Proteins (1. LF UK, NT)

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Proteins, or polypeptides, are organic macromolecular substances. Their molecular weight exceeds 10,000. They consist of amino acids in numbers greater than 100. A typical protein contains 200-300.

Construction[edit | edit source]

peptide binding

amino acids in the peptide are bound to each other by peptide binding. Peptide binding combines with simple covalent binding amino group of one amino acid and a carboxylic group of the other amino acid. The Gibbs energy value of this reaction is equal to G = 10 kJ/mol.

Polycondensation produces an arbitrarily long chain of amino acids. The end of the chain, which has a free (unreacted) amino group, is called the N-end. On the opposite side of the chain, on the other hand, we find a free carboxyl group. This end is called the C-end.

Structure[edit | edit source]

The structure of proteins is based on the arrangement of amino acids in the chain. Protein structure is very important for their function.

Primary structure

The primary structure is defined by the exact sequence of amino acids in the chain.

Secondary structure
β-folded sheet

Secondary structure means the spatial arrangement of amino acids in a chain and stabilization by hydrogen bonds.

They are two basic secondary structures:
  1. α-helix: The string is curled into a right-hand helix. The length of a single screw thread is equal to 3,6 amino acid residues. The structure of α-helix is found primarily in fibrous proteins (keratins) or muscle proteins.
  2. β-folded sheet: Two parallel and antiparallel chains resembling a folded sheet of paper.
Tertiary structure

The tertiary structure is characterized by other intramolecular binding interactions. For example, disulfide bridges, ionic bonds and van der Waals forces. Other H-bridges may also form in the molecule.

Quarternary structure

The quaternary structure is formed in proteins consisting of two or more polypeptide chains. Their connections provide extramolecular binding interactions with each other. The quaternary structure is found, for example, in hemoglobin. In contrast, myoglobin does not possess quaternary structure.

Protein denaturation[edit | edit source]

Protein denaturing is the process by which secondary and tertiary structures change. The protein thereby loses its biological activity. Denaturation can be achieved, for example, by heating or changing the pH.

Main nutrients[edit | edit source]

  • peptide bonds
  • other bindings
    • disulphide -S-S-
    • ester
    • amide
  • Ingredients other than amino acids (physically or chemically)


Division[edit | edit source]

By origin[edit | edit source]

  • animal (meat, milk, eggs) - 60% of food protein
  • vegetable (cereals, legumes, fruits, vegetables) - 30% of food protein
  • non-traditional food protein (seaweed, micro-organisms)

By function[edit | edit source]

By structure[edit | edit source]

(the presence of a non-protein component)

  1. Simple proteins (they contain only a protein chain - globular, fibrillary proteins)
  • globular, spherical proteins (albumins, globulins)
  • fibrillary (fibrous), scleroproteins, stromatic proteins (collagen, keratins, elastins)
  1. Compound proteins (contain protein chain and non-protein part - prosthetic group - lipoproteins, glycoproteins)

By solubility[edit | edit source]

  • albumins − milk (lactalbumin), egg whites (ovalbumin, conalbumin), wheat (leucosin)
  • globulins − meat (myosin, actin), milk (lactoglobulin), eggs (ovoglobulin)
  • gliadins, or prolamines − wheat (gliadin), barley (hordein), maize (zein)
  • glutelines − wheat (glutenin), rice (oryzenin)
  • protamines − soft roe (cyprimin, salmin, klupein, skombrin)
  • histones − blood (globins of haemoglobin and myoglobin

By status[edit | edit source]

  • native (natural, biological functions)
  • denatured
  • adjusted (modified, additives)

Nutritional aspect[edit | edit source]

  • full-bodied (essential amino acids in optimal quantities)
    • egg and milk
  • almost full-bodied (some essential amino acids deficient)
    • animal, muscle
  • incomplete (some essential amino acids deficient)
    • all plant, animal connective tissues
Food deficient in certain amino acids
  • Lysine − cereals (generally vegetable proteins)
  • Methionine − milk, meat
  • Threonine − wheat, rye
  • Tryptophan − casein, maize, rice
Content in food
  • 0–100 % P (in dry matter)
  • animal foods > vegetable
  • legumes, oilseeds > fruits, vegetables
eggs – 75 % H2O, 13 % P (whole), 52 % P in dry matter
legumes – 12 % H2O, 24 % P (soya 32-45 %), 27 % in dry matter
meat (H) – 69 % H2O, 21% P, 68 % in dry matter
bread – 38 % H2O, 7 % P, 11 % in dry matter
milk – (3,5 % L) 87-90 % H2O, 3,4 % P, 28 % in dry matter
potatoes – 78 % H2O, 2 % P, 9 % in dry matter

Covering energy needs: ~ 10 % Recommended daily dose: 1-1,2 g/kg Nutrient ratio

  • protein : lipids : carbohydrates (weight = 1 : 1 : 4)
  • energy = < 14 : < 14 : <56 %

Physiology and nutrition[edit | edit source]

  • minimum need for full protein 0,5-0,6 g·kg-1
  • recommended dose 1,0-1,2 g·kg-1 (not used optimally)
~ 2,4 g·kg-1 growth period, breastfeeding women, recuperation etc.
  • nutritional value (nutritional, biological)
  • total intake

Availability of peptide bonds to digestive enzymes

Other factors

  • BV (Biological Value) (= g P produced in organism / 100 g P in food)
  • NPU (Net Protein Utilization)
  • PER (Protein Efficiency Ratio) aj. (animals)

Depends on:

  • absolute content of essential amino acids
  • relative ratio
  • ratio to non-essential amino acids
  • digestion
  • AAS (Amino Acid Score)
  • EAAI (Essential Amino Acid Index) − more accurate
AAS (%) = 100 Ai /Asi


  • Ai = essential amino acid content in protein
  • Asi = same amino acid content in standard (reference) protein

Standard protein = fictitious protein with optimal composition of essential amino acid (AAS = 100%)


Physico-chemical qualities[edit | edit source]

  • solubility, hydration and swelling
  • dissociation
  • optical activity
  • formation of gel
  • formation of emulsions
  • stabilization of foam
  • denaturation
    • physical factors − changes in temperature, pressure, ultrasound, penetrating, penetrating electromagnetic radiation
    • chemical factors− salts, changes in pH (acids, bases), surfactants
  • concequences
    • more accessible to digestive enzymes of digestive tract
    • denaturalization of antinutritional factors, toxic substances (proteas inhibitors, amylas, lectins)
    • inhibition of undesirable enzymes and microorganisms
Meat, meat products, poultry, fish

Meat, meat products, poultry, fish[edit | edit source]

4 main tissues types (additional blood)

  • epithelial
  • supporting (connective)
  • muscle (transversely striped, smooth)
  • neural
Parts of warm-blooded animals in fresh, processed state
In the narrower sense: skeletal muscle tissue − number of muscles, bone tendrils, blood supply, nerves, skin, cartilage, bone, fat
Other components
  • vitamins
  • free amino acids 0,1–0,3 %
taurine (0,02-0,1 %), bile acid component, nerve excitation transfer Taurin.jpeg
  • quaternary ammonium compounds
choline 0,02-0,06 %, phospholipids, transmethylation reactions, acetylcholine Choline.jpeg
carnitine 0,05-0,2 %, fatty acid transport Carnitine.jpeg
  • guanidine compounds
  • glycogen
  • phosphate sugars and free sugars
  • lactic acid and other acids
  • purines and pyrimidines

Use for food and non-food purposes.

Myofibrillar proteins[edit | edit source]

In vivo reaction
Post mortem reaction
  • ATP anaerobic glykolysis from glycogen
  • lactic acid › decrease in pH 6,8 to < 5,8
  • inhibition of glycolytic enzymes
  • Ca2+ / actin reaction with myosin, without ATP › post-mortem stiffening (rigor mortis)

Effect on meat quality[edit | edit source]

Maturation of meat
  • cleaving actomyosin by endogenous proteas (mainly cathepsins)
  • cleaving collagen with collagen
Meat defects
  • DFD (dry-firm-dark) a DCB (dry-cutting-beef)
    • dark, high binding, low maintenance
    • removal of lactic acid during exsanguination, pH~ 6
  • PSE (pale-soft-exudative)
    • light, low binding, grey-green surface
    • increased glycolysis stimulated by hormones, pH~ 5,6

Changes in processing[edit | edit source]

  • ~35° C association of sarcoplasma proteins, reduction in binding, increase in stiffness
  • ~45° C visible changes, shortening=denaturation of myosin
  • ~50-55° C denaturation of actomyosin
  • ~55-65° C denaturation of sarcoplasma proteins, associated structure and gel
  • ~60-65° C changes in collagen conformation (shortening 1/3-1/4)
  • ~80° C oxidation of SH-groups
  • ~90° C gelation of collagen (release of tropocolagen fibres, sol gelatine)
  • ~100 ° C elimination NH3, H2S, other substances, aromatic substances, change of colour

Milk and milk products[edit | edit source]

Complicated dispersion system
  • globular protein of whey − colloidal dispersion
  • casein molecule − micellar dispersion
  • fat − adipose globule (microsomes, φ 0,1-10 µm): emulsion
  • molecules od lipoproteins − colloidal suspension molecular substance (lactose, amino acids, minerals, hydrophilic vitamins)
  • molecular substance (lactose, amino acids, minerals, hydrophilic vitamins) − genuine solution

Protein composition of cow's milk[edit | edit source]

Amino acid content of milk

  • caseins
    • α-caseins = phosphoproteins, αS1, αS2, phospherin Alfa-casein.jpeg
    • ß-caseins = phosphoproteins
    • γ-caseins = products of ß-caseins degradation
    • κ-caseins = glycoproteins (2 genetic variants, B), sugar = tetra-, tri-, di-, mono-, GalNAc, Gal,

NeuAc, binding to Thr (133) Kappa-kasein.jpeg

  • caseins − αS-, ß-, κ-caseins aggreation into submicel and micel, casein moleculs › submicel › micel

Changes in storage and processing[edit | edit source]

Heat treatment
  • clumping of fatty globules in raw milk, ~ makroglobulin
  • whey causes proteins to thermable, denature, caseins practically don't denature
  • 72-74 °C (20-40 s): denatures about 50-90 % of serum proteins
  • > 75 °C:
    • most enzymes are inactivated
    • reduction of disulfide-binding
    • elimination of H2S (ß-lactoglobulin)
    • sulphides, disulphides simmer ingestion (Met)
    • degradation of thiamine
    • formation of lactones and methyl ketones
  • sterilisation 140 °C (4 s)
    • denaturation 100 % of the proteins
    • reaction of lactose with lysine
    • loss of lysine (Maillard reaction), fragrances − raw and pasteurised milk ~ 400 fragrances (1-100 mg/kg)
Casein clotting and proteolysis
  • fresh milk − pH 6,5-6,75
  • casein precipitation − pH 4,6 (contaminant, culture microbes)

Hard cheeses[edit | edit source]

  • microorganisims (lactic acid), acidifaction (pH 5,5)
  • proteolytic enzyme rennin (chymosine, rennet), specific hydrolysis of κ-casein − para-κ-kasein = hydrophobic part, part of micells, κ-caseinmacropeptide = hydrophilic part, coagulation
  • curds, (storage › rigidity, acidity, centrifugation of whey, salting, maturation (in the case of Emmental-type cheese, conversion of lactic › propionic acid + CO2), proteolysis, lipolysis › hard cheese

Soft cheeses, yoghurts[edit | edit source]

  • precipitation, low pH (fermentation of lactose, lactic acid), partial coagulation of caseins, in yoghurts is association of micells (gel structure)
Insoluble acid casein
Sweet casein (rennet clotting)
Caseinates (soluble)
Na, K, NH4; dispersible: Ca, Mg)
Insoluble coprecipitates

Egg[edit | edit source]

Proteins of egg white[edit | edit source]

~ 40 proteins (globulins, glycoproteins and phosphoproteins)
  • enzymes (lysozyme, N-acetylmuramide activity, murein, cell walls of bacteria)
  • protein components of enzymes (flavoprotein/riboflavin, avidine/biotine)
  • protease inhibitors (ovomucoid, ovoinhibitor)
  • viscosity and gel-like consistency of egg white − ovomucoid a ovomucin
  • stability of whipped white foam − ovoglobulins G2 a G3
  • antimicrobial effects −lysozyme (ovoglobulin G1)
  • antinutritional action − avidine

Yolk proteins (emulsion of fat in water)[edit | edit source]

  • 1/3 = proteins, 2/3 = lipids
  • glyco-, lipo-, glycophospho- and glycophospholipoproteins
  • granules − lipovitellin and phosvitin
  • plasma − lipovitellenin and livetin

Changes in storage and processing[edit | edit source]

  • partial denaturation of white proteins when whipped
  • denaturation by heat
    • 57 °C − beginning
    • 60-65 °C − most proteins denature (not ovomucoid)
    • 65-70 °C − most yolk proteins (not phosvitin)
Foods of vegetable origin

Foods of vegetable origin[edit | edit source]

  • main sources − plant seeds
  • limited sources − fruits, leaves, tubers and other parts of plants (fruits, vegetable, root crop)

Cereals and pseudocereals[edit | edit source]

Wheat proteins[edit | edit source]

Flour 7-13 (up to 15) % of proteins

  • 15 % albumins (water soluble) leucosin
  • 7 % globulins (0,4 M-NaCl) edestin
  • 33 % prolamines (70 % ethanol) gliadine
  • 46 % glutelines (remainder) glutenin
prolamine/gluteline ratio = 2 : 3
  1. Strong flour = bread (12-14 %)
  • (dough elastic, stiff, necessary intense mixing, retains carbon dioxide, air, bulkier products)
  1. Weak flour= biscuit production, confectionery (< 10 %)
  • gluten = viscoelastic mass, 2/3 of water, 1/3 hydrated gluteline (viscosity),
  • gliadines (elasticity), gluten solids = 90 % of proteins, 8 % of lipids, 2 % of sugars
Gluten-free products
  • allergic disease - celiac disease (~ 0,05 % of children in Europe)
  • changes in the epithelial cells of the intestinal wall, impaired absorption of nutrients
  • prolamine fraction of wheat, rye, barley, sequence: Pro-Ser-Gln-Gln a Gln-Gln-Gln-Pro
limits < 100 mg gliadine/kg (dry matter)

Rye proteins[edit | edit source]

  • no gluten
  • bakery properties: pentoses, certain proteins (bottling in an acidic environment)
  • formation of acids by microorganisms (S. cerevisiae, S. minor, L. plantarum, L. brevis)

Legume and oilseed proteins[edit | edit source]

  • high in globulins, germination function
Searchtool right.svg For more information see amino acid content in legumes.
Utilization of non-traditional protein sources
Textured herbal proteins
Protein-rich preparation

Reactions[edit | edit source]

  • elimination, isomerization, additive, oxidation
  • complex reaction
  • influence of food composition, conditions: temperature, pH, O2, other substances
  • impairment of biological value
    • breakdown of essential amino acids
    • formation of non-metabolizable products
    • reduction of digestion
    • formation of antinutritional and toxic substances
  • formation of aromatic substances
    • mainly Cysteine, Methionine, Ornitine, Proline
    • amines, aldehydes, alcohols, S-compounds

Elimination reaction[edit | edit source]

  • dekarboxylation (elimination of carbon dioxide) Decarboxylation.jpeg
aromatic substances
biologically active substances (biogenic amines)
Elimination of ammonia and water
  • formation of 2,5-dioxopiperazines (cyclical dipeptides) Formation 25.jpeg
  • formation of alc-2-enoic acidsFormation alc2en.jpeg
  • formation of γ-lactams from γ-amino acids, γ-amino acid Glu, creatine Formation gamalactam.jpeg
Elimination of function groups of side chains
  • reaction in an acidic environment or thermal reactions
    • protein deamideation, hydrolysis
  • reaction in neutral environment or thermal reactions
    • formation of unusual bindings
  • reaction in alkaline environment or thermal reactions
    • formation of unusual binding, unusual amino acids, D-amino acids (abiogenic)
  • reduction of digestion
  • reduction in the nutritional value
  • formation of potentially toxic amino acids
  • formation of aromatic substances
Acidic environment
  • Production of protein hydrolysates
    • enzymes ofautolysis, yeast autolysates, food hydrolysates, soy sauce
    • acids of food hydrolysates
Neutral environment
  • formation of transverse bindings and unusual amino acids
ε-amino group Lys, the carboxamide group Asn, Gln Transverse.jpeg
Alkaline environment
  • loss of Lysin, Cystein, Serin, Threonin, Arginin aj.
  • 1,2-elimnation of H-X (Ser, Thr, Cys, SySSCy) and hydrolysis Reaction alkaline.jpeg

X = OH, SH, SR, SSR etc.

Cys, Ser›2-aminoacrylic acid (dehydroalanine), Thr › 2-aminocrotonic acid (dehydrobutyrine) Formation to aminocrotonic.jpeg

  • addition of functional groups of amino acids (intra- and intermolecular transverse bindings)

Addition transverse binding.jpeg

  • hydrolysis of cross-linked protein and formation of unusual amino acids, lysinoalanine, lanthionine

Hydrolysis crosslinked protein.jpeg

  • isomerisation and formation of D-amino acids, reduced reusability

Isomeration formation D-AMK.jpeg

Additive reactions[edit | edit source]

  • carbohydrate reaction (aldehydes, ketones), Maillard reaction

Maillard reaction.jpeg

    • coloured substances, aromatic substances, biologically active substances

Oxidative reactions[edit | edit source]

  • oxidative deamination and transamination
    • Enzyme reactions
      • deaminases or transminases, hydrolasses


      • aldehydes − aroma of fruits and vegetables
      • alcohols − aroma of alcoholic beverages (alcohols congeners)
Strecker degradation (oxidative decarboxylation)
  • formation of strecker aldehydes
    • non-enzyme reactionStrecker degradation.jpeg
Oxidizing agents
  • dicarbonyl compounds
  • carbohydrates
  • quinones
  • inorganic substances (hypochlorites)
Formation of other products
  • N- and S-heterocyclical compounds
More oxidations
  • oxidised lipids and phenols, O2 (photosensitisers)
  • cysteine and cystine
    • oxidation of Cys na sulfenic, sulfinic, sulfonic (cysteic) acid (unusable)Cysteic.jpeg
    • oxidation of Cys to CySSCy Oxidation to CySSCy.jpeg
    • oxidation of CySSCy Oxidation CySSCy.jpeg
    • oxidation of Met Oxidation Met.jpeg

Reaction with food ingredients[edit | edit source]

  • reaction with polyphenols
    • dark colour of scrap isolates
    • unusable products, reduced digestion
  • reaction with oxidized lipids
    • unusable products, reduced digestion

References[edit | edit source]

Related articles[edit | edit source]

Sources[edit | edit source]

  • KOOLMAN, Jan – RÖHM, Klaus-Heinrich. Barevný atlas biochemie. 1. edition. Grada, 2012. ISBN 978-80-247-2977-0.
  • MATOUŠ, Bohuslav, et al. Základy lékařské chemie a biochemie. 1. edition. Galén, 2010. 540 pp. ISBN 978-80-7262-702-8.
  • MURRAY, Robert K. (Robert Kincaid) – BENDER, David A – BOTHAM, Kathleen M, et al. Harper’s illustrated biochemistry : [object Object]. 29. edition. McGraw-Hill Medical ; New York [u.a.] : McGraw-Hill Lange, 2012. ISBN 9780071765763.
  • LEDVINA, Miroslav, et al. Biochemie pro studující medicíny. 2. edition. Karolinum, 2009. 548 pp. ISBN 978-80-246-1414-4.