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Week 5: Proteins - Coggle Diagram
Week 5: Proteins
Part II - Properties of Proteins
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Protein Hydration: Water holding capacity refers to the ability of protein to imbibe water and retain it against gravitational force within a protein matrix :check:
Increasing it
May increase with denaturation
Increased by large amount of charged amino acid groups
Increase in net charge, and repulsive forces, protein swell
Uses & Benefits
Dough formation
Increase water absorption of meat emulsion
Decrease cooking losses of sausage fillings
Retarding syneresis : Extraction or expulsion of liquid from a gel
E.g. whey protein and gelatine are commonly used to bind water and to provide texture
Ability to entrap water (important for gelation)
Solubility :check:
Most proteins are soluble in water, in alcohol, in dilute base or in salt solutions
Albumins are soluble in distilled water
Globulins are soluble in dilute solutions of neutral pH
Prolamins are solubles in alcoholic solutions
Protein solubility will affect thickening, foaming, emulsifying and gelling properties
Solubility of protein is the equilibrium between protein-protein and protein-solvent interactions
Hydrophobic interactions promote protein-protein interactions and decrease solubility
Ionic interactions promote protein-water interactions and increase solubility
Denaturation of protein can both increase or decrease solubility of proteins
e.g. very high or very low temperature on the other hand will lead to loss in solubility since exposed hydrophobic groups of denatured protein lead to aggregation (may be desirable or undesirable in food products)
Emulsion: Proteins can be excellent emulsifiers because they contain both hydrophobic and hydrophilic groups
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To form a good emulsion, protein has to be able to
Rapidly adsorb the oil-water interface
Rapidly and readily open up and orient its hydrophobic groups towards the oil phase and its hydrophilic groups to the water phase
Form a stable film around the oil droplet
Foaming: Similar to emulsion where air is the hydrophobic phase instead of oil
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typically formed by
Injecting gas/ air into a solution through small orifices
Mechanically agitate a protein solution (whipping)
Gas release in food e.g. leavening breads
Flavour Binding
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Natural ability of proteins to bind flavour compounds has both desirable and undesirable implications
E.g. grassy and beany off-flavours of soy protein concentrates and isolates are mainly due to bound hexanal
Can also be used as flavour carriers or flavour modifiers in foods
Proteins do not bind all flavourants with equal affinity
Protein can modify flavour profile of a food product by selectively binding some flavourants more tightly than others
Protein-bound flavourants do not contribute to taste and aroma unless they are released readily in the mouth during mastication
Viscosity: the resistance to flow under an applied force ( or shear stress)
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Factors that cause proteins to unfold can increase their viscosity
Protein solutions are generally pseudoplastic
Viscosity decreases with increasing shear rate (protein molecules interact and align at rest results in high viscosity; disruption between molecules will improve flow)
Proteins re-associate on standing (thixotropic)
Applications
Whey protein concentrates and isolates as viscosity stabilisers in sauces
Gelatine – stabilisation of ice foam
Casein an whey proteins in yogurt
Gelation: transformation of a protein from the “sol” state to a “gel-like” state
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gel is an intermediate phase between solid and liquid
Made up of polymers cross-linked via either covalent and non-covalent bonds to form a network that is capable of entrapping water and other low molecular weight substances
Can be formed by
Addition of salts
Action of enzymes
Changes in pH
Application of heat
Two types of gels
Translucent
Coagulum (opaque) : Contain large amount of nonpolar amino acids residues
Undergo hydrophobic aggregation upon denaturation
Rate of aggregation and network formation is faster than the rate of denaturation, protein readily set into a gel network even while being heated
Opaqueness of these gels is due to light scattering caused by unordered network of insoluble protein aggregates
E.g. egg white
E.g. whey protein isolate
Two stage process
limited aggregation
Formation of a protein network occur
Limited entrapment of water or unfolded proteins
When progel is cooled, decrease in kinetic energy, stable noncovalent bonds are form among exposed functional groups, results in gelation.
Initial denaturation (unfolding)
This stage causes unfolding of protein and exposure of a number of critical functional groups such as H bonds and hydrophilic groups
Protein in sol state is first transformation into a progel stage (a viscous liquid state) by denaturation
Protein Denaturation
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Principles: Often irreversible
Involves a loss of ordered structure
Changes from globular to extended rod or random coil conformation
Destabilization of H-bonds, electrostatic interactions, Van der waals interactions
Any changes in environment such as pH, ionic strength, temperature, solvent compositions etc. will force the molecules to assume a new equilibrium structure
Results
Changes in functionality of food
Can lead to insolubilisation and/ or coagulation of proteins
Loss of physiological function
Factors Causing denaturation
Heat: primarily destablization of major non-covalent interactions
H-bonds, electrostatic and Van der waals interactions destabilised at high temp, stable at low temp
Highly hydrophobic proteins often more heat stable than highly hydrophilic ones
Highly hydrophilic proteins more cold stable than highly hydrophobic ones
Dry protein has limited mobility, therefore high denaturation temperature. As water content increases, molecular mobility and capacity to change shape in response to heat increases
Extreme cold
Agitation: occurs because of the incorporation of air bubbles and adsorption of protein molecules to the air-liquid interface
Extent of conformational change depends on the flexibility of proteins
The non-polar residues of denatured protein orient toward the gas phase and the polar residues orient toward the aqueous phase
Highly flexible proteins denature more readily at air-liquid interface than do rigid proteins
Food processing operations that involves high pressure, shear, and high temperature for example, extrusion, high speed blending, and homogenisation
The combination of high temperature and high shear force causes irreversible denaturation of protein
The greater the shear, the greater the degree of denaturation
High salt concentration
High sugar concentration
Dehydration
Changes in pH: Proteins are more stable against denaturation at their isoelectric point
At neutral pH, most proteins are negatively charged and a few are positively charged
At extreme pH values, strong intermolecular electrostatic repulsion caused by high net charge results in swelling and unfolding of the protein molecules
pH-induced denaturation is mostly reversible
Irreversible denaturation of proteins can occur when
Partial hydrolysis of peptide bonds
Deamidation of Asn (Asparagine) and Gln (Glutamine); or
Destruction of sulfhydryl groups at alkaline pH occurs
Oxidation/ reduction
High pressure
Part 1 – Protein Structure
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Amino Acid composition: characterised by the –CH(NH2)COOH
Classification of AAs :check:
All proteins are formed from 20 different amino acids
Amino acids can be classified based on their chemical nature of their side chains or R group of amino acid
Classification based on polarity (interaction with water) would be more meaningful
Aliphatic R groups: non-polar and hydrophobic
Hydrophobicity increases with increasing number of C atoms.
Polar, uncharged R groups
Both basic and acidic amino acids are also strongly hydrophilic
Positively charged R groups: have side chains that often make salt bridges
basic in nature
Aromatic R groups
Negatively charged R groups: form aspartic acid and glutamic acid. Acidic nature
Structure of AAs :check:
The carbon atom with the amino group is attached to four different groups (except glycine)
Act as ampholytes: behave both as acids and bases
(Principle) At around neutral pH, both the -carboxyl groups are ionized and the molecule is a dipolar ion or a zwitterion
The pH at which the dipolar ion is electrically neutral is called the isoelectric point (pI)
When the zwitterion is titrated with an acid, the COO- group becomes protonated
The pH at which the concentrations of COO- and COOH are equal is known as pKa1
(due to) a carboxyl group (acidic) and an amino group (basic)
Peptides bonds: Peptides are formed by joining amino acids together via amide bonds
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Oligopeptides are small peptides containing less than a couple of dozen amino acids
Longer peptides having long acids of amino acids are called polypeptides
Reactions
Amide bonds are formed from condensation of a carboxylic acid and an amine group with subsequent removal of water
Reverse reaction (i.e. the breakdown of peptide bonds into the component amino acids) is achieved by hydrolysis
Classification of Proteins:
on the basis of their composition and solubility :check:
Simple proteins
Globulins – water- insoluble proteins, soluble in dilute salt solutions e.g. egg ovoglobulin, legumin (peas)
Glutelins – insoluble in water and dilute salt solutions, but soluble in dilute acids and alkalis e.g. glutenin (wheat) and oryzenin (rice)
Prolamines – soluble in 70% alcohol but insoluble in water and absolute alcohol e.g. gliadin (wheat), zein (corn)
Scleroproteins – insoluble in all solvents. They have structural and protective functions e.g. keratin, collagen, elastin
Albumins – water soluble proteins which coagulate on heating e.g. egg albumin
Conjugated Proteins are simple proteins combined with non-protein material in the body. on hydrolysis yield a non protein substance
Glycoproteins – contain carbohydrates
Flavoproteins – proteins with riboflavin
Phosphoproteins – proteins with a phosphorus radical e.g. caesin,
Lipoproteins – proteins with lipids e.g. lipoproteins of egg, yolk, and milk
Based on Physical Configuration :check:
Secondary
Polypeptide chain is folded into either - helix or β- sheet Held in place by H bonds between AA residues
Structure determined by patterns of H-bonds
β-sheet
4 properties
Can be parallel or anti parallel
Antiparallel
Hydrogen bonds are straight in the antiparallel conformation and therefore results in a straighter structure which is the most stable
Parallel
Stabilised by inter chain H bonds
Peptides chains stretched to maximum resistance
More hydrophobic than -helix
-helix
Major form in proteins and is most stable
3 properties
Stabilised by nearly-straight intra-chain H bonds between an NH group in the polypeptide chain and a C=O group
R groups of the amino acids all extend to the outside
Amphiphilic in nature: One side of helical structure is hydrophobic, the other side is hydrophilic residues
Very compact and allow no room for water or other small molecules on the inside
Tertiary Structure : when a linear protein chain with secondary structure segments folds further into a compact 3D structure
Aim is to attain the most thermodynamically favourable conformation (i.e. lowest free energy state)
Several non-covalent interactions such as salt linkages, hydrogen bonds, dipole-dipole interactions stabilising the tertiary structure
Hydrogen bonding involving groups from both the peptide backbone and the side chains are important in stabilising tertiary structure
Can also be stabilised by disulphide bonds between cysteine residues
Major driving force in determining the tertiary structure of globular proteins is the hydrophobic effect
3D structures are disturbed in nature due to the following reasons:
Presence of proline in the protein chain does not allow for normal hydrogen bonding because of the absence of an amine group
Formation of disulphide bridges between cysteine molecules can join various parts o the polypeptide chains, disturbing the regular helix formation
Sides chains have different configurations
Primary : Linear sequence where Amino acids are covalently linked through amide bonds also known as peptide bonds
The terminus with the free -amino group is known as the N-terminal, and that with free -COOH group is known as the C-terminal
Quaternary Structure: Association of more than one polypeptide chain in well defined structures
Often involved other classes of smaller molecules (e.g. hemoglobins)
Stabilised mainly by non-covalent interactions (H-bond, van der waals interactions and ionic bonding)
Formation of quaternary structure is primarily driven by thermodynamic requirements to bury exposed hydrophobic surfaces of subunits
Stabilisation of Proteins :check:
Slow but spontaneous transformation of an unfolded state to a folded state is facilitated by several intramolecular noncovalent interactions
Forces that contribute to protein folding may be grouped into two categories:
Intramolecular interactions affected by surrounding solvent e.g. hydrogen bonding, electrostatic, hydrophobic interactions
Hydrogen bonds : hydrogen atom that is covalently attached to an electronegative atom (such as N, O, or S) with another electronegative atom
H-bonded groups carry a partial charge
Increase No of H bonds
Hydrogen bonds increase at low temperature and low polarity solvents
Presence of high concentrations of Asn and Glu in cereal proteins allow high degrees of H-bonding
3 scenarios
Greatest number of hydrogen bonds are formed between the N-H and C=O groups of the peptide bonds in -helix and β-sheet structures
Surface-placed H-bonds are normally disrupted by water which is a strong H-bonding agent
Electrostatic Interactions
Proteins contain several amino acids residues with ionisable groups
The pH at which the net charge is zero is called the isoelectric pH (pI)
Depending on the number of negatively and positively charged residues, proteins assume either a net negative or net positive charge at neutral pH
This interaction do not act as the primary force to determine protein folding
Surface charges on the protein do not contribute significantly to protein stability
Buried interior charged groups usually form salt bridges with strong interaction energy
Protein’s affinity to remain expose to aqueous environment would influence the folding pattern
Hydrophobic interaction
Major driving force for protein folding
Proteins fold in order to remove non-polar amino acid chains from contact with water
Non-polar amino acid chains are squeezed from contract with water thereby organising a protein’s shape
Hydrophobic interactions are endothermic: Interactions are stronger at higher temperatures
Intramolecular interactions emanating forces intrinsic to the protein molecule e.g. van der waals and steric interactions
Van der waals interactions: Dipole- induced dipole and induced dipole- induced dipole interactions between neutral atoms in protein molecules
Disulfide Bonding
Interchange reaction can occur within protein mixtures. This can reduce the stability of protein molecules
Only covalent side chain cross-links found naturally in proteins
Conformation and Adaptability of protein :check:
Proteins are not rigid molecules
Highly flexible
Native state is metastable state
Breakage of one to three hydrogen bonds or a few hydrophobic interactions can cause a conformational change
Conformational adaptability is necessary to enable proteins to carry out critical biological functions
Proteins that require structural stability are stabilised by intramolecular disulfide bonds
Proteins III – Food Proteins
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Animal Proteins
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Meat Proteins
3 types :check:
Stroma is insoluble :check:
3 examples
Collagen: Main component of intercellular tissue of muscle
Each collagen is composed of three polypeptide chains designated -chains which form a triple helical structure stabilised by H-bonds
Crosslinks formed between -chains in the tropocollagen molecules and between the tropocollagen monomers that make up the collagen fibrils
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Each collagen molecules contains about 33% glycine, 12% proline, and 11% hydroxyproline
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elastin
reticulin
Sarcoplasmic :check:
Relatively low molecular weight, high isoelectric pH, have globular or rod-shaped structures
Partially responsible for high solubility of these proteins in water or dilute salt solutions
Responsible for meat colour and meat quality
Two essential parts – a heme group and a protein moiety called globin
Most water soluble
Myofibrillar is salt-soluble :check:
3 subgroups
Regulatory proteins includes tropomyosin, troponin complex and several other minor proteins
Cytoskeletal or scaffold proteins
Major contractile proteins including
2 mechanisms
Muscle contraction: Produces Actomyosin – protein complex
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Muscle Relaxation: With small amount of overlap between thick and thin filaments
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2 examples
actin (Thin filaments, 2nd most abundant)
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myosin (Thick filaments)
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Functions
Creates thermally induced cohesive structure and the firm texture of meat products
produce 3D viscoelastic gel matrices via protein-protein interactions
bind water
Form cohesive and strong membranes on the surface of fat globules in emulsions/ flexible films around the air/ water surfaces
Contribute to product consistency as in the case of meat based soups and gravies
Structural proteins that make up the myofibrils
Can also form structural components, stabilizing restructured meat products
Physiochemical changes in Muscle Proteins during Meat Processing
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Muscle proteins has ability to form viscoelastic network to bind water, to entrap flavours, to emulsify fat and oils, to form deformable yet stable foams.
External factors that can bring about protein changes
Heat (thermal energy)
Mechanical (shear)
Physical and chemical environment – pH ionic strength, redox potential
Functionality :check:
Water Retention : Ability of meat and meat proteins to retain moisture before, during and after processing or cooking. Plays a crucial role in palatability and consumer acceptance of product (water holding capacity)
Bound water is tightly associated with proteins through charged groups and dipolar sites on protein surface
Amount of meat is primarily influenced by amino acid composition of proteins
Myofibrillar proteins are responsible for much of water-holding capacity of meat because of both their structural organisation
Formation of cross-bridges between myosin and actin in rigor state is believed to constrain myofibril swelling and their water holding ability
Viscosity
Proteins are charged polymers capable of binding water and causing fibre swelling by the uptake of water and loosening of polypeptide matrix
influence texture and stability as well as handling of meat batters
Gelation: gel is a form of matter intermediate between a solid and a liquid, consisting of strands or chains cross-linked to create a continuous network immersed in a liquid medium
Solubility: function of protein structure, the structure of myofibrils, pH, concentration (ionic strength) of salt added to meat, temperature, time of mixing meat with salt
Foaming: Desirable and important functionality of proteins in many prepared foods when protein play a crucial role in air entrapment
Emulsification
Types of emulsion – macroemulsion, miniemulsion, microemulsion
Meat batters are stabilised through two mechanisms
1st mechanism – physical entrapment of fat globules within the protein matrix formed largely by protein-protein interaction
2nd mechanism – fat globules are stablilised by an interfacial protein film (membrane) that surrounds them
Egg Proteins
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Egg white contains at least eight different proteins
Lysozymes – antibiotic
Ovomucoid – binds iron, trypsin inhibitor
Ovalbumin (aka Monomeric phosphoglycoprotein): Predominant
Complete amino acid has 385 amino acid residues, with 4 cysteine residues
Conalbumin – renders iron unavailable to microorganisms
Ovomucin – inhibits hemagglutination
Avidin – bind biotin
3 Main functions in Preparation undergo denaturation, coagulation and gelation
Uses of Isolated egg white and yolk
enrich nutritional quality
prove functional properties such as gel strength and water-holding capacity of foods
Milk Protein
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Casein: are highly digestible in the intestine
Casein micelles: exhibit unique interactions with calcium ions and calcium salts
Structure of casein micelles are also affected by heat
calcium phosphate hold micelles together, dissolving calcium phosphate due to pH changes in solution will dissociate the micelles
Whey: less digestible in the intestine
Plant Proteins
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Rice Proteins : High content of lysine :check:
in the form of encapsulated protein bodies
Protein bodies are insoluble and remain intact during cooking
Large protein bodies contain prolamin and glutelin
Small protein bodies contain primarily glutelin
Maize Proteins : Main proteins of maize are the storage proteins of the endosperm
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High molecular weight – glutelins: several subunits joined together by disulphide bonds
contain higher lysine, arginine, histidine, tryptophan and lower glutamic acid content than zeins
Low molecular weight - zein : asymmetric molecules
– 45% -helix and 15% β-sheet, the rest are aperiodic (irregular)
Soybean Proteins
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Good source of all essential amino acids except methionine and tryptophan
contain neither gliadin nor glutenin, does not improve loaf volume in bread
have relatively high solubility in water or dilute salt solution at pH values below or above their iso-electric point
(affected by) Application of heat to soybeans make the proteins progressively more insoluble
(why) Hydrogen bonds and hydrophobic bonds play a role in decreasing the solubility of proteins during heating
Wheat proteins :check:
(Salt soluble) globulins
(Alcohol soluble) gliadins: has intra-molecular disulphide linkage
(Water soluble) albumins
(Acid soluble) glutenins: both inter- and intra- molecular disulphide linkages
Formation of S-S bond: greatly influence solubility and rheological properties such as extensibility and elasticity
play an important role in cross-linking polypeptide chains
Functional Properties & Uses :check:
