Module 5 Proteins

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Module 5Proteins

• Food Chemistry 2
• ND Food Technology

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Table of Contents

• Introduction
• Classification
• Amino acids
• Protein structure
• Protein denaturation
• Chemical changes
• Proteins in food
• Proteins, amino acids flavour

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1. Introduction

• What are proteins?
• Linear, complex polymers of about 20 different
  amino acids (monomers) joined together with
  peptide bonds. Proteins participate in virtually
  every biol. process
• Number of amino acids joined give different
  groups
• Dipeptides 2 amino acids joined
• Oligopeptides 2-10 amino acids joined
• Polypeptides 10 amino acids joined
• A variety of side chains when amino acids link ?
  different proteins have different chem.
  properties, secondary tertiary structures
• Biol. functions of proteins
• Are enzymes catalise all reactions in living
  organisms
• Bind other molecules for storage support
  (hemoglobin binds transports O2 CO2 in red
  blood cells
• Structural proteins support cells, thus tissues,
  thus organisms
• Assembles to perform mechanical work (flagell,
  muscle contraction)
• Decode info in cell regulate gene expression
• Are hormones regulate biochem. activities in
  larger cells
• Speciallised functions, e.g. antibodies in immune
  system

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2. Classification

• Based on their solvents or unique structure
• Simple proteins Contain only amino acids, no
  other groups added

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2. Classification
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2. Classification

• Conjugated proteins contains amino acid part,
  combined with non-protein part e.g. lipid,
  carbohydrate, nucleic acid

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2. Classification

• Derived proteins Proteins modified by chemical /
  enzymatic methods. Divided into primary
  secondary derivatives, depending on extent of
  change

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3. Amino acids

• Amino acids are put together by a process
  involving genes, that specifies the exact
  structure of a protein
• All amino acids contain an amino group a
  carboxyl (or acid) group
• NB Know basic structure of amino acid!
• The difference lies in the R-group the R-group
  places the amino acid in a certain chemical
  category
• Catagories
• Negatively charged
• Positively charged
• Polar uncharged
• Hydrophobic

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3. Amino acids

• The amount of essential amino acids in a protein
  determines the protein quality
• High red meat, fish, soybeans, eggs
• expensive
• Intermediate legumes (peas, beans)
• Low wheat, potatoes
• protein malnutrition ? Kwashiorkor
• Fortification
• High protein white bread, Soya beans
• The peptide bond
• The bonds that link amino acids together
• Bond between the amide hydrogen (N-H) carbonyl
  oxygen (CO)
• Can be hydrolysed by enzymes, acids bases
• A covalent bond with little rotation around the
  bond
• Di-, oligo- or polypeptides can form
• Peptides short string of amino acids
  (oligopeptide)
• Peptones group of breakdown products from larger
  proteins

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3. Amino acids

• Most amino acids are stereoisomers (enantiomers)
  
• compounds with the same molecular formula, but
  differ in arrangement (configuration)
• All amino acids (except glycine) are symmetric
  (the central (a) carbon is chiral has 4
  different groups attached to it)
• L (levo)-form amino group on left
• D (dextro)-form amino group on right
• (look at an example!)

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4.1 Protein structure

• Proteins are either globular or fibrous
• Protein structure has 4 levels primary,
  secondary, tertiary, quaternary
• Primary structure
• The number sequence of arrangement of amino
  acids, peptide bonds
• Thus also determines type of R-groups (chemical
  character) in protein chain
• Determines interaction between R-groups, thus
  also the other levels of protein structure

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4.2 Protein structure

• Secondary structure
• Folding of the primary structure
• H-bonds between amide nitrogen carbonyl oxygen
  are major stabilising force
• H-bonds are between different areas of
  polypeptide chain / between 2 adjacent chains
• In aqueous media, VdWaals hydrophobic
  interaction between apolar side chains contribute
  to stability of sec structure
• Sec structure either in a-helix or ß-sheet
  structure

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4.2 Protein structure

• a-helix forms compact coils (each residue 0.15
  mm) like DNA
• Stabilised by intramolecular H-bonds between N-H
  CO in chain
• Requirements for stability translation of 0.54
  mm per turn along central axis, 1 complete turn
  for every 3.6 amino acid residues
• Either globular or fibrous
• E.g. collagen (tripple helix)
• ß-Sheet (each residue 0.32-0.34 mm)
• Stabilised by intermolecular H-bonds
• Polypeptide chains almost fully extended form
  H-bonds between N-H CO of adjacent chains
• Fibrous

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4.3 Protein structure

• Tertiary structure
• Folding of polypeptide chain into closely packed,
  almost spherical shape
• Takes place according to 2nd law of
  thermodynamics
• Conforms to structure that is thermodynamically
  most feasible has highest entropy
• Amino acid residues that are far apart in primary
  structure brought together ? interaction along
  side chains
• Stabilised primarily by noncovalent interactions
  between side chains of amino acid residues (also
  has H-, covalent-, ionic- disulphide bonds)
• Globular proteins

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4.3 Protein structure

• Hydrophillic R-groups of globular prot. on outer
  surface (soluble)
• Hydrophobic R-groups of globular prot. on inner
  surface (insoluble)
• Covalent bonds are for rigidity in protein
  structure
• Disulphide bonds between insulin is reduced to
  break prot. chain e.g. in bread baking reduce
  wheat prot. To smaller chains for easier mixing
  with best dough structure
• Enzymes are globular with specific active sites
• Certain internal areas of globular protein are
  highly specialised (specific amino acids)
• Where reactions are catalised
• E.g. serine proteases (proteolytic enzyme) has
  serine in a hydrophobic active site to
  hydrolyse peptide bonds of proteins

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4.4 Protein structure

• Quaternary structure (globular proteins)
• For proteins with multiple subunits (many
  different chains) deals with the organisation
  of subunits
• Each subunit is a polypeptide chain monomer
• Multiple subunits - oligomer
• Subunits of oligomers held together mainly
  noncovalently hydrophobic interactions
• Also some covalent, ionic bonds
• Monomers of oligomers can be identical (simple
  enzymes) or different (multienzyme complexes -
  each monomer has specific function)
• Structure-function relationship any small change
  in structure causes change in function

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4.5 Protein structure

• Synthesized protein coils folds spontaneously,
  because it is exogonic tends to be in most
  relaxed form possible
• Each random coil consists of
• Elements of helix pleated sheets (if amino acid
  sequence is right)
• Bends /kinks at end of a helix section (due to
  proline)
• Disulphide bonds stabilise tertiary quaternary
  structures
• Active sites in enzymes
• Binding sites for metal ions, cofactors,
  coenzymes
• Protein structure is denatured when external
  factors disrupt its natural structure
  conformation

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5. Protein denaturation

• Denaturation process (e.g. during food
  processing) that changes the molecular structure
  without alteration of amino acid sequence
• Leads to loss of biol. activity, changes in
  physical properties (e.g. solubility)
• Affects different proteins in different degrees
  (depending on structure of a protein
• Agents causing denaturation
• Temperature
• Cause change in tertiary structure ? polypeptide
  chain less ordered
• Prot. denatures _at_ 45C ? prot. precipitates /
  loses water binding capacity
• Advantages denature whey prot. for production of
  milk powder, egg white prot. denatured to make
  foam
• Disadvantages Freezing destabilise fish
  (become tough, loose moisture), caseinate in milk
  destabilise coagulate (caseinate is stable
  against high temp.)

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5. Protein denaturation

• pH
• Changes in sec, ter, quaternary structure, due to
  addition of acid / base
• When pH changes, R-groups change their degree of
  ionization ? affects inter interachain bonding
• At any pH, proteins carry specific charge
• At isoelectric point (specific for each protein),
  nr of pos nr of neg charges ? total charge 0
• No charge no repulsion clotting of protein
  e.g. production of dairy products (milk
  isoelectric point _at_ pH 4.5
• Ionic strength (salts)
• Salts in medium use water for its own dissolution
  ? no water for prot. to stay in solution ? prot.
  precipitates
• Concept used in lab to purify prot. (increase
  ammonium sulphate content to 75)
• Enzymes
• Proteolytic enz. (pepsin, rennin, trypsin)
  hydrolyse peptide bond of prot. ? can lead to
  large changes in structure function

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6. Chemical changes

• Desirable undesirable changes during processing
  storage of proteins
• Lead to changes in peptide side chains, make some
  amino acids unavailable, undigestible
• Maillard (nonenz. browning)
• Heat damage or in presence of reducing sugar
• Leads to decomposition of certain amino acids ?
  brown pigments, flavours form
• Heat
• Mild heat in presence of water can improve prot.
  nutritional value
• Sulfur-containing can become more available
• Antinutritional factors (trypsin inhibitor in
  soybeans) deactivated
• Excessive heat ? disadvantage fish prot. (try,
  arg, met, lys) damaged
• Decomposition, dehydration (Ser, Thr), sulfur
  loss (Cys, Met), cyclization (Glu, Asp, Thr)

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6. Chemical changes

• Oxidation
• Amino acids oxidized by reacting with free
  radicals formed during lipid oxidation
• E.g. Met reacts with lipid peroxide ? Methionine
  sulphoxide
• Light-induced oxidation ? destruction of
  essential amino acids in milk, off flavours
• Reaction with polyphenols
• Prot. React with tannins, phenolic acids,
  flavonoids in plant products ? decrease in
  availability, digestibility, biol. value
• Racemization
• The result of heat alkaline treatment of food
  prot.
• Reduced digestibility protein quality

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7. Proteins in food

• Functional properties of food proteins in food
  systems Table, 3.12, 3.13, p.135!
• Most commom food proteins

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7. Proteins in food

• Animal proteins
• Meat proteinsconsist of fibrillar myosin, actin
  (split ATP ? energy for muscle contraction),
  collagen water soluble proteins
• Egg proteins
• Egg white
• Ovalbumin, conalbumin, ovomucoid, lysozyme,
  avidin
• Egg yolk
• Livetin, phosvitin, lipoproteins
• Milk proteins
• Consist of casein (phosphoproteins) serum
  proteins
• Yoghurt production fermentation by Streptococcus
  cremori produce lactic acid ? pH? to isoelectric
  point ? viscosity?
• Cheese production Rennet (stomach enzyme
  containing rennin) added to milk ? milk prot.
  coagulates ? curd (whey is removed to purify whey
  protens)

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7. Proteins in food

• Plant proteins
• Obtained from leaves, cereals, oilseeds, nuts
• Leaf portein easily denatured (_at_ pH 4.5-4.6,
  50C)
• Cereal seed protein low in lysine
• Legume seeds low in cys, met
• Peanut protein low in lys, try, met, thr
• Cereal grain proteins vary depending on species,
  soil, fertilizer, climate
• Wheat proteins
• Have bread-making properties
• Albumin, (water soluble, coagulated by heat),
• globulin (soluble in neutral salt solution),
• gliadin (soluble in ethanol),
• glutelin (soluble in dilute acid/alkali
• Gluten
• provides good crumb structure (holds together
  starch, gas bubbles)
• High in Glu, low in lys, try, met (essential)
• Has low solubility

Forms gluten when flour is mixed with water
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7. Proteins in food

• Maize proteins
• Varies depending on variety, climate, soil,
  fertilizer
• Mostly zein, glutelin, low in albumin, globulin,
  essential amino acids (Lys, Try)
• Rice proteins
• High content of lysine (essential amino acid)
• Main storage protein glutelin
• present in encapsulated protein bodies
• Protein bodies insoluble, intact during cooking
• Proteins in rice bran albumin globulin
• lost in bran polish during milling (polished
  white rice)
• Byproducts valuable food source animal feed

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7. Proteins in food

• Soybean proteins
• Contained in protein bodies
• Good source of essential amino acids (except met,
  thr)
• Contains no gliadin, glutelin (cannot be used in
  bread making without additives to improve loaf
  volume
• High solubility in water / dilute salt solutions
  _at_ pH above / below isoelectric point (classify as
  globular proteins)
• Used in soymilk, tofu, bean curd

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Proteins, amino acids flavour

• 4 Primary tastes localised to specific areas on
  tongue (see sketch)
• Flavour taste odour
• Flavour influenced by
• Texture (smoothness, roughness, granularity,
  viscosity), hottness, spiciness, coolness
  (menthol), fullness of certain amino acids
• Umami
• Japanese for deliciousness
• flavour enhancer by amino acids e.g. glutamic
  acid (MSG)
• Glutamic acid is primary taste because
• Receptor different from receptors for sweet,
  bitter, salty, sour
• Not affect taste of other 4
• Taste quality different from that of the 4
  primary tastes
• Cannot be reproduced by mixing of chemicals or of
  any of the 4 primary tastes