2-casein
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strongly negative N-terminus, positive Cterminus
Acid-soluble, heat-stable, highly functional
fraction
Smallish peptides, some are fragments of βcasein, some glycosylated
Component 3 (most hydrophobic)
Component 5 (1-105)
Component 8-fast (1-28)
Component 8-slow (29-105)
Caseins
Property
αSl‑B
Molecular Weight 23,614
Amino acids/Molec
199
Proline
17
Cys
0
‑S‑S‑
0
Phospho‑
8
Carbohydrate
0
Hydrophobicity (kJ/res) 4.9
Charge
mol % residues
34
Net charge/residue
‑0.10
Charge distribution uneven
αSl -A
25,230
207
10
2
?
11
0
4.7
36
‑0.07
uneven
β‑A2
23,983
209
35
0
0
5
0
5.6
23
‑0.06
very
uneven
k‑B
19,023
169
20
2
?
1
+
5.1
21
‑0.02
very‑uneven
Properties of the Other Caseins
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β-casein
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Phosphoprotein
Neg charge on N-term, neutral elsewhere
35 Pro residues
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k-casein - Micelle stabilizer
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γ-casein (& others)
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Glycoprotein, phosphoprotein, on C-terminal side
C-terminal fragment of β-casein
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11.10.12
Casein Micelle - Composition
Component
αs1-Casein
αs2-Casein
β-Casein
k-Casein
ϒ- & others
Calcium
Phosphate
g/100g micelle
35.6
10
34
12
~3
2.9
2.9
Casein Micelle - Structure
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The conc of
Ca++ and
HnPO4
exceeds the
solubility
product of
several CaPO4 salts
Micelle Cartoon
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Caseins lack rigid structure
Associate with hydrophobic regions on the inside,
phosphoserine residues on the outside (sub-micelles)
Polar portion of k-casein holds -OPO3-2 apart
Submicelles are crosslinked by Ca(PO4)6 chains
bridging between -OPO3-2 groups on submicelles
Digestible protein, available Ca++ & Phosphate
Clotting of Milk
core
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OPO3
Ser
-casein
Ca9(PO4)6
OPO3
-casein
Ser
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OPO3
Ca9(PO4)6
OPO3
Ser
Acid (HCl, HOAc, Lactic acid…)
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Ser
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Chymosin (works better at low pH)
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core
Isoelectric precipitation of caseins
Solubilization of Ca++
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Clips k-casein between F105-M106
Disruption of micelle structure
Precipitation of by Ca++
Coprecipitation with other caseins
Further (slower) proteolysis in curd
Enzymes
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Xanthine Oxidase
Phosphatases
Lactoferrin, Transferrin
Lactoperoxidase
Plasmin
Catalase
Lipases
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