Stairs. Entry group. Materials. Doors. Locks. Design

Slide 12

Classification of proteins according to their functions

• Types of proteins
• Enzymes
• Hormones
• Transport
• Protective
• Spares
• Toxins
• Structural
• Structural
• Catalytic
• Regulatory
• Contractives
• Contractive
• Transport
• Protective
• Spare
• Protective
• Keratin
• Insulin
• Myosin
• Hemoglobin
• Antibodies
• Casein
• Plant toxins
• Protein functions examples

Slide 13

• Intermediate knowledge control
• Study the diagram of amino acid diversity.
• How are amino acids different from each other?
• Describe the essential components of amino acids.
• Indicate those sections of amino acids. Between which a peptide bond occurs.
• What accounts for the wide variety of proteins?

Slide 14

Intermediate knowledge control

Fill in the blanks in the table.

Structural organization of protein

1. What bonds exist in a protein molecule?

2. Thanks to what bonds does the protein chain form turns?

3. What connections underlie the tertiary structure of a protein?

4. What structure provides the diversity of protein functions?

Slide 15

Thank you for your attention!

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"Life is a way of existence of protein bodies"

F. Engels.

None of the living organisms known to us can do without proteins. Proteins serve nutrients, they regulate metabolism, playing the role of enzymes - metabolic catalysts, promote the transfer of oxygen throughout the body and its absorption, play an important role in the functioning nervous system, are the mechanical basis of muscle contraction, participate in the transmission genetic information etc.

Proteins (polypeptides) – biopolymers built from α-amino acid residues connected peptide(amide) bonds. These biopolymers contain 20 types of monomers. Such monomers are amino acids. Each protein is a polypeptide in its chemical structure. Some proteins consist of several polypeptide chains. Most proteins contain an average of 300-500 amino acid residues. There are several very short natural proteins, 3-8 amino acids long, and very long biopolymers, more than 1500 amino acids long. The formation of a protein macromolecule can be represented as a polycondensation reaction of α-amino acids:

Amino acids combine with each other to form new connection between carbon and nitrogen atoms – peptide (amide):

From two amino acids (AA) a dipeptide can be obtained, from three - a tripeptide, from a larger number of AAs polypeptides (proteins) are obtained.

Functions of proteins

The functions of proteins in nature are universal. Proteins are part of the brain internal organs, bones, skin, hair, etc. Main sourceα - amino acids for a living organism are food proteins, which, as a result of enzymatic hydrolysis in the gastrointestinal tract, giveα - amino acids. Manyα - amino acids are synthesized in the body, and some are necessary for protein synthesis α - amino acids are not synthesized in the body and must come from outside. Such amino acids are called essential. These include valine, leucine, threonine, methionine, tryptophan, etc. (see table). In some human diseases, the list of essential amino acids expands.

· Catalytic function - carried out with the help of specific proteins - catalysts (enzymes). With their participation, the speed of various metabolic and energy reactions in the body increases.

Enzymes catalyze the reactions of breakdown of complex molecules (catabolism) and their synthesis (anabolism), as well as DNA replication and RNA template synthesis. Several thousand enzymes are known. Among them, such as pepsin, break down proteins during digestion.

· Transport function - binding and delivery (transport) various substances from one organ to another.

Thus, hemoglobin, a protein in red blood cells, combines with oxygen in the lungs, turning into oxyhemoglobin. Reaching organs and tissues with the bloodstream, oxyhemoglobin breaks down and releases the oxygen necessary to ensure oxidative processes in tissues.

· Protective function - binding and neutralization of substances entering the body or resulting from the activity of bacteria and viruses.

The protective function is performed by specific proteins (antibodies - immunoglobulins) formed in the body (physical, chemical and immune defense). For example, the protective function is performed by the blood plasma protein fibrinogen, participating in blood clotting and thereby reducing blood loss.

· Contractile function (actin, myosin) – as a result of the interaction of proteins, movement in space, contraction and relaxation of the heart, and movement of other internal organs occur.

· Structural function - Proteins form the basis of the structure of the cell. Some of them (connective tissue collagen, hair, nails and skin keratin, vascular wall elastin, wool keratin, silk fibroin, etc.) perform almost exclusively a structural function.

In combination with lipids, proteins participate in the construction of cell membranes and intracellular formations.

· Hormonal (regulatory) function - the ability to transmit signals between tissues, cells or organisms.

Proteins act as metabolic regulators. They refer to hormones that are produced in the endocrine glands, some organs and tissues of the body.

· Nutritional function - carried out by reserve proteins, which are stored as a source of energy and substance.

For example: casein, egg albumin, egg proteins ensure the growth and development of the fetus, and milk proteins serve as a source of nutrition for the newborn.

The diverse functions of proteins are determined by the α-amino acid composition and structure of their highly organized macromolecules.

Physical properties of proteins

Proteins are very long molecules that consist of amino acid units linked by peptide bonds. These are natural polymers; the molecular weight of proteins ranges from several thousand to several tens of millions. For example, milk albumin has a molecular weight of 17,400, blood fibrinogen - 400,000, viral proteins - 50,000,000. Each peptide and protein has a strictly defined composition and sequence of amino acid residues in the chain, which determines their unique biological specificity. The number of proteins characterizes the degree of complexity of the organism (E. coli - 3000, and in human body more than 5 million proteins).

The first protein we encounter in our lives is protein chicken egg Albumin is highly soluble in water, when heated it coagulates (when we fry eggs), and when stored in a warm place for a long time it collapses and the egg goes rotten. But protein is hidden not only under the eggshell. Hair, nails, claws, fur, feathers, hooves, the outer layer of skin - they are all almost entirely composed of another protein, keratin. Keratin does not dissolve in water, does not coagulate, does not collapse in the ground: the horns of ancient animals are preserved in it just as well as bones. And the protein pepsin, contained in gastric juice, is capable of destroying other proteins, this is the process of digestion. The inferferon protein is used in the treatment of runny nose and flu, because kills the viruses that cause these diseases. And the protein from snake venom can kill a person.

Protein classification

From point of view nutritional value proteins, determined by their amino acid composition and the content of so-called essential amino acids, proteins are divided into full-fledged And inferior . Complete proteins include mainly proteins of animal origin, except for gelatin, which is classified as incomplete proteins. Incomplete proteins are mainly of plant origin. However, some plants (potatoes, legumes, etc.) contain complete proteins. Among animal proteins, proteins from meat, eggs, milk, etc. are especially valuable for the body.

In addition to peptide chains, many proteins also contain non-amino acid fragments; according to this criterion, proteins are divided into two large groups - simple and complex proteins (proteids). Simple proteins contain only amino acid chains; complex proteins also contain non-amino acid fragments ( For example, hemoglobin contains iron).

Based on their general type of structure, proteins can be divided into three groups:

1. Fibrillar proteins - insoluble in water, form polymers, their structure is usually highly regular and is maintained mainly by interactions between different chains. Proteins that have an elongated thread-like structure. The polypeptide chains of many fibrillar proteins are located parallel to each other along one axis and form long fibers (fibrils) or layers.

Most fibrillar proteins are not soluble in water. Fibrillar proteins include, for example, α-keratins (they account for almost the entire dry weight of hair, proteins of wool, horns, hooves, nails, scales, feathers), collagen - the protein of tendons and cartilage, fibroin - the protein of silk).

2. Globular proteins - water soluble, the general shape of the molecule is more or less spherical. Among globular and fibrillar proteins, subgroups are distinguished. Globular proteins include enzymes, immunoglobulins, some protein hormones (for example, insulin), as well as other proteins that perform transport, regulatory and auxiliary functions.

3. Membrane proteins - have domains that cross the cell membrane, but parts of them protrude from the membrane into the intercellular environment and the cytoplasm of the cell. Membrane proteins function as receptors, that is, they transmit signals and also provide transmembrane transport of various substances. Transporter proteins are specific; each of them allows only certain molecules or a certain type of signal to pass through the membrane.

Proteins – an integral part of animal and human food. A living organism differs from a nonliving one primarily by the presence of proteins. Living organisms are characterized by a huge variety of protein molecules and their high orderliness, which determines high organization a living organism, as well as the ability to move, contract, reproduce, the ability to metabolize and perform many physiological processes.

Protein structure

Fischer Emil German, German organic chemist and biochemist. In 1899 he began work on protein chemistry. Using the ether method of analyzing amino acids, which he created in 1901, F. was the first to carry out qualitative and quantitative determinations of protein breakdown products, discovered valine, proline (1901) and hydroxyproline (1902), and experimentally proved that amino acid residues are linked to each other by peptide bonds; in 1907 he synthesized an 18-membered polypeptide. F. showed the similarity of synthetic polypeptides and peptides obtained as a result of protein hydrolysis. F. was also involved in the study of tannins. F. created a school of organic chemists. Foreign corresponding member of the St. Petersburg Academy of Sciences (1899). Nobel Prize (1902).

Topic – 50: Proteins as biopolymers of amino acids. The structure of the peptide group. Biological functions of proteins.

The student must:

Know:

· Name of protein structure, properties of proteins with different structures and their applications.

· Purpose of protein.

Be able to:

· Explain the presence of protein compounds using qualitative reactions.

Proteins , or protein substances, are high molecular weight (molecular weight varies from 5-10 thousand to 1 million or more) natural polymers, the molecules of which are built from amino acid residues connected by an amide (peptide) bond.

Proteins are also called proteins(from the Greek “protos” - first, important). The number of amino acid residues in a protein molecule varies greatly and sometimes reaches several thousand. Each protein has its own inherent sequence of amino acid residues.

Proteins perform a variety of biological functions:

catalytic (enzymes),

regulatory (hormones),

· structural (collagen, fibroin), motor (myosin), transport (hemoglobin, myoglobin),

protective (immunoglobulins, interferon),

In protein molecules, α-amino acids are interconnected by peptide (-CO-NH) bonds:

Polypeptide chains constructed in this way or individual sections within a polypeptide chain can, in some cases, be additionally linked to each other by disulfide (-S-S-) bonds, as they are often called disulfide bridges.

Ionic (salt) and hydrogen bonds, as well as hydrophobic interaction, play a major role in creating the structure of proteins - special kind contacts between hydrophobic components of protein molecules in an aqueous environment. All these bonds have varying strengths and ensure the formation of a complex, large protein molecule.

Despite the difference in the structure and functions of protein substances, their elemental composition varies slightly (in% by dry weight): carbon - 51-53; oxygen - 21.5-23.5; nitrogen - 16.8-18.4; hydrogen - 6.5-7.3; sulfur - 0.3-2.5. Some proteins contain small quantities phosphorus, selenium and other elements.

The sequence of amino acid residues in a polypeptide chain is called primary protein structure.

A protein molecule can consist of one or more polypeptide chains, each of which contains a different number of amino acid residues. Given the number of possible combinations, the variety of proteins is almost limitless, but not all of them exist in nature.

Total number various types proteins in all types of living organisms is 1010-1012. For proteins whose structure is characterized by exceptional falsity, in addition to the primary one, more high levels structural organization: secondary, tertiary, and sometimes quaternary structures. Secondary structure most proteins possess, although not always throughout the entire polypeptide chain (Fig. 38). Polypeptide chains with a certain secondary structure can be differently located in space.

This spatial arrangement is called tertiary structure.

In the formation of the tertiary structure, in addition to hydrogen bonds, ionic and hydrophobic interactions play an important role. Based on the nature of the “packaging” of the protein molecule, they are distinguished globular, or spherical, and fibrillar, or filamentous, proteins.

For globular proteins, an α-helical structure is more typical; the helices are curved, “folded.” The macromolecule has a spherical shape. They dissolve in water and saline solutions to form colloidal systems. Most proteins in animals, plants and microorganisms are globular proteins.

For fibrillar proteins, a filamentous structure is more typical. They are generally insoluble in water. Fibrillar proteins usually perform structure-forming functions. Their properties (strength, stretchability) depend on the method of packing the polypeptide chains. Examples of fibrillar proteins are proteins of muscle tissue (myosin), keratin (horny tissue). In some cases, individual protein subunits form complex ones with the help of hydrogen bonds, electrostatic and other interactions.

In this case, it is formed quaternary structure of proteins.

It should be noted once again that in the organization of higher protein structures, an exclusive role belongs to the primary structure

50.2. Classification

There are several classifications of proteins. They are based on different features:

Degree of complexity (simple and complex);

Shape of molecules (globular and fibrillar proteins);

Solubility in individual solvents (water-soluble, soluble in dilute saline solutions - albumins, alcohol-soluble - prolamins, soluble in dilute alkalis and acids - glutelins);

The function performed (for example, storage proteins, skeletal proteins, etc.).

50.3. Properties

Proteins are amphoteric electrolytes. At a certain pH value (called the isoelectric point), the number of positive and negative charges in the protein molecule is the same. This is one of the main properties of protein. Proteins at this point are electrically neutral, and their solubility in water is the lowest. The ability of proteins to reduce solubility when their molecules reach electrical neutrality is used to isolate them from solutions, for example, in the technology for producing protein products.

50.3.1. Hydration

The process of hydration means the binding of water by proteins, and they exhibit hydrophilic properties: they swell, their mass and volume increase. The swelling of the protein is accompanied by its partial dissolution. The hydrophilicity of individual proteins depends on their structure. The hydrophilic amide (-CO-NH-, peptide bond), amine (NH2) and carboxyl (COOH) groups present in the composition and located on the surface of the protein macromolecule attract water molecules, strictly orienting them on the surface of the molecule. The hydration (aqueous) shell surrounding protein globules prevents aggregation and sedimentation, and therefore contributes to the stability of protein solutions. At the isoelectric point, proteins have the least ability to bind water; the hydration shell around the protein molecules is destroyed, so they combine to form large aggregates. Aggregation of protein molecules also occurs when they are dehydrated using certain organic solvents, such as ethyl alcohol. This leads to the precipitation of proteins. When the pH of the environment changes, the protein macromolecule becomes charged and its hydration capacity changes.

With limited swelling, concentrated protein solutions form complex systems called jellies. Jellies are not fluid, elastic, have a certain plasticity mechanical strength, are able to maintain their shape. Globular proteins can be completely hydrated by dissolving in water (for example, milk proteins), forming solutions with low concentrations. The hydrophilic properties of proteins, i.e. their ability to swell, form jellies, stabilize suspensions, emulsions and foams, have great importance in biology and food industry. A very mobile jelly, built mainly from protein molecules, is the cytoplasm - the semi-liquid contents of the cell. Highly hydrated jelly is raw gluten isolated from wheat dough, it contains up to 65% water. The different hydrophilicity of gluten proteins is one of the signs characterizing the quality of wheat grain and flour obtained from it (the so-called strong and weak wheat). The hydrophilicity of grain and flour proteins plays an important role in the storage and processing of grain and in baking. The dough, which is obtained in bakery production, is a protein swollen in water, a concentrated jelly containing starch grains.

50.3.2. Denaturation of proteins

During denaturation under the influence of external factors (temperature, mechanical stress, the action of chemical agents and a number of other factors), a change occurs in the secondary, tertiary and quaternary structures of the protein macromolecule, i.e. its native spatial structure. The primary structure, and therefore the chemical composition of the protein, does not change. Are changing physical properties: solubility, hydration ability decreases, biological activity is lost. The shape of the protein macromolecule changes and aggregation occurs. At the same time, the activity of certain chemical groups increases, the effect of proteolytic enzymes on proteins is facilitated, and therefore it is easier to hydrolyze.

In food technology, thermal denaturation of proteins is of particular practical importance, the degree of which depends on temperature, duration of heating and humidity. This must be remembered when developing heat treatment regimes for food raw materials, semi-finished products, and sometimes finished products. Thermal denaturation processes play a special role in blanching plant materials, drying grain, baking bread, and producing pasta. Protein denaturation can be caused by mechanical impact(pressure, rubbing, shaking, ultrasound). Finally, the denaturation of proteins is caused by the action of chemical reagents

(acids, alkalis, alcohol, acetone). All these techniques are widely used in food and biotechnology.

50.3.3. Foaming

The foaming process refers to the ability of proteins to form highly concentrated liquid-gas systems called foams. The stability of foam, in which protein is a foaming agent, depends not only on its nature and concentration, but also on temperature. Proteins are widely used as foaming agents in the confectionery industry (marshmallows, marshmallows, soufflés). Bread has a foam structure, and this affects its taste.

Protein molecules, under the influence of a number of factors, can be destroyed or interact with other substances to form new products. For the food industry, two very important processes can be distinguished:

1) hydrolysis of proteins under the action of enzymes and

2) interaction of amino groups of proteins or amino acids with carbonyl groups of reducing sugars.

Under the influence of proteases - enzymes that catalyze the hydrolytic breakdown of proteins, the latter break down into simpler products (polypeptides) and ultimately into amino acids. The rate of protein hydrolysis depends on its composition, molecular structure, enzyme activity and conditions.

50.3.4. Protein hydrolysis

Hydrolysis reaction to form amino acids in general view can be written like this:

50.3.5. Combustion

Proteins burn to produce nitrogen, carbon dioxide and water, as well as some other substances. Combustion is accompanied by the characteristic smell of burnt feathers.

50.3.6. Color reactions

Use following reactions:

xanthoprotein , in which the interaction of aromatic and heteroatomic cycles in a protein molecule with concentrated nitric acid occurs, accompanied by the appearance of a yellow color;

biuret, in which weakly alkaline solutions of proteins interact with a solution of copper (II) sulfate to form complex compounds between Cu2+ ions and polypeptides. The reaction is accompanied by the appearance of a violet-blue color.

Biopolymers- a class of polymers found naturally in natural form components that make up living organisms: proteins, nucleic acids, polysaccharides. Biopolymers consist of identical (or different) units - monomers. Monomers of proteins are amino acids, nucleic acids are nucleotides, and in polysaccharides they are monosaccharides.

There are two types of biopolymers - regular (some polysaccharides) and irregular (proteins, nucleic acids, some polysaccharides).

Squirrels

Proteins have several levels of organization - primary, secondary, tertiary, and sometimes quaternary. The primary structure is determined by the sequence of monomers, the secondary structure is determined by intra- and intermolecular interactions between monomers, usually through hydrogen bonds. Tertiary structure depends on the interaction of secondary structures, quaternary, as a rule, is formed by combining several molecules with a tertiary structure.

The secondary structure of proteins is formed by the interaction of amino acids using hydrogen bonds and hydrophobic interactions. The main types of secondary structure are

α-helix, when hydrogen bonds occur between amino acids in the same chain,

β-sheets (folded layers), when hydrogen bonds are formed between different polypeptide chains running in different directions(antiparallel,

disordered areas

Computer programs are used to predict secondary structure.

Tertiary structure or "fold" is formed by the interaction of secondary structures and is stabilized by non-covalent, ionic, hydrogen bonds and hydrophobic interactions. Proteins that perform similar functions usually have similar tertiary structures. An example of a fold is a β-barrel, where the β-sheets are arranged in a circle. The tertiary structure of proteins is determined using X-ray diffraction analysis.

An important class of polymeric proteins are fibrillar proteins, the best known of which is collagen.

In the animal world, proteins usually act as supporting, structure-forming polymers. These polymers are built from 20 α-amino acids. Amino acid residues are linked into protein macromolecules by peptide bonds resulting from the reaction of carboxyl and amino groups.

The importance of proteins in living nature is difficult to overestimate. This construction material living organisms, biocatalysts - enzymes that ensure reactions occur in cells, and enzymes that stimulate certain biochemical reactions, i.e. ensuring selectivity of biocatalysis. Our muscles, hair, skin are made of fibrous proteins. A blood protein that is part of hemoglobin promotes the absorption of oxygen in the air; another protein, insulin, is responsible for the breakdown of sugar in the body and, therefore, for providing it with energy. The molecular weight of proteins varies widely. Thus, insulin, the first protein whose structure was established by F. Sanger in 1953, contains about 60 amino acid units, and its molecular weight is only 12,000. To date, several thousand protein molecules have been identified, the molecular weight of some of them reaches 106 or more.

Nucleic acids

The primary structure of DNA is a linear sequence of nucleotides in a chain. As a rule, the sequence is written in the form of letters (for example, AGTCATGCCAG), and the recording is carried out from the 5" to the 3" end of the chain.

Secondary structure is a structure formed due to non-covalent interactions of nucleotides (mostly nitrogenous bases) with each other, stacking and hydrogen bonds. The DNA double helix is ​​a classic example of secondary structure. This is the most common form of DNA in nature, which consists of two anti-parallel complementary polynucleotide chains. Antiparallelism is realized due to the polarity of each of the circuits. Complementarity is understood as the correspondence of each nitrogenous base of one DNA chain to a strictly defined base of another chain (opposite A is T, and opposite G is C). DNA is held in a double helix due to complementary base pairing - the formation of hydrogen bonds, two in pair A-T and three in the G-C pair.

In 1868, the Swiss scientist Friedrich Miescher isolated a phosphorus-containing substance from cell nuclei, which he called nuclein. Later, this and similar substances were called nucleic acids. Their molecular weight can reach 109, but more often ranges from 105-106. The starting substances from which nucleotides are built - units of macromolecules of nucleic acids are: carbohydrate, phosphoric acid, purine and pyrimidine bases. In one group of acids, ribose acts as a carbohydrate, in the other, deoxyribose.

In accordance with the nature of the carbohydrate they contain, nucleic acids are called ribonucleic and deoxyribonucleic acids. Common abbreviations are RNA and DNA. Nucleic acids play the most important role in life processes. With their help, two important tasks are solved: storage and transmission. hereditary information and matrix synthesis of DNA, RNA and protein macromolecules.

Polysaccharides

3-dimensional structure of cellulose

Polysaccharides synthesized by living organisms consist of large quantity monosaccharides connected by glycosidic bonds. Often polysaccharides are insoluble in water. These are usually very large, branched molecules. Examples of polysaccharides that are synthesized by living organisms are storage substances starch and glycogen, as well as structural polysaccharides - cellulose and chitin. Since biological polysaccharides consist of molecules of different lengths, the concepts of secondary and tertiary structure do not apply to polysaccharides.

Polysaccharides are formed from low molecular weight compounds called sugars or carbohydrates. Cyclic molecules of monosaccharides can bond with each other to form so-called glycosidic bonds through the condensation of hydroxyl groups.

The most common are polysaccharides whose repeating units are residues of α-D-glucopyranose or its derivatives. The best known and most widely used is cellulose. In this polysaccharide, an oxygen bridge links the 1st and 4th carbon atoms in adjacent units, such a bond is called α-1,4-glycosidic.

The chemical composition similar to cellulose is starch, consisting of amylose and amylopectin, glycogen and dextran. The difference between the former and cellulose is the branching of macromolecules, and amylopectin and glycogen can be classified as hyperbranched natural polymers, i.e. dendrimers of irregular structure. The branch point is usually the sixth carbon of the α-D-glucopyranose ring, which is linked by a glycosidic bond to the side chain. The difference between dextran and cellulose is the nature of the glycosidic bonds - along with α-1,4-, dextran also contains α-1,3- and α-1,6-glycosidic bonds, the latter being dominant.

Chitin and chitosan have a chemical composition different from cellulose, but they are close to it in structure. The difference is that at the second carbon atom of α-D-glucopyranose units linked by α-1,4-glycosidic bonds, the OH group is replaced by –NHCH3COO groups in chitin and –NH2 group in chitosan.

Cellulose is found in the bark and wood of trees, plant stems: cotton contains more than 90% cellulose, trees coniferous species– over 60%, deciduous – about 40%. The strength of cellulose fibers is due to the fact that they are formed by single crystals in which macromolecules are packed parallel to one another. Cellulose forms the structural basis of representatives not only flora, but also some bacteria.

In the animal world, polysaccharides are “used” only by insects and arthropods as supporting, structure-forming polymers. Chitin is most often used for these purposes, which serves to build the so-called exoskeleton in crabs, crayfish, shrimp. From chitin, deacetylation produces chitosan, which, unlike insoluble chitin, is soluble in aqueous solutions of formic acid, acetic acid and hydrochloric acid. In this regard, and also thanks to the complex valuable properties combined with biocompatibility, chitosan has great prospects for a wide range of practical application soon.

Starch is one of the polysaccharides that act as a reserve food substance in plants. Tubers, fruits, and seeds contain up to 70% starch. The stored polysaccharide of animals is glycogen, which is found mainly in the liver and muscles.

The strength of plant trunks and stems, in addition to the skeleton of cellulose fibers, is determined by the connective plant tissue. A significant part of it in trees is lignin - up to 30%. Its structure has not been precisely established. It is known that this is a relatively low molecular weight (M ≈ 104) hyperbranched polymer, formed mainly from phenol residues substituted in the ortho position by –OCH3 groups, in the para position by –CH=CH–CH2OH groups. Currently, a huge amount of lignins has been accumulated as waste from the cellulose hydrolysis industry, but the problem of their disposal has not been solved. The supporting elements of plant tissue include pectin substances and, in particular, pectin, which is found mainly in cell walls. Its content in apple peels and the white part of citrus peels reaches up to 30%. Pectin belongs to heteropolysaccharides, i.e. copolymers. Its macromolecules are mainly built from residues of D-galacturonic acid and its methyl ester, linked by α-1,4-glycosidic bonds.

Among the pentoses, the most important are the polymers arabinose and xylose, which form polysaccharides called arabins and xylans. They, along with cellulose, determine the typical properties of wood.