Protein Functionality

1
Protein Functionality

• NTRS 525
• H. Singh, Ph.D.

2
Protein Functions

• Biological?
• Enzymes
• Structural
• Hormonal
• Transport

• Food?
• Wheat gluten
• Corn
• Egg
• Whey
• Milk
• Other cereals
• Soy

3
Protein Structure
20 Amino Acids
Coded in DNA
Primary
Secondary
Self assembly to a single (native) structure.
Depends on primary structure and solution
conditions
Tertiary
( )
Quaternary
Common in foods. Many non-native forms depending
on protein structure, solution conditions (
history) and ingredient interactions
Denatured
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Amino Acids

• The monomer unit of proteins

• R is the side chain.
• One of 20 different chemical compounds
• Some R-groups are acid (other alkali)
• Some R-groups are water soluble (others are not)

Chiral carbon (L-series)
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Amino Acids

• Polar
• Uncharged. Ser, Thr, Asn, Gln, Cys
• Positive (basic). Arg, Lys, His
• Negative (acidic). Asp, Glu,

• Non-Polar
• Aliphatic. Ala, Ile, Leu, Met, Pro, Val
• Aromatic. Phe, Trp, Tyr

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Example Amino Acids
Alanine
Phenylalanine
Glutamic acid
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Peptide Bonds
Amino acids
Water
8
Disulfide Bonds

• Two cysteine molecules under oxidizing conditions
• Intermolecular or intramolecular cross-link

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a-Helix

• N-H to CO hydrogen bonds in 4th succeeding A.A.
• Hydrogen bonds parallel to axis
• Typically amphiphilic

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b-Sheet

• CO and N-H perpendicular to chain form
  inter-segment H-bonds
• Parallel or antiparallel
• b-strands typically 5-15 A.A.
• More stable than a-helix

b-sheet
11
Protein Folding
Hydrophobic amino acids
Peptide chain
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Tertiary Structure
13
Types of Tertiary Structure
Globular
Disordered
Fibrous
Many insoluble amino acids, protein tends to
minimize surface/volume ratio
Interacts well with water and takes up a random
configuration
Strong secondary structure allows protein to
retain a non-spherical shape
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Quaternary Structure
Folded protein unable to contain some hydrophobic
residues
Dimerized protein shields the hydrophobic amino
acids from water
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Denaturation

• Denaturation
• Balance of forces
• Consequences of denaturation

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Effect of Temperature on Rate of Enzyme Action
rate
denaturant
17
Denaturation

• Denaturation is a phenomenon that involves
  transformation of a well-defined, folded
  structure of a protein, formed under
  physiological conditions, to an unfolded state
  under non-physiological conditions.
• Occurs suddenly and completely over a narrow
  range of conditions
• Slowly reversible (if at all)

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Thermal Denaturation

• Trypsinogen 55C
• Pepsinogen 60C
• Lysozyme 72C
• Myoglobin 79C
• Soy Glycinin 92C
• Oat globulin 108C

Affected by pH, water, solutes
Table 11
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Why is Denaturation Sudden?
COOPERATIVE PROCESS Partly denatured structure
is weaker so begins to change faster
100
Native Structure
0
Critical value
Concentration of denaturant or temperature
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Types of Denaturation

• Temperature
• Organic solvents
• Surface
• pH
• Shear

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Reversibility?
One native form
Refolding is a complex process particularly for
large proteins or complex proteins
Many denatured forms
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Energy Surface
Many secondary minima amongst denatured states
Free energy
One native state (true energy minimum)
Changes in Conformation
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Denaturation

• The conversion of a biologically functional
  molecule into a non-functional form
• There are many denatured states but one native
  state
• Proteins can regenerate to their native state but
  slowly
• Denatured proteins have a greater tendency to
  aggregate.

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Behavior of Denatured Protein
Hydrophobic core Hydrophilic surface
DENATURED
Fast under non-physiological conditions
Slow under physiological conditions
NATIVE
Unfolding forces some hydrophobic AA to surface
AGGREGATED or other ingredient interactions
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Consequences of Denaturation

• Loss of enzymatic activity (death)
• Destruction of toxins
• Improved digestibility
• Loss of solubility
• Changes in texture

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Physical properties

• Hydrodynamic-Aggregation
• Viscosity, Elasticity, Viscoelasticity
• Solubility, Water holding capacity
• Hydrophobic- Surface Active
• Emulsion and foam stabilization
• Flavor binding

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Hydrodynamic Functionality
Dilute
Semi-dilute
rg
Viscosity
Concentrated
Concentration
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Viscosity

• A property of LIQUIDS
• Viscosity is the resistance to flow. The amount
  of energy you need to expend to get a given flow
  rate.
• Stress (force per unit area) is proportional to
  rate of strain (i.e., flow rate)
• Particles of any type in a fluid will increase
  its viscosity
• Large, well hydrated polymers contribute most to
  viscosity

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Elasticity

• A property of SOLIDS
• Elasticity is the force to achieve a given
  percentage change in length
• Stress (force per unit area) is proportional to
  strain (fractional deformation)
• An elastic material must have some solid-like
  network throughout the structure
• The more load bearing structures the more elastic
• The more inter-structure links the more elastic

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Viscoelasticity

• Many materials simultaneously show solid and
  liquid like properties
• If they are stretched they will partly and slowly
  return to their original shape
• Elastic solids would completely recover
• Viscous liquids would retain their shape

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Creep
Dough stretched
Dough released
Length
Time
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Mechanisms of Link formation

• Hydrophobic (following denaturation or
  hydrolysis)
• Thiol-disulfide interchange
• Enzymic crosslinking
• Also consider repulsive effects (charge)

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Water Binding

• Gel contains pores
• Water can flow out of the pores
• If the gel contracts it may expel liquid
  ?SYNERESIS
• Due to closer association of protein with protein

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Protein hydration

• Water binding capacity
• g H2O/g dry protein at 90-95 ERH
• Water holding capacity
• imbibe remain H2O against gravitation force
  within a protein matrix
• Dispersibility

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Protein solubility

• Functionalities of proteins are affected by
  solubility
• pH effects on solubility
• Ionic strength effects on solubility
• Temperature effects
• Effects of organic solvents

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Solubility
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Hydrophobic/Surface Functionality

• Emulsion stabilization
• Foam destabilization
• Flavor binding

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Whey vs. Casein

• Dense, ordered globular proteins
• 2D Gel

• Loose, disordered, flexible chains
• Loop-train-tail model

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Emulsion Stabilization

• Proteins adsorbed to surfaces can stabilize
  emulsions and foams by various mechanisms
• Proteins are often denatured on binding

electrostatic
steric
-
-
-
-
-
-
-
-
-
-
-
-
-
-
(a magnification of the lamella between two
touching bubbles or two emulsion droplets)
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Emulsification

• Emulsions dispersions of one liquid in another
• Properties of emulsions
• - type o/w w/o
• - droplet size
• - composition thickness of the continuous
  phase the surface layer around the droplets

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Protein in emulsion

• Emulsifiers of o/w food emulsions
• Preventing coalescence
• Increase viscosity of continuous phase

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Protein effects on foaming

• Protein stabilize foams
• Desired properties for foaming
• rapidly adsorb to an interface
• rapidly unfold reorient at an interface
• form viscoelastic film at an interface
• Structural factors flexibility amphiphilic
  molecule

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Gelling property

• Gel soft solid with network entrapped small
  molecules
• Protein gelation

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Protein gels

• Types coagulum-type translucent
• Protein gels highly hydrated system
• Stability types numbers of cross-links per
  units
• LCE least concentration endpoint
• Effects of pH
• Effects of salt
• Effects of other additives

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Flavor binding

• Proteins odorless
• Binding off-flavor components
• Carriers for desired flavors
• Structural dependence

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Dough formation

• S-S formation
• Hydroxyl groups
• Hydrophobic interactions

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Nutritional value of proteins

• Nutritional value of protein
• Content of essential AA digestibility
• Essential AA

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Protein Digestibility

• Definition
• proportion of food nitrogen that is absorbed
  after ingestion
• Protein from animal source better digestibility
• Factors affect protein digestibility
• 1. protein conformation
• 2. antinutritional factors
• 3. binding with other food components
• 4. processing

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Loss of nutritional value

• Maillard reactions
• Oxidation of Met, Cys, Trp, and Tyr
• Free radical oxidation
• Effects of polyphenolics

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Chemistry of enzymes

• Enzymes proteins with catalytic activity due to
  their power of specific activation and conversion
  of substrates to products
• Enzyme catalysis
• ? rates of reactions (by 103 to 1011)
• high selectivity or specificity

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Food Enzymes

• Biological catalysts
• Proteins (can be denatured)
• Specific (can be inhibited)

intermediate
Enzyme stabilized intermediate
DG
Substrate
Product
reaction
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Polyphenol oxidase

• Founds in plants, animals and some
  microorganisms, requires Cu2, O2

diphenol
phenol
MELANINS
diphenol
catechol
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Lipoxygenase

• Catalyzes peroxidation of fatty acids
• Abstracts an H., allows reaction with O2, free
  radicals produced
• Leads to rancidity and flavor
• Co-oxidations
• Bleaching flour
• Thiols
• Vitamins

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Amylase
a-amylase (endo-splitting)
glucoamylase
Starch Dextrins Glucose
Highly viscous suspension of polymerized corn
starch
Glucose isomerase
Fructose
105C