Dr. MEHREZ Ikram BIOCHEMISTRY Chapter 1: Carbohydrates

2 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram Chapter (I): Carbohydrates Introduction Definition Carbohydrates are molecules of organic origin, characterized by the presence of carbon chains carrying hydroxyl groups (OH) and aldehyde (-CHO) or ketone (-C=O) carbonyl groups. They are known as carbohydrates (former name) or carbohydrates in English. 1. Origins and biological importance In plants, carbohydrates are produced by the combination of CO2 and water (H2O) through the process of photosynthesis. In animals, most carbohydrates are obtained through food, however, there is a process that allows carbohydrates to be obtained from non-carbohydrate molecules called gluconeogenesis. Carbohydrates are very important; they are the most abundant biomolecules in nature. They are important for: - Metabolic and energetic: as energy reserves (glycogen in animals and starch in plants), as a source of energy through carbohydrate oxidation, or as fundamental molecules in nucleic acids and enzymes (coenzymes). - Structural: they are fiber- or gel-shaped structural elements that act as support and protection (cellulose in plants, chitin in invertebrates, polysaccharides in bacterial walls). - Functional: they are involved in intercellular recognition signals, differentiation mechanisms, and the expression and reception of antigenic determinants. 2. Classification: Carbohydrates are divided into two classes according to their complexity: monosaccharides (simple carbohydrates) and disaccharides (complex carbohydrates). Monosaccharides: they have a basic structure Cn(H2O)n, which has: - (n) number of carbons. - (n-1) alcoholic functions. - A carbonyl function (aldehyde or ketone). They are classified according to two criteria: the number of carbon atoms (triose, tetrose, pentose, hexose, or heptose) and the nature of the carbonyl function (aldehyde ⇒ aldosis, ketone⇒ketosis).

3 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram ➢ Classification according to the number of carbon atoms: 3 Carbons: Triosis; 4 Carbons: Tetraosis; 5 Carbons: Pentosis; 6 Carbons: Hexosis; 7 Carbons: Heptosis. ➢ Classification according to the nature of the reducing function: * An osose with an aldehyde function is an aldose; * An osose with a ketone function is a ketose. Combining the two classification criteria gives us: Table 1: Classification of oses C3: Triosis C4: Tetrosis C5: Pentosis C6: Hexosis C7: Heptosis Aldosis Aldotriosis Aldotetrosis Aldopentosis Aldohexosis Aldoheptosis Ketosis Ketotriosis Ketotetrosis Ketopentosis Ketohexosis Ketoheptosis Osides: these are combinations resulting from the association of several oses molecules, with the possible presence of certain non-carbohydrate substances. 3. OSES a. Linear structure of oses There are more than 200 known natural oses, all chemically related to glucose (aldohexose) or fructose (ketohexose). They are generally solid compounds, appearing as white crystals that are soluble in water. Some do not have a sweet taste. Although most oses share the same molecular formula, they do not have the same physical and chemical properties due to their spatial conformation. It is therefore important to review the concepts of isomerism in order to better understand them. i. Representation of oses All monosaccharides except dihydroxyacetone (DHA) contain one or more asymmetric carbon atoms, giving rise to optically active isomeric compounds. The simplest aldose, glyceraldehyde, has a chiral (asymmetric) center at the at the middle carbon, which gives rise to two optical isomers called ENANTIOMERS. By convention, one of the enantiomers is designated as the D isomer and the other as the L isomer. As with most biomolecules with a chiral center, the absolute configuration of sugars is known through X-ray crystallography. To represent the 3D structure of a sugar on paper,

4 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram 90° Fischer Projection or Fischer projection. In this projection or representation, horizontally oriented lines represent bonds that exist in the plane perpendicular to the plane of the sheet and point toward the observer, while vertically oriented lines represent bonds that exist in the same plane but point away from the sheet. For oses with a longer carbon chain, and therefore a greater number of asymmetric carbons, the following steps are followed: - Pass the projection plane (the sheet) through the asymmetric carbon C* (center of chirality). - The longest carbon chain is behind the projection plane (vertical line) - The carbon atom at the top of the vertical chain is the one involved in the functional group with the highest oxidation state. - The other two non-carbon substituents of the asymmetric carbon are placed in front of the projection plane (represented by horizontal lines). Note: by conventional carbons carrying an a II ary lcohol function and their hydroxyl groups are represented by horizontal lines in the Fischer projection. According to Fischer's convention, if the OH carried by carbon (n-1) is on the right, the oses belong to the D series (natural series of oses); otherwise, if it is on the left, the oses belong to the L series

5 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram b. D and L nomenclature of oses All oses (aldoses and ketoses) are prefixed with the letters D or L. This means that the series of oses is indicated by the position of the OH carried by the preterminal carbon (Cn-1). We refer to the D series when the OH of Cn-1 is oriented to the right and the L series when it is oriented to the left. Generally, natural oses are of the D series. Figure 1. The D and L series of glyceraldehydes. c. Concept of chirality and optical activity of oses • Concept of chirality Any object that cannot be superimposed on its mirror image is a chiral or asymmetric object. This chirality is due to the presence of a center of asymmetry called an asymmetric carbon or chiral carbon, which has four different substituents and is often denoted C*. A very simple example of oses is glyceraldehyde, which contains a center of chirality, the central carbon atom (C2), which has four different substituents: CHO, CH2OH, OH and H

6 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram Organic substances with an asymmetric carbon are chiral molecules and have optical activity because their molecules lack an element of symmetry (axis or plane). Dihydroxyacetone has no asymmetric carbon and therefore no optical activity; it occurs in only one form. • Concept of isomers An oses with n asymmetric carbons will have 2n stereoisomers; glyceraldehyde, which has a single asymmetric carbon, will therefore have two (21) stereoisomers: D-glyceraldehyde and L-glyceraldehyde. It therefore has two optical isomers, or enantiomers, whose 3D structures are mirror images of each other. For aldoses, the number of stereoisomers is 2n-2 where n is the number of carbons in the chain. Example: Aldohexoses (such as glucose) where n is equal to 6. The total number of stereoisomers is equal to 24 = 16 (8 from the L series and 8 from the D series). Similarly, for ketoses, the number of stereoisomers is 2n-3. Certain concepts are widely used to characterize isomers: Isomer: Compounds are referred to as isomers if they have the same general formula but a different spatial arrangement in the plane (2D). Enantiomers: one molecule is the mirror image of the other.

7 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram Epimers: When two sugars differ only in the position of a hydroxyl group on a single asymmetric carbon. D-glucose and D-galactose are epimer oses at carbon 4. Diastereoisomers: two isomers are called diastereoisomers if the difference is in the number of chiral carbons C* between 1 and their total number n of C*. Example: D-glucose and D- gulose are diastereoisomers because they differ in the configurations of 2 (C3 and C4) of their 4 chiral carbons.

8 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram exp: D-Erythrose (a) and L-Erythrose (b) are enantiomers, as are D-Threose (d) and L-Threose (c). Therefore, the diastereoisomer pairs are (a and c), (a and d), (b and c), (b and d). Functional isomers: two functional isomers have the same linear structure and the same number of carbon atoms, but differ in their carbonyl group. Example: D-glucose and D- fructose have the same formula C6H12O6 but not the same structural formula (because they differ in their function).

9 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram • The optical rotation of sugars The optical rotation of a molecule is its ability to deflect the plane of vibration of polarized light passing through it. We say that the molecule has optical activity. Sodium D-line was used as a polarizer to orient the light horizontally, and measurements were taken at a temperature of 20°C. When a light beam is directed at a crystal called a polarizer, a ray emerges from this crystal on a single plane. This light is called polarized light. Measuring the angle of rotation α allows us to characterize the optical rotation of an optically active substance. Specific optical rotation is defined by BIOT's relation as follows: Α: Angle of rotation (deviation) of the plane of polarization measured in degrees (°) on the polarimeter. C: Concentration of the substance (g/ml). L: Length of the polarimeter tube containing the solution expressed in decimeters (dm). The abbreviations D and L placed before the name of the sugar molecule are therefore only an indication of the series and do not imply the direction of the optical rotation. This can only be determined experimentally using a polarimeter. The direction of deviation of polarized light is indicated by the following signs: (+): deviation of polarized light is to the right, (-) deviation of polarized light is to the left. Example: D-glyceraldehyde is dextrorotatory: D (+) deviates light to the right, i.e., the compound deviates the plane of polarized light to the right (d) or, more precisely, clockwise, and D-fructose is levorotatory: D (-) deviates light to the left, or counterclockwise. Note: The abbreviations D and L do not refer to the nature of the optical rotation, dextrorotatory (+) or levorotatory (-). Figure 2. Principle of polarimetry. C.1 Chemical classification of sugars according to Fischer From an oses with n number of carbons, it is possible to obtain oses with n+1 carbons by Kiliani-Fisher synthesis. Symmetrically, Wohl degradation allows the transition from the sugar with n C to one with n-1 C.

10 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram C.1.a Aldose pathway Starting with glyceraldehyde (D or L), the number of carbon atoms can be increased by extending its C1 end: we move from aldotriose to aldotetrose, then to aldopentose and finally to aldohexose. To lengthen the carbon chain of glyceraldehyde, it must be treated with hydrocyanic acid (HCN). Hydrocyanic acid is added to the aldehyde function to form a cyanhydrin. Through hydrolysis, the aldose (n+3) C can be converted to amide, then to aldonic acid, and from there, through reduction, to aldehyde by sodium amalgam in an acidic environment, leading to the formation of a new aldose with one more carbon atom. Thus, from D- glyceraldehyde, we obtain: 2 aldotetroses, 4 aldopentoses, and 8 aldohexoses. In conclusion, we can say that: The chain is elongated at the reducing end of the sugar (CHO). This elongation creates a new asymmetric carbon atom. This chemical synthesis will give rise to two isomers (racemization). The number of stereoisomers for a given oses will therefore be equal to 2n (n being the number of asymmetric carbons).

11 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram Figure 3. Aldose family tree. NB: Not all 16 hexose isomers exist in nature. There are three known natural aldohexoses: D- glucose, D-mannose, and D-galactose. Of these three isomers, glucose is widely distributed in either free or polymerized form.

12 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram C.1.b Ketose pathway Dihydroxyacetone (ketotriose) is the starting point for the ketose family tree. This triose has no asymmetric carbons, so the differentiation of ketoses only begins with erythrulose, which has an asymmetric carbon. Figure 5. Ketose lineage. D. Cyclic structure of oses (HAWORTH cyclic representation) The linear form of oses is a simple but incomplete structure; it does not explain the

13 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram physicochemical characteristics of oses. Sugars with more than 4 carbon atoms in their skeleton usually occur in solution in cyclic form, in which the reducing function (aldehyde or ketone) is engaged in a covalent bond with one of the free hydroxyl groups of the skeleton. The Haworth representation facilitates the representation of different cyclic forms. The most oxidized carbon (aldonic or ketonic function) is positioned at the right end. The distribution of hydroxyl groups (OH) depends on their position in the Fisher representation. The OH groups on the right in the Fisher representation will be found below the plane cycle. For glucose, the configuration of C5 determines the D or L series in the Fisher representation. Thus, in the Haworth representation, the position relative to the plane of the primary alcohol function will determine the series: D series for CH2OH above the plane of the cycle, L series for CH2OH below the plane. In the simplified representation, carbons and hydrogens are not shown, an OH groups are represented by vertical lines. Table 2. Difference between the linear and cyclic structures of oses. Linear structure Cyclic structure OH on the right. OH on the left. OH below the cycle. OH above the cycle Reducing carbonyl function aldehyde or ketone Reductive hemiacetal function Non-asymmetric reducing C Asymmetric reducing C Hemiacetal OH at the bottom: α (trans position). Hemiacetal OH at the top: β (cis position) • Cyclization of aldoses An osidic bridge is formed by the formation of an internal hemiacetal bond between the aldehyde group and one of the alcohol groups of the same oside, thus forming a ring. This reaction can take place with the C1-C5 carbon pair to form a 6-membered oxygen heterocycle: pyranose (this is the stable form) or with the C1-C4 carbon pair to form a 5- membered oxygen heterocycle: furanose (this is the unstable form).

14 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram Figure 6. Cyclization of C1-C5 Aldoses Figure 7. Cyclization of C1-C4 aldoses

15 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram • Cyclization of ketoses Like aldoses, ketoses can also undergo cyclization. In this case, internal hemiacetalization occurs between the ketone group and a hydroxyl group carried by one of the carbons of the same sugar. Between carbons C2-C6: a 6-membered heterocycle called the pyran form (pyranose) is obtained (this is the unstable form) or between carbons C2-C5: a 5-membered heterocycle called the furan form (furanose) is obtained (this is the stable form). Figure 8. Cyclization of C2-C6 ketoses Figure 9. Cyclization of C2-C5 ketoses.

16 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram 4. Physicochemical properties of oses 4.1 Physical properties of oses 4.1.1 Solubility and crystallization Their high hydroxyl (OH) group content gives them polar properties capable of multiple hydrogen bonds: - with water: they are water-soluble - with other compounds such as proteins Concentrated aqueous solutions are viscous, like syrups (crystallization is very difficult). Crystallization is facilitated by the addition of alcohol (methanol or ethanol), in which oses are poorly soluble. Oses are soluble in methanol and insoluble in ether. 4.1.2 Optical rotation Compounds with asymmetric carbons deviate the plane of polarization of polarized light; these compounds are said to have optical activity. All sugars (except dihydroxyacetone) have optical rotation. Examples of the specific optical rotation of some oses: Glucose: +52.5°, Fructose: -93°, Sucrose: +66.5°, Lactose: +55.5°, Cellobiose: +35°. 4.1.3 Absorption spectrum Sugars do not absorb visible or ultraviolet light. However, they do have a characteristic infrared absorption spectrum. 4.2 Chemical properties of oses The chemical properties of oses are characteristic of alcoholic hydroxyl (OH) functions and carbonyl functions. 4.2.1 Chemical properties due to the carbonyl function 4.2.1.1 Reduction of oses The carbonyl function of aldoses and ketoses can be converted into an alcohol function by chemical reduction with an alkali borohydride (NaBH4 or LiBH4) to produce polyalcohols called alditols. The nomenclature of alditols is obtained by replacing the suffix -ose with the suffix -itol. For example, D-glucose produces D-glucitol.

17 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram The reduction of a ketose produces two epimeric alditols. D-fructose reduced by NaBH4 produces an equimolecular mixture of D-glucitol and D-mannitol, which are C2 epimeric alditols. 4.2.1.2 Oxidation of oses All oses have a reducing function (reducing power): aldehyde function or ketone function. These are reducing agents that undergo oxidation during their reaction. • Mild oxidation in an alkaline medium Mild oxidants, such as bromine or iodine (Br2, I2) in an alkaline medium, or highly diluted nitric acid, oxidize the aldehyde function of aldoses into a carboxylic group (C1), leading to the formation of an aldonic acid (R-COOH). The ketone function of ketoses cannot be oxidized by iodine or bromine in an alkaline environment. For their nomenclature, simply change the suffix "ose" to the suffix "onic."

18 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram Examples: glucose yields gluconic acid, mannose yields mannonic acid, galactose yields galactonic acid. • Strong oxidation This is a more advanced oxidation carried out by nitric acid, producing aldaric acids, which are diacids with a carboxylic function on carbons 1 and 6. The product of advanced oxidation is named by replacing the suffix "ose" with the suffix "aric," for example, glucose gives glucaric acid, galactose gives galactaric acid.

19 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram The oxidation of ketoses by HNO3 breaks down at the ketone function, leading to an oxidative cleavage of the carbon skeleton. • Oxidation with heavy metal salts Certain sugar compounds have reducing power (they supply protons and electrons). In an alkaline environment, metal salts such as copper, iron, silver, mercury, etc. are reduced by the pseudo- aldehyde function. An example of this is Fehling's solution, obtained by mixing solutions of copper sulfate (CuSO4) and double sodium and potassium tartrate. In the presence of Fehling's solution, the sugar is oxidized by cupric oxide (blue), which is reduced to insoluble cupric oxide (brick red), and the aldose is oxidized to aldonic acid according to the following reaction: 4.2.1.3 Properties related to the alcohol function • Phosphoric ester formation (esterification) The primary and secondary alcohol functions of oses can be esterified by phosphoric acid phosphoric acid (H3PO4) to form phosphoric esters. This reaction is called "Esterification," which occurs through the combination of alcohol and acid, leading to the formation of esters. These compounds are very important in most metabolic reactions. Examples: monophosphoric esters of oses, e.g., glucose 6 phosphate;

20 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram diphosphoric esters: fructose 1-6 diphosphate; polyphosphoric esters: ATP. • Methylation of oses This is a type of etherification. Methylation produces ethers (R-O-CH3) by attaching a -CH3 to an OH. Methylation of oses is carried out using methylating agents such as methyl iodide (ICH3) with silver oxide (Ag2O) or dimethyl sulfate (CH3)2SO4 in an alkaline medium (NaOH). • Oxidation by periodic acid (HIO4) Periodic acid nder a hydrated hydrated (IO6H5 =metaperiodate) has the property of cutting the carbon chain by breaking the covalent bond carrying the free α glycol (OH). Two carbonyl functions appear as follows: When there are several free alcoholic functions nearby, the breakage by HIO4 molecules occurs between: HIO4 molecules occurs: - The primary alcohol function will give rise to formic aldehyde HCHO.

21 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram - The secondary alcohol functions will give rise to formic acid HCOOH. • Oxidation of the primary alcohol function If the aldehyde function was protected during oxidation, the oxidized uronic acids are obtained only on the primary alcohol group. To determine the nomenclature of the uronic acid formed, replace the suffix "ose" with the suffix "uronic": glucose forms glucuronic acid, which is the precursor in the vitamin C synthesis pathway. 5 Ose derivatives 5.1 Osamines These are osidic molecules in which a C2 alcohol function has been replaced by an amine (NH2) or N-acetylamine (NH-CO-CH2). The most important are hexosamines, derivatives of glucose such as "glucosamine" or "N-acetylglucosamine," or derivatives of galactose such as "galactosamine."

22 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram Osamines have the same characteristics as oses. They are found: - in polymerized form, for example in chitin in arthropods; - in the structure of murein (bacterial cell wall); - in glycoproteins. 5.2 Uronic acids The oxidation of the primary alcohol function on C6 leads to the formation of uronic acids; glucose produces glucuronic acid, galactose produces galacturonic acid, etc. They play an essential role in detoxification processes by giving rise to water-soluble glucurono-conjugated derivatives. 5.3 Deoxyoses In their structure, a hydrogen atom (H) replaces the hydroxyl group (OH). The most important is 2-deoxy-D-ribose, which is a constituent of DNA. 5.4 Ascorbic acid or Vitamin C A hexonic acid with an enediol group between carbons 2 and 3. This bond is very unstable

23 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram and easily oxidized to form dehydroascorbic acid. This iswhy it participates in intracellular oxidation-reduction reactions. 6 Osides Osides are polysaccharides linked by osidic bonds. The osidic bond is formed by the condensation of two or more oses molecules through the elimination of a water molecule. There are two types of osidic bonds: - Oligosaccharide-oligosaccharide bond: - Oligosaccharide-oligosaccharide bond: between two carbons, one of which carries a hemiacetal OH group and the other does not. The resulting oligosaccharides are reducing. These include heterosides, whose degradation releases oses and non-carbohydrate compounds (such as aglycone), and holosides, whose degradation releases oses. There are also oligosides and polyosides, which differ in terms of the number of monomers forming the polymer. 6.1 The osidic or glycosidic bond This bond is formed between two carbons carrying the hemiacetal OH: C1-C1 between two aldoses, C1-C2 between an aldose and a ketose, and C2-C2 between two ketoses. Two types of bonds can be defined: - Osidic-ose bond: hemiacetal OH + alcohol OH (reducing diholoside). The diholoside

24 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram therefore retains a free hemiacetal alcohol -OH group, which is responsible for the molecule's reducing power. - Osid-oside bond: hemiacetal OH + hemiacetal OH (non-reducing diholoside, no free hemiacetal OH). 6.2 Nomenclature The oside bond is defined by the anomeric position of the oside that fulfills its hemiacetal function, and by the number of the atom of the other oside. Osides are named from top to bottom or from right to left. Generically, the name will be: - Name of 1st oside + osyl/osido (α/ß 1 (anomer) →n) name of 2nd + oside (n different from anomeric carbon = 2, 3, 4, 5, or 6). - Name of 1st sugar + osyl/osido (α/ß 1 (anomer)→ α/ß 1 (anomer)) name of 2nd sugar + oside.

25 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram 6.3 Holosides 6.3.1 Oligosides Three diholosides are found in free form, their molecular formula is C12H22O11, these are lactose, sucrose, and trehalose (insect hemolymph, fungi). The others come from the breakdown of polysaccharides. 7.3.1.1. Reducing diholosides This is a diholoside that carries a free hemiacetal OH function. • Lactose This is the sugar found in the milk of all mammals. It is a reducing diholoside containing one galactose molecule and one glucose molecule linked by a β 1-4 osidic bond. Lactose is a: β - D- galactopyranosyl-(1,4) -D-glucopyranose. • Maltose This is a product obtained by the breakdown of starch and glycogen. Through hydrolysis, it

26 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram yields two molecules of D-glucose. Maltose is an α D-glucopyranosyl (1–4) D-glucopyranose. • Isomaltose This is a product obtained by the breakdown of starch and glycogen. It is formed of two glucose molecules linked by an (α1–6) bond. Isomaltose is an α D-glucopyranosyl (1–6) D- glucopyranose. • Cellobiose This is a product of cellulose degradation. Through hydrolysis, it yields two molecules of D- glucose. It is formed of two glucose molecules linked by a (β1–4) bond. Cellobiose is a β D- glucopyranosyl (1–4) D-glucopyranose.

27 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram 7.3.1.2. Non-reducing disaccharides A non-reducing disaccharide is an osido-oside in which the bond locks the two oses in one of the cyclic anomeric forms (α or ß). No hemiacetyl OH is free and the diholoside has no reducing power. • Sucrose Sucrose is an osido-oside. It is found in sugar cane stalks and in roots of sugar beets. Sucrose is an α-D-glucopyranosyl-(1,2) β-D-fructofuranoside. • Trehalose This is a diholoside widely distributed in fungi, bacteria, and even in the hemolymph of insects. Many organisms accumulate it in response to thermal shock (cold) or desiccation. It is an α D- glucopyranosyl (1-1) α D-glucopyranoside. • Raffinose Raffinose is a trisaccharide containing galactose, glucose, and fructose molecules found in beets and removed during sugar refining. Raffinose is α-D-Galactopyranosyl-(1→6)-α-D- glucopyranosyl-(1→2)-ß-D-fructofuranoside.

28 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram 6.3.2 Polysaccharides These are polysaccharides consisting of chains of several dozen to several thousand sugar units. 6.3.2.1 Starch This is a widely distributed plant polysaccharide that plays a very important nutritional role in humans and animals. It consists of: - Amylose, which accounts for 20% of starch (300 to 1000 D-glucose residues, linked by an glycosidic bond (α1–4), is soluble in warm water and crystallizes when cooled. - Amylopectin, which accounts for 70 to 95% of starch, forms a viscous gel when heated, repeating approximately every 20 to 30 residues, formed by an (α1–6) bond on the main chain (α1–4) of the branch points. This is a molecule that animals store in the cytosol of hepatocytes and in muscles. It is a more branched polysaccharide than starch, with more branches: D-glucose units >50,000, with one

29 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram branch every 10 units containing α-1,6 glucosidic bonds. • Inulin This is a β-D-fructofuranose polysaccharide of 30 to 100 units linked by β (2–1) bonds and linked by an α (1 → 2) bond to a glucopyranose molecule. It is found in certain plants. • Dextran These are polymers of α-D-glucose linked by (α1 → 6) bonds, with branches on C3 or C4. They are found in dental plaque, produced by oral bacterial growth. 6.4 Heterosides These consist of a sugar moiety and a non-carbohydrate moiety called an aglycone. • Glycolipids These are the combination of oligo- or polysaccharides (hydrophilic part) with the lipids of animal or bacterial cell membranes (hydrophobic part).

30 BIOCHEMESTRY Chapter 1 : Carbohydates Dr. MEHREZ Ikram • Proteoglycans (PG): Proteoglycans are essential molecules in the extracellular matrix, the plasma membrane, and the glycocalyx, playing a role in cell-matrix interactions. • Glycoproteins (GP) These are formed by the association of a carbohydrate part (at least 5% of the total) and a protein part. • Peptidoglycans These are a network of polysaccharides linked by numerous small peptides. • Glycated proteins These are molecules with a glucose unit chemically attached to them. Hyperglycemia in insulin- dependent diabetes promotes the attachment of glycated proteins to plasma proteins (a marker of diabetes). Bibliographic references - Meriem, Zerriouh. Biochemistry Course. 2019. - Leïla, Halitim. Structural Biochemistry Course, 1st Year Medicine. 2014. - Mourad, Nechi. STRUCTURAL & METABOLIC BIOCHEMISTRY/CARBOHYDRATES. 2020. - Nadia, Besbes. Linear Structure of Sugars. Structural Biochemistry/Carbohydrates. 2021 - Nadia, Besbes. Cyclic structure of sugars. Structural Biochemistry/Carbohydrates. 2021 - Cummings, J. H., and A. M. Stephen. "Carbohydrate terminology and classification." European journal of clinical nutrition 61.1 (2007): S5-S18. - Horton, Derek. "The development of carbohydrate chemistry and biology." Carbohydrate chemistry, biology and medical applications. Elsevier, 2008. 1- 28. - Maughan, Ron. "Carbohydrate metabolism." Surgery (Oxford) 27.1 (2009): 6-10.