Download AMINO ACIDS IN PROTEINS

Chapter 5
Food Chemistry II:
Proteins
1
CHAPTER OBJECTIVES

describe structure of food proteins and functional
properties

relationship between isoelectric point and protein
functionality

composition of casein micelle and functional properties
of polypeptides in the micelles
2
FUNCTION AND COMPONENTS
OF PROTEIN
Protein is essential to building
and maintaining body tissues
 Amino acids are building blocks
of protein
 Plant and animal proteins are
made of up of 20 common amino
acids

3
CLASSIFICATION OF AMINO ACIDS
IN FOOD PROTEIN
Amino acids in food protein can be
classified as
l
l
l
Essential (indispensable) - body synthesis
inadequate to meet needs
Non-essential (dispensable) - can be
synthesised by the body
Conditionally essential (indispensable) become essential under certain conditions
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ESSENTIAL AMINO ACIDS
Histidine***
Methionine* (and Cysteine)
Isoleucine
Phenylalanine** (and Tyrosine)
Leucine
Threonine
Lysine
Tryptophan
Valine
* necessary for synthesis of cysteine
** necessary for synthesis of tyrosine
*** necessary only for infants
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COMPLETE AND
INCOMPLETE PROTEINS

Complete (balanced) protein – single
food protein containing all 9 essential
amino
acids in concentrations sufficient to
meet effectively the requirements of
humans

Incomplete (unbalanced) protein – single
food protein deficient in 1 or more of 9
essential amino acids
6
COMPLEMENTARY
PROTEINS1


People can meet their minimum daily
requirements for protein and essential amino
acids by –

Consuming sufficient quantities of complete protein(s)

Consuming a sufficient amount of a variety of
incomplete proteins

Combining complete proteins with incomplete proteins2
Complementary proteins can be consumed over
the course of a day
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PROTEIN COMPOSITION
l
l
l
Proteins consist of 20 -amino acids
Amino acids consist of carbon,
hydrogen, oxygen, nitrogen, and
partly sulphur
Amino acids differ in their side
chains
8
BASIC STRUCTURE OF AN
AMINO ACID
H
R
C
COOH
NH2
R = sidechains of different composition
9
PROPERTIES OF AMINO
ACIDS
l
l
Characteristic side chain (R) influences
physiological and physico-chemical properties
of amino acids and also those of proteins
Division into five groups in relation to the
different side chains:
–
–
–
–
–
non-polar aliphatic side chains
polar, not charged (hydrophilic) side chains
positively charged side chains
negatively charged side chains
aromatic side chains
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AMINO ACIDS IN PROTEINS
l
1: Non polar aliphatic side chains
–
–
–
–
–
–
l
alanine (Ala)
glycine (Gly)
isoleucine (Ile)
leucine (Leu)
methionine (Met)
valine (Val)
l
–
–
–
l
2: Polar, not charged (hydrophilic)
side chains
–
–
–
–
–
–
asparagine (Asp-NH2)
cysteine (Cys)
glutamine (Glu-NH2)
proline (Pro)
serine (Ser)
threonine (Thr)
3: Positively charged side
chains
4: Negatively charged side
chains
–
–
l
arginine (Arg)
histidine (His)
lysine (Lys)
aspartic acid (Asp)
glutamic acid (Glu)
5: Aromatic side chains
–
–
–
phenylalanine (Phe)
tryptophan (Trp)
tyrosine (Tyr)
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AMINO ACIDS IN PROTEINS
12
LINKING AMINO ACIDS INTO PEPTIDES
and PROTEINS
l
O
H
C
CR2
H
R1C
+NH
3
l
N
H
COOl
Amino acids are
linked by amide
bonds (also peptide
bonds)
Up to 10 amino acids:
peptides
More than 10 amino
acids: proteins
R1 and R2 are side chains of amino acids
13
5.3 Food Proteins
A. The Structure of Proteins
14
PROTEIN TYPES
l
Proteins can be divided into two groups:
–
Homoproteins, containing amino acids only
–
Heteroproteins, containing an extra nonprotein part or prosthetic group
l nucleo-, lipo-, glyco-, phospho-, hemo-,
flavo-, metalo-proteins
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PROTEIN STRUCTURE
l
Proteins are characterised by their amino
acid sequence and their conformation or
three-dimensional structure:
l
l
l
Primary structure: the amino acid sequence
Secondary and tertiary structure: the three
dimensional arrangement of the polypeptide
chain
Quarternary structure: the arrangement of
several polypeptide chains together
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Primary
Structure
Secondary
Structure
Tertiary
Structure
Quaternary Structure
Proteins are
characterised by their
amino acid sequence and
their conformation or
three-dimensional
structure:
Primary structure: the amino acid
sequence
Secondary and tertiary structure:
the three dimensional arrangement of the
polypeptide chain
Quarternary structure:
the arrangement of several polypeptide
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MAIN PROTEIN CLASSES
l
Fibrous proteins
l
l
l
l
Mainly structural tasks
Consist of simple and repeating secondary
structures (-helix and -sheet structure)
For example: keratin, collagen (-helix), silk fibrin
(-sheet structure)
Globular proteins
–
–
Biologically active proteins
Complex tertiary structure often with several types
of secondary interactions within the same
polypeptide chain
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5.3 Food Proteins
B. The Chemical Reactions and Functional Properties of
Proteins
1) Buffering
2) Denaturation
3) Emulsification
4) Enzymes
5) Fat Reduction
6) Foaming
7) Gelation
8) Hydrolysis
9) Solubility
10) Water-Holding Capacity
23
5.3 Food Proteins
1)Buffering
• a process where pH change are
prevented through ionization
• Amino acids may form ions in aqueous
solution
• at certain pH, excess charge in
protein, +ve or –ve helps
• to be polar
• soluble in water
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AMINO ACID PROPERTIES
l
l
Some amino acid properties follow from the
uneven charge distribution
In aqueous solution, amino acids appear as
amphoteric moleculesbehave as both acid
& base
R CH
COO¯
+NH
3
25
5.3 Food Proteins
2) Denaturation
•unfolding of protein structure
•H bonds breaking
•without disrupting protein covalent
bond
26
Process of coagulation
heat
whipping
acid
salts
alkalies
Denaturation of protein
and loss of solubility
sol
Aggregation of protein to form a
crossed linked matrix
Coagulation
Change from fluid (sol) to solid
or semisolid (gel) state
gel
27
PROTEIN DENATURATION
l
Physical:
– Heating
– Cooling
– Mechanical treatment
– Hydrostatic pressure
– Radiation
l
Chemical:
–
Acids
–
Bases
–
Metals
–
Organic
solvents
28
Coagulation: Process of denaturation
Denaturation of protein when heated up
1. Protein structure will open up
2. Refold in new arrangement
3. New permanent bonds formed
Heat
Heat
29
Critical thinking

Between denaturation & coagulation :
-which bonds are broken : H, covalent, peptide ?
-reversible
-compare similarities & differences
30
PROTEIN DENATURATION EFFECTS
l
Change in solubility by exposure of hydrophilic
or hydrophobic peptide units
l
Change in water-binding capacity
l
Loss of biological activity
l
Higher risk of chemical attack because of
exposure of other peptide bonds
l
Change in the viscosity of solutions
l
Loss of crystallisation properties
31
5.3 Food Proteins
3) Emulsification
- oil water interface are stabilized by
hydrophilic & hydrophobic groups in
proteins
Oil
Interface
Water
32
Protein Stabilized Emulsions

In order to form and stabilize an emulsion,
a protein must:
 Diffuse
to the interface
 Unfold
 Expose
hydrophobic groups
 Interact with lipid
33
3) Emulsification
34
4)Enzymes
-protein
molecules
which speed
up
chemical
reactions
without being
used up in the
process
-pH,temprature
moisture
35
5.3 Food Proteins

5) Fat Reduction
 microparticulated protein Simpless
 reduced calorie 1-2 kcal/gm
 whey protein, milk, egg protein






bakery
creamers
dairy products
salad dressing
sauces
soups
36
5.3 Food Proteins
6) Foaming
Foaming Characteristics
 Similar in some respects to
emulsification.
 Denaturation of protein at
an air-liquid interface.
 Hydrophobic groups
interact the air, hydrophilic
amino acids remain in the
water.
37
5.3 Food Proteins
7) Gelation
 A gel
is a continuous
network of macroscopic
dimensions immersed in a
liquid medium exhibiting no
steady-state flow.
Stages in heat induced
gelation
 Protein unfolding
 Water binding
 Protein-protein interactions
 Water immobilization
38
5.3 Food Proteins
8) Hydrolysis
-enzymatic & non enzymatic hydrolysis
-amino acids-breakdown products
9) Solubility
-affected by pH, temperature
10) Water-Holding Capacity
-ability of proein molecule to
bind water
39
5.3 Food Proteins
10) Water-Holding Capacity
•WHC determined by pH, salt, temperature
•Eg :- meat
•ve charge = +ve charge on a protein, protein-protein P-P
interactions maximum
•adding salt-reduce P-P interactions
•more water-protein interactions
•Na + and Cl- ions bind with charged groups on protein fiber
molecules
•Temp 80 C, water binding in proteins form thermally
induced gels
•3-dim gel network
40
FUNCTIONAL PROPERTIES
OF FOOD PROTEINS
l
Hydration
l
Dough formation
l
Solubility
l
Emulsification
l
Viscosity
l
Foaming
l
Gel formation
l
Aroma binding
l
Texture
l
Interaction with
other food
components
41