Proteins, peptides and amino acids

Overview

The proteins were discovered by the Dutch chemist Gerhard Mulder (1802-1880). The word protein comes from the Greek word protos which means first, essential. This is probably referring to the fact that proteins are essential to life and they often constitute the majority of the dry weight of organisms (more than 50% of their dry weight).

Proteins are macromolecules consisting of amino acids which, in turn, are composed of carbon, hydrogen, oxygen, nitrogen and sometimes sulfur. These amino acids are linked by peptide bonds in the form of long filaments (polypeptide chains). They wrap themselves in a virtually infinite number of spherical or helical forms, which explains the wide variety of functions performed by proteins. They are present in all living organisms and are essential to their operations. It is estimated that there are about thirty miles different proteins in humans, with only 2% have been described.

The form of the protein is highly variable: it is the long fibers found in connective tissue and hair to the compact, soluble globules that can cross the cell membrane. They serve to build and maintain the cells and their chemical degradation provides energy, producing about 5.7 kilocalories per gram. Apart from their role in the growth and maintenance of cells, proteins are also responsible for muscle contraction. Digestive enzymes are proteins, as well as the insulin and most other hormones, and antibodies of the immune system and the hemoglobin. The chromosomes that carry all of the hereditary form of genes, consist of nucleic acids and proteins (histones).

Amino acids

Amino acids (or amino acids) are organic compounds containing the amino group (-NH2) and carboxyl group (-COOH), and are the basic constituents of proteins.

Among amino acids, there are those who enter into the composition of proteins and are called alpha-amino acids and those that exist in nature but are not part of the composition of proteins.

Alpha-amino acids

Among amino acids, twenty are in the composition of proteins. These are the alpha-amino acids, molecules electrically charged or neutral: the alanine, the arginine, the asparagine, the aspartic acid, cysteine, the glutamic acid, glutamine, glycine, the histidine, the isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine and valine.

These amino acids have the formula developed semi-General as follows: (Figure 1)

Aminoacid

As shown in Figure 1, the amino and carboxyl groups are bound to the same atom, the carbon atom alpha. It is through the R group that molecules of the twenty amino acids differ from each other. In the simplest molecules of amino acids, glycine, R is a hydrogen atom. Other amino acids have more complex R groups, consisting of atoms of carbon, hydrogen, and in some cases, oxygen, nitrogen or sulfur.

Depending on the nature of group R, can be distinguished: aliphatic amino acids, aromatic amino acids, hydrophilic amino acids, hydrophobic amino acids, amino acids, alcohols, sulfur amino acids and amino acid nitrogen.

Amino acids are also classified according to their isoelectric pH, and there are three categories:

  • Amino acids: Asparagine and glutamic acid (glutamate).
  • The basic amino acids: Lysine, arginine, and histidine.
  • The neutral amino acids: the other amino acids.

The isoelectric pH or isoelectric point (IP), is the pH where the molecule is in its zwitterionic form (ion mixed), its overall load being zero. If pH <IP, the overall load is positive, because the molecule tends to keep its protons or capture of acidic. If pH> IP, the overall load is negative, because the molecule tends to transfer its proton to the alkaline conditions.

Isoelectric pH

Main amino acids

On Tables 1 and 2, amino acids are classified according to their hydrophobic or hydrophilic.

Table 1: Hydrophobic amino acids

Nature of the group R

Formula of the group R

Name of amino acid (abbreviation)

Properties / Role in protein structure

Aliphatic

H–

Glycine (Gly, G)

Non-polar / steric The small footprint provides flexibility to the protein chain

Aliphatic

CH3

Alanine (Ala, A)

Non-polar / steric The small footprint provides flexibility to the protein chain

Aliphatic

(CH3)2–CH–

Valine (Val, V)

Non-polar / steric The small footprint provides flexibility to the protein chain

Aliphatic

(CH3)2–CH–CH2

Leucine (Leu, L)

Non-polar / steric The small footprint provides flexibility to the protein chain

Aliphatic

CH3–CH2–C(CH3)H–

Isoleucine (Ile, I)

Non-polar / steric The small footprint provides flexibility to the protein chain

Indole nucleus

In–CH2

Tryptophan (Trp, W)

Non-polar. The indole nucleus (In) strongly absorbs UV.

Aliphatic (folded)

(N)–CH2–CH2–CH2

Proline (Pro, P)

Nonpolar / The heterocyclic ring (the last carbon is connected to the N amine) confers rigidity to the protein chain

Aromatique

Phe–CH2

Phenylalanine (Phe, F)

Nonpolar

Sulfur group

CH3–S–CH2–CH2

Methionine (Met, M)

Nonpolar

 

Table 2: Hydrophilic amino acids

Nature of the group R

Formula of the group R

Name of amino acid (abbreviation)

Properties / Role in protein structure

Alcohol

OH–CH2

Serine (Ser, S)

Polar, not charged

Alcohol

CH3–CH(OH)–

Threonine (Thr, T)

Polar, not charged

Aromatique

PheOH–CH2

Tyrosine (Tyr, Y)

Polar, not charged

Sulfur group

HS–CH2

Cysteine (Cys, C)

Polar unloaded / Form bridges "disulfides between chains that stabilize the structure.

Amide

NH2–CO–CH2

Asparagine (Asp, N)

Polar, not charged

Amide

NH2–CO–CH2–CH–

Glutamine (Gln, Q)

Polar, not charged

Amine

NH2–(CH2)4

Lysine (Lys, K)

Polar, positively charged

Amine

NH=C(NH2+)–NH–(CH2)3

Arginine (Arg, R)

Polar, positively charged

Imidazole

Im–CH2

Histidine (His, H)

Polar, positively charged

Carbonate

COO-–CH2

Aspartate (Asp, D)

Polar, negatively charged

Carbonate

COO-–CH2–CH2

Glutamate (Glu, E)

Polar, negatively charged

Essential amino acids

Most plants and microorganisms can synthesize, from inorganic compounds, all the amino acids that are necessary for their growth. Animals, they can not get that certain amino acids in their food, and these amino acids are called essential.

In humans, essential amino acids are in number nine: lysine e, tryptophan, valine, the histidine, leucine, the isoleucine, phenylalanine, threonine and methionine. The histidine is an essential amino acid only for children. In addition to these nine amino acids, amino acids added considered as conditionally essential (essential in some conditions). For example, glutamine and the arginine may become essential during metabolic stress.

Essential amino acids from food. They are found in foods of animal origin, rich in protein, and in associations of vegetable protein.

Other amino acids

In addition to the amino acids in proteins, more than 150 others have been found in nature, the amine and carboxyl groups of some are linked to different carbon atoms. There are unusual amino acids in fungi or plants more complex.

Peptides

The peptide chains are products of the covalent polymerization of amino acids by a peptide bond. They differ in the number, nature and sequence of amino acids. Is arbitrarily defined:

  • Peptides: sequence of a number of less than 50 amino acids. Among these, one speaks of the oligopeptides for a number of less than 10 amino acids and polypeptides for a number greater than 10.
  • Protein sequencing of a number of amino acids beyond 50.

Formation of the peptide bond

When a living cell synthesizes a protein, the carboxyl group of one amino acid reacts with the amino group of another amino acid to form a peptide bond (CO-NH). The second carboxyl group of amino acid reacts in the same manner with the amino group of a third, and so on, until the formation of a chain, or peptide, which may contain two to several hundreds of amino acids. The two amino acids at the ends of the chain are called N-terminal to that which has its aminée free α-and C-terminal to that which has its free α-COOH.

Formation of the peptide bond

Hydrolysis of the peptide bond

The peptide bond is very stable, its spontaneous hydrolysis is almost nil, but it can be hydrolysed by chemical or enzymatic.

The chemical hydrolysis may be complete or specific

  • The action of hydrochloric acid (HCl) 6M on a peptide, to a boil for at least 24 hours, resulting in a hydrolyzate containing amino acids. However, some amino acids such as tryptophan may be destroyed.
  • Some reagents hydrolyze peptide with a link to a specific amino acids involved in the affair. For example, cyanogen bromide (BrCN) hydrolysis of the peptide bond carboxyl side of methionine and 2-nitro-5-thiocyanobenzoate (NTCB) hydrolyse the peptide bond on the side of the cysteine amine.

The enzymatic hydrolysis of peptide bonds can be achieved by proteolytic enzymes (or proteases or peptidases) are hydrolases. The main specificity of this group of enzymes is the hydrolysis of peptide bonds. Their specificity allows secondary to classify them into two groups:

  • Exopeptidases: These enzymes n'hydrolysent that the first peptide bond (aminopeptidase) or the last peptide bond (carboxypeptidase) by releasing the terminal amino acid. Of course, the process begins again on the cut of a peptide amino acid (a hydrolysis time court allows release of a single amino acid).
  • Endopeptidases: These enzymes hydrolyze internal peptide bonds between amino acids i and (i +1) according to their specificity (specificity of the residue in position i or i +1). The hydrolysis of a peptide by endopeptidase peptide fragments give more if you coupures am (m hydrolysed peptide bonds), the peptide will be degraded in (m 1) peptide fragments.

Protein

A protein, known as proteins, is a macromolecule composed of a chain (or sequence) of amino acids linked by peptide bonds (Figure 3). In general, we speak of protein when the channel contains more than 50 amino acids.

Classification of proteins

Several bases are used for classification of proteins: their chemical composition, their biological role in the body and their solubility.

Depending on their chemical composition, proteins can be divided into two groups: holoprotéines and hétéroprotéines. The holoprotéines or simple proteins contain only amino acids. They are divided into protamines, histones, gluténines, prolamins, albumins, globulins and scléroprotéines. The hétéroprotéines contain more amino acids, some non-protein, known as prosthetic group and plays an important role in the function of the protein. Depending on the type of prosthetic group, these proteins are further divided into nucleoproteins, lipoproteins, glycoproteins, phosphoproteins, hemoproteins, and flavoprotéines metalloproteins. The most metal ions found in metalloproteins are iron, zinc, calcium, molybdenum and copper.

Depending on their biological role in the body, there are two main classes of proteins: fibrous proteins and globular proteins. Proteins are found in fibrous tissue and structural elements such as muscles, bones, skin, components or cell membranes. They perform tasks and structural units are composed of simple structures and repetitive secondary (alpha helix and beta sheet). Collagen and keratin are fibrous proteins with alpha helical conformation, silk fibroin (fibrous protein) has a pleated sheet conformation in beta. Globular proteins play an essential role in metabolism and fulfill several other functions. The enzymes, a group of proteins with a highly specific catabolic activity, constitute an essential subgroup of globular proteins. Other globular proteins are hormones (eg insulin), motor proteins (eg myosin muscle), protein transport (hemoglobin), the immunity protein in the blood of vertebrates (eg immunoglobulin) and all major storage proteins (proteins reserve) as the soybean glycinin and the egg white of the egg.

Depending on their solubility, there are soluble proteins and insoluble proteins. Soluble proteins are divided into two groups: proteins soluble in water (such as albumin) and proteins that dissolve in the presence of salts in a neutral or slightly acidic medium or slightly alkaline (such as globulins ). As for the insoluble proteins, such as scléroprotéines, they are insoluble in any aqueous medium.

Structure of proteins

Each protein consists of a linear sequence of several amino acids. This sequence (eg Lys-Ala-Ile-Thr-...) determines what is called "primary structure" of the protein. However, a protein does not care a strictly linear. The energy contained in hydrogen bonds, disulphide bridges, the attraction between positive and negative charges, and hydrophobic or hydrophilic radical, require the protein secondary structure, mainly helical or sheet. molecules become more compact by adopting a tertiary structure. When a protein is made up of more than one polypeptide chain, such as hemoglobin and certain enzymes, it is said that it has a quaternary structure (Figure 4).

Structure of proteins

Denaturation of protein structure

The structure of proteins is very sensitive to physico-chemical treatments. Many processes can lead to denaturation of proteins by affecting the secondary structures, tertiary and quaternary. The physical treatment may induce a distortion are heating, cooling, mechanical, hydrostatic pressure and ionizing radiation. Interactions with certain chemicals can also denature proteins: acids and bases, metals and high salt concentrations, organic solvents, etc..

Denaturation of proteins is a complex process that induces changes in conformation (secondary structure, tertiary and quaternary) important. During the process of denaturation, intermediaries may be partially unstable. Any change in structure of the native protein is called denaturation. Denaturation of proteins may be reversible or irreversible. The effects of distortion are varied:

  • Change of solubility associated with a different exposure units hydrophobic or hydrophilic peptide,
  • Changing the power of water retention,
  • Loss of biological activity for example enzymes,
  • Increased risk of chemical degradation due to exposure of peptide bonds less stable,
  • Changing the viscosity of solutions,
  • Modification or loss of properties of crystallization.

Examples of protein

Histones

The histone proteins are globular in basic character due to the presence of high proportion of lysine and arginine. They are found associated with DNA of the nucleus of somatic cells (cells not related to reproduction). Histones are soluble in water and not clottable by heat. Their isoelectric point (PI) is between 10 and 12.

Protamines

The protamines are proteins of low molecular weight, rich in basic amino acids like arginine and contain no sulfur or aromatic amino acids. They are associated with the DNA of fish and plants. The protamines are soluble in water and not clottable by heat. The PI is between 10 and 12.

Prolamins and gluténines

The prolamins and gluténines are proteins specific to plants and are particularly experienced in cereal grains as they are part of the reserve protein substances.

The prolamins are rich in proline, aspartate and in glutamate, but poor in lysine and tryptophan. Their relative wealth in their hill attributes the property of solubility in alcohol at 70%. The corn zein, wheat gliadin and the hordein of barley are, for instance, prolamins. Thanks to their elastic and spiral structures, these proteins provide the baked goods moist and excellent cooking. Family proteins are the prolamins toxic amounts in gluten intolerance and mainly the alpha-gliadin, which is present in wheat.

Rate prolamins in cereals

Cereals

Prolamins

Rate (%)

Oats

Avenin

16

Wheat (wheat)

Alpha-gliadin

69

Maize

Zein

55

Millet

Panicin

40

Barley

Hordein

46-52

Rice

Orzenin

5

Rye

Secalinin

30-50

The glutenin have a composition similar to that of prolamins and are distinguished by the proportions contained in glutamate (slightly lower in glutenin) and lysin (slightly higher in glutenin). They are insoluble in water but soluble in dilute acids and alkalis.

Gluten (elastic protein mass remaining after extraction of starch from wheat), which is the main constituent of the protein of cereals (85% of the quantity of wheat proteins) and a factor in their ability to bread, is consists of these two proteins: prolamin and glutenin.

Albumins and globulins

The albumins and globulins are blood proteins (spheroproteins) found both in animals than in plants.

The albumins are rich in glutamic acid (glutamate), aspartic acid (aspartate), lysine and leucine, but low in tryptophan. They have a molecular weight between 50,000 and 100,000 and an isoelectric point in the acid zone (PI <7). Albumins are soluble in water, coagulation by heat and precipitate by adding ammonium sulfate between 66% and 100% saturation. The white egg white egg, lactalbumin of milk and legumes are léguméline, for example, albumin.

Globulins are proteins that contain the whole range of amino acids and are particularly rich in glutamate and aspartate. The globulins are insoluble in water but soluble in dilute salt solutions. They precipitate by adding ammonium sulfate to 50% saturation. They are often glycoproteins or lipoproteins. The ovoglobulin egg white and milk lactoglobulin are, eg, globulin.

Casein

Casein is a phosphoprotein rich in lysine (one of the essential amino acids). In milk, it takes the form of calcium phophoscaseinate and represents approximately 80% of its protein and is present in the form of microparticles in suspension, also called micelles. Casein is coagulated in acid and rennet. It is in the casein we must curd cheese (the action of ferments and rennet) and yoghurt (actions ferments).

In the dry state, the casein is in the form of a white powder, amorphous, tasteless and odorless. The casein dissolves slightly in water, largely in alkaline solutions or strong acids.

Fibroin

Fibroin is a fibrous protein rich in glycine and alanine, but also serine and tyrosine. It occurs, for example, in silk and the spider's web.

Keratins

Keratin is a tough, fibrous protein rich in proline and cysteine, which forms the main material of the skin (external surface tissues such as skin) and dander (skin productions, such as hair, nails, scales, feathers, beak, horns and hooves) animals.

Collagen

Collagen is the most abundant protein in vertebrates. It is in the bones, skin, tendons and cartilage. Its molecule usually contains three long polypeptide chains, each composed of about a thousand amino acids. These chains curl into a regular triple helix, responsible for the elasticity of the skin and tendons.

Aqueous solution and under the action of heating, collagen turns into gelatin. Gelatin is a substance soluble in water and gives a viscous solution. Besides its use as glue, gelatine is used as thickener in the food industry.

Protein degradation

Unlike the distortion affects the quaternary structure, tertiary and secondary protein; degradation in turn affects the primary structure of proteins and hence leads to the formation of other, sometimes undesirable. The degradation of proteins can be chemical or enzymatic.

Chemical degradation of proteins

Chemical degradation of proteins may occur during the process technology or culinary preparations. Damage the best known are:

  • Tagħrif isopeptides: by heating of intramolecular or intermolecular bridges between the amino-terminal lysine and the acid-terminal glutamine / asparagine. The consequence of this reaction is the loss of digestibility.
  • Training lysinoalanine: by replacing the function of the serine OH, or SH of cysteine, the function of amino-terminal lysine after heating in basic (cooked egg white). The consequence of this reaction is the loss of nutritional value due to the formation of abnormal amino acids.
  • Maillard reaction, reaction between a reducing sugar and an amino group. This reaction is the main responsible for the production of fragrances, flavors and pigments characteristics of cooked foods. It may also give rise to carcinogenic compounds and also reduce the nutritional value of foods by degrading the essential amino acids. The Maillard reaction is developed in part related to chemical weathering reactions of food.

Enzymatic degradation of proteins

Enzymatic degradation of proteins may occur during the process of degradation of organic foods such as fermentation or putrefaction. The most important enzymatic degradation of proteins are: the formation of higher alcohols and the formation of biogenic amines.

Formation of higher alcohols

The degradation of proteins in higher alcohols is an enzymatic process that occurs during the fermentation of fruits, cereals, potatoes, etc.. The distribution of higher alcohols formed is partly an image of the distribution of amino acids available. For example, for the production of spirits, yeasts use amino acids as nitrogen source, this activity led to the formation of higher alcohols involved in the aromatic fraction of spirits and analyzed for authentication of these products.

Degradation of proteins in higher alcohols


Formation of biogenic amines

The formation of biogenic amines involved in microbial degradation of foods rich in proteins (putrefaction of meat / fish, cheese maturation, various fermentations). This decarboxylation reaction of amino acids is catalyzed by enzymes (decarboxylases) present in some bacteria (Proteus morganii, Escherichia coli, Lactobacillus Bulgarian, ...).

Degradation of proteins in biogenic amines

The biogenic amines are responsible for several food poisoning. The most dangerous is the scombroides poisoning due to ingestion of products, especially fish containing high doses of histamine. That is why the level of histamine in fish is regulated and must be controlled to avoid this type of poisoning in consumers.

L' histamine se développe dans la chair de plusieurs espèces de poisson suite à la décarboxylation de l'histidine . Cette réaction de décarboxylation est catalysée par l'enzyme histidine-décarboxylase (figure 7). L' histamine est aussi présente naturellement dans divers types d’aliments comme les tomates et les épinards.

Décarboxylation de l'histidine en histamine

 

Nutritional quality of protein

The food proteins are often classified as high or low nutritional quality in terms of their amino acid profile. Proteins of high nutritional quality contain the 9 essential amino acids whose proportions are sufficient to cover the needs of the human species. The lower quality proteins are deficient in one or more of the nine essential amino acids that must be made by the rest of the food.

Functional properties of food proteins

The proteins have a major role in the organoleptic qualities of fresh food and many manufactured goods, such as the consistency and texture of meat and meat products, milk and derivatives, pasta and bread. These qualities of food very often depend on the structure and physicochemical properties of protein components or simply functional properties of proteins.

The term "functional property" applied to food ingredients is defined as any property not influence the nutritional value of an ingredient in food. The various properties will help to achieve the desired characteristics of the food. Some of the functional properties of proteins are: solubility, the hydration, viscosity, coagulation, texturing, the formation of dough, the emulsifying and foaming properties.

The property of protein solubility

The solubility of proteins is their ability to dissolve in water. This solubility is a function of pH, ionic strength and temperature of the environment. The solubility of proteins is minimal in a pH around the isoelectric point (PI). This property is exploited for the preparation of protein isolates from food, by isoelectric precipitation.

The solubility of proteins decreases during heat treatment. This loss of solubility has major consequences for the emulsifying and foaming properties of proteins.

The solubility of proteins is gaining in importance when the clarity of the product, as in beverages, is crucial.

The hydration property of proteins: Retention of water

The ownership of hydration of a protein on its ability to absorb water. The absorption and retention of water by protein ingredients play a major role in the quality and texture of various foods.

The absorption of water is influenced by the presence of ionisable groups, the pH, the presence of salts and temperature. The amino acid composition also affects the ability of proteins to absorb water. The polar groups of proteins (carboxyl, hydroxyl and thiol) tend to easily bind water molecules, which contributes to their ability to absorb water molecules. PI, the ability of proteins to absorb water molecules is minimal since the net charge of the protein is zero. Low salt concentration in the medium improves the absorption of water, but a high concentration decreases. The absorption of water is an exothermic phenomenon, the increase in temperature decreases the absorption of water.

The water absorption of proteins is especially important in foods such as beverages, soups and sausages, it plays a key role on the texture (viscosity, gelation) of these foods.

The viscosity property of protein

The viscosity of a protein is its property which tends to prevent its flow when subjected to the application of a force. Solutions of high viscosity to resist the flow and low viscosity flows easily.

Variations of pH, temperature and ionic strength may alter the viscosity of protein solutions. However, it increases alkaline because the negative electric charges leads to a dislike and maximum elongation of the protein. This phenomenon plays an important role in food liquids such as beverages, soups, sauces and creams.

The clotting property of proteins

When denatured molecules aggregate to form an ordered protein network, the phenomenon is called coagulation, or gel. The coagulation of proteins is obtained by the action of physical agents (temperature, agitation, ...) and chemical (pH, enzymes, ...).

The action of these agents on proteins leads to modification of their structure and promotes the formation of disulphide bridges between their sulfur amino acids: the coagulation. The protein network formed traps between its meshes, the water in the food which gives the food its consistent: the gel or coagulum.

The property of clotting proteins is used for cooking (eggs, meat, ...) and in the preparation of many other products: gels of soy protein (eg tofu), yogurt, cheese. It is also used to improve water absorption (thickening) and to stabilize emulsions and foams.

The texturising property of proteins

Proteins are the basis of the structure and texture of many foods. There are texturing processes that lead to fibrous structures or in a film with a texture mastics and good water retention capacity.

Textured proteins possess certain physical properties of meat and can replace at least a portion (meat, hamburger, etc.).

The property of forming the dough

During the kneading of the flour of certain cereals such as wheat, we are witnessing the formation of an extensible dough (ownership of prolamins) and elastic (ownership of glutenin) is the property of formation of the dough. This property is used primarily in the manufacture of bread and biscuits.

The emulsifying property of proteins

The amphiphilic nature of proteins confers their property to be good surfactants and are therefore used for the stabilization phase oil / water emulsion. Many food products are emulsions (milk, cream, ice cream, ...) and protein components play a role in their stabilization.

Factors such as protein concentration, pH, protein solubility, the presence of salts or other solutes as well as temperature influence the emulsifying properties of proteins.

The foaming properties of proteins

Under the action of a solution combining protein molecules denature proteins, take place and trap the air. This operation led to the formation of a foam (dispersion of gas in a liquid) and the abundance of the solution (increase the volume by adding air).

The foaming properties of proteins is used in the preparation of many foods such as mayonnaise and foams used in confectionery. For the preparation of these foams are frequently used proteins in the egg white, but other proteins such as whey proteins and soy proteins also have this property foaming.

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