Proteins (1. LF UK, NT)

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
150px|thumb|right| peptidová vazba 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 Gibbson 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
The structure of proteins is based on the arrangement of amino acids in the chain. Protein structure is very important for their function.

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

α-helix|thumb β-skládaný list|thumb Secondary structure means the spatial arrangement of amino acids in a chain and stabilization by hydrogen bridges.
 * Secondary structure


 * 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 strings resembling a folded sheet of paper.

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.
 * Tertiary 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 haemoglobin. In contrast, myoglobin does not possess quaternary structure.
 * Quarternary structure

Protein denaturation
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

 * peptide bonds
 * other ties
 * disulphide -S-S-
 * ester
 * amide
 * Ingredients other than amino acids (physically or chemically)
 * water
 * inorganic ions lipids
 * lipids, sugars, nucleic acids, color compounds

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By origin

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

By function

 * structural (cell building components, collagen)
 * catalytic (enzymes, hormones)
 * transport (transfer of compounds, myoglogin)
 * motion (muscle proteins, actin, myosin)
 * defence (antibodies, immunoglobulins, lectins)
 * stock (ferritin)
 * sensory (rhodopsin)
 * regulating (histones, hormones)
 * nutritional (source of essential amino acids, source of nitrogen, mass to build and restore tissues)

By structure
(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) Compounds proteins (contain protein chain and non-protein part - prosthetic group - lipoproteins, glycoproteins)
 * nucleoproteins (nucleid acids)
 * lipoproteins (neutral lipids, phospholipids, sterols)
 * glycoproteins (carbohydrates)
 * phosphoproteins (phosphoric acid)
 * chromoproteins (porphyrin derivatives, flavin)
 * metalloproteins (coordinating boud metals)

By solubility

 * Soluble
 * 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)
 * gluteline − wheat (glutenin), rice (oryzenin)
 * protamines − soft roe (cyprimin, salmin, klupein, skombrin)
 * histones − blood (globins of haemoglobin and myoglobin


 * Insoluble
 * collagen, elastin, keratin

By status

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

Nutritional aspect

 * 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

 * 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


 * Previously
 * 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


 * Today
 * AAS (Amino Acid Score)
 * EAAI (Essential Amino Acid Index) − more accurate
 * AAS (%) = 100 Ai /Asi

where: Standard protein = fictitious protein with optimal composition of essential amino acid (AAS = 100%)
 * Ai = essential amino acid content in protein
 * Asi = same amino acid content in standard (reference) protein

Soubor:EIAA.jpg

Physico-chemical property

 * solubility, hydration and swelling
 * dissociation
 * optical activity
 * formation of gel
 * formation of emulsions
 * stabilization of foam
 * denaturalization
 * 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


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Meat, meat products, poultry, fish
4 main tissues types (additional blood)
 * epithelial
 * supporting (connective)
 * muscle (transversely striped, smooth)
 * neural


 * Definition
 * 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 Soubor:taurin.jpg


 * quaternary ammonium compounds
 * choline 0,02-0,06 %, phospholipids, transmethylation reactions, acetylcholine Soubor:cholin.jpg
 * carnitine 0,05-0,2 %, fatty acid transport Soubor:karnitin.jpg


 * 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

 * muscle fiber
 * myofibrill (contractile fibers)
 * microfilaments (microfibers)
 * myosin
 * actin
 * other proteins


 * In vivo reaction


 * Post mortem reaction
 * ATP anaerobic glycolysis 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

 * 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

 * ~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 myosin
 * ~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 gelatine collagen (release of tropocolagen fibres, sol gelatine)
 * ~100 ° C elimination NH3, H2S, other substances, aromatic substances, discolouration

Milk and milk products

 * Nutrient content in milk
 * Water by type of milk (origin) 63 - 88 %


 * 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


 * Coloration

Protein composition of cow's milk
Amino acid content of milk NeuAc, binding to Thr (133) Soubor:kapa-kasein.jpg
 * Protein composition of cow's milk
 * caseins
 * α-caseins = phosphoproteins, αS1, αS2, phospherin Soubor:alfa-kasein.jpg
 * ß-caseins = phosphoproteins
 * γ-caseins = products of ß-caseins degradation
 * κ-caseins = glycoproteins (2 genetic variants, B), sugar = tetra-, tri-, di-, mono-, GalNAc, Gal,
 * caseins − αS-, ß-, κ-caseins aggreation into submicel and micel, casein moleculs › submicel › micel

Changes in storage and processing

 * Heat treatment
 * clumping of fatty globules in raw milk, ~ makroglobulin
 * whey causes proteins to thermable, denature, caseins practically don't denature


 * Pasteurization
 * 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

 * 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

 * 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


 * Whey
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Egg

 * proteins of whites 53 %, yolk 47 %
 * nutrient content in hens eggs
 * protein composition of hens egg whites and yolk

Proteins of egg whites

 * ~ 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 inhinitors (ovomucoid, ovoinhibitor)


 * Concequences
 * viscosity and gel-like consistency of whites − 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)

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

Changes in storage and processing

 * 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)
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Foods of vegetable origin

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

Cereals and pseudocereals

 * Basic chemical composition of cereals
 * Cereal proteins and their composition

Wheat proteins
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 %)


 * Dough
 * dough protein (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

 * 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

 * high in globulins, germination function


 * Utilization of non-traditional protein sources


 * Textured herbal proteins


 * Protein-rich preparation
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Reakce

 * eliminační, isomerační, adiční, oxidační
 * komplexní reakce
 * vliv složení potravin, podmínek: teplota, pH, O2, další látky


 * Důsledky
 * snížení biologické hodnoty
 * rozklad esenciálních aminokyselin
 * vznik nemetabolizovatelných produktů
 * snížení travitelnosti
 * vznik antinutričních a toxických látek
 * vznik aromatických látek
 * hlavně Cystein, Methionin, Ornitin, Prolin
 * aminy, aldehydy, alkoholy, S-sloučeniny

Eliminační reakce

 * dekarboxylace (eliminace oxidu uhličitého) Soubor:dekarboxylace.jpg
 * aromatické látky
 * biologicky aktivní látky (biogenní aminy)


 * histamin (His) Soubor:histamin.jpg, kadaverin (Lys) Soubor:kadaverin.jpg


 * Eliminace amoniaku a vody
 * vznik 2,5-dioxopiperazinů (cyklické dipeptidy) Soubor:vznik_25.jpg
 * vznik alk-2-enových kyselin Soubor:vznik_alk2en.jpg
 * vznik γ-laktamů z γ-aminokyselin, γ-aminokyselina Glu, kreatin Soubor:vznik_gamalaktamu.jpg


 * Eliminace funkčních skupin postranních řetězců
 * reakce v kyselém prostředí nebo termické reakce
 * deamidace proteinů, hydrolýza
 * reakce v neutrálním prostředí nebo termické reakce
 * vznik neobvyklých vazeb
 * reakce v alkalickém prostředí nebo termické reakce
 * vznik neobvyklých vazeb, neobvyklých aminokyselin, D-aminokyseliny (abiogenní)


 * Důsledky
 * snížení travitelnosti
 * snížení výživové hodnoty
 * vznik potenciálně toxických aminokyselin
 * vznik aromatických látek


 * Kyselé prostředí
 * Výroba hydrolyzátů bílkovin
 * enzymy autolýza kvasničné autolyzáty, potravinářské hydrolyzáty, sójová omáčka
 * kyseliny potravinářské hydrolyzáty
 * Neutrální prostředí
 * vznik příčných vazeb a neobvyklých aminokyselin
 * ε-aminoskupina Lys, karboxamidová skupina Asn, Gln Soubor:příčné_neutrální.jpg

Soubor:reakce_alkalické.jpg
 * Alkalické prostředí
 * ztráty Lysin, Cystein, Serin, Threonin, Arginin aj.
 * 1,2-eliminace H-X (Ser, Thr, Cys, SySSCy) a hydrolýza

X = OH, SH, SR, SSR aj.

Cys, Ser›2-aminoakrylová kyselina (dehydroalanin), Thr › 2-aminokrotonová kyselina (dehydrobutyrin) Soubor:vznik_k_aminokrotonové.jpg

Soubor:adice-příčné_vazby.jpg Soubor:hydrolyza_zesítěných_proteinů.jpg Soubor:isomerace_vznik_D-AMK.jpg
 * adice funkčních skupin aminokyselin (intra- a intermolekulární příčné vazby)
 * hydrolýza zesítěného proteinu a vznik neobvyklých aminokyselin, lysinoalanin, lanthionin
 * isomerace a vznik D-aminokyselin, snížená využitelnost

Adiční reakce
Soubor:Maillardova_reakce.jpg
 * reakce se sacharidy (aldehydy, ketony), Maillardova reakce
 * barevné látky, aromatické látky, biologicky aktivní látky

Oxidační reakce
Soubor:Hydrolasy.jpg
 * oxidační deaminace a transaminace
 * Enzymové reakce
 * deaminasy nebo transminasy, hydrolasy
 * aldehydy − aróma ovoce a zeleniny
 * alkoholy − aróma alkoholických nápojů (alkoholy přiboudliny)


 * Streckerova degradace (oxidační dekarboxylace)
 * tvorba Streckerových aldehydů
 * Neenzymová reakce Soubor:Streckerova_degradace.jpg


 * Oxidační činidla
 * dikarbonylové sloučeniny
 * sacharidy
 * chinony
 * anorganické látky (chlornany)


 * Vznik dalších produktů
 * N- a S-heterocyklické sloučeniny


 * Další oxidace
 * oxidované lipidy a fenoly, O2 (fotosenzibilizátory)
 * cystein a cystin
 * oxidace Cys na sulfenovou, sulfinovou, sulfonovou (cysteovou) kyselinu (nevyužitelná) Soubor:cysteová.jpg
 * oxidace Cys na CySSCy Soubor:oxidace_na_CySSCy.jpg
 * oxidace CySSCy Soubor:Oxidace_CySSCy.jpg
 * oxidace Met Soubor:Oxidace_Met.jpg

Reakce se složkami potravin

 * reakce s polyfenoly
 * tmavá barva izolátů ze šrotů
 * nevyužitelné produkty, snížená travitelnost
 * reakce s oxidovanými lipidy
 * nevyužitelné produkty, snížená travitelnost
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Související články

 * Peptidy (1. LF UK, NT)
 * Aminokyseliny (1. LF UK, NT)
 * Aminokyseliny, peptidy, bílkoviny (1. LF UK, NT)