Chapter 2: Glucides | Document Summary

# Chapter 2: Glucides ## I. Glucides (Carbohydrates) ### 1. Introduction Carbohydrates are organic molecules characterized by the presence of carbon chains carrying hydroxyl groups, aldehyde or ketone functions, and possibly carboxyl or amine functions. They are divided into oses and osides. ###...

# Chapter 2: Glucides ## I. Glucides (Carbohydrates) ### 1. Introduction Carbohydrates are organic molecules characterized by the presence of carbon chains carrying hydroxyl groups, aldehyde or ketone functions, and possibly carboxyl or amine functions. They are divided into oses and osides. ### 2. Classification of Carbohydrates - **Oses:** Also called simple sugars or monosaccharides, have the gross formula: *C_nH_2nO_n*. - They are non-hydrolysable and usually contain 3 to 7 carbon atoms. - **Osides:** These are hydrolysable sugars and can be: - **Holosides:** Their hydrolysis releases only oses. A distinction is made between: - **Oligosides:** Association of 2 to 10 oses by osidic bonds. - **Polyosides:** Polymer made up of 10 to several thousand oses. - Homogeneous polyoside (or homo-polyoside) for a polymer of the same ose. - Mixed polyoside (or hetero-polyoside) for a sequence of different units. - **Heterosides:** Hydrolysis releases oses and non-carbohydrate compounds (aglycones). ## Classification of Glucides | Glucides | oses | osides | |---|---|---| | | Monosaccharides | Holosides | | | Unités simples | hydrolyse | | | non hydrolysables | 1 ou plusieurs types d'oses | | | + | Héterosides | | | dérivés d'oses | Polyosides | | | | (Polysaccharides) | | | | X unités > 10 | | | | O-Héterosides | | | | N-Héterosides | | | | S-Héterosides | | | | Homopolyosides | | | | (1 seul type d'oses) | | | | Hétéropolyosides | | | | (Pls types d'oses) | | | n-1 fonctions alcools | hydrolyse | | | = (CH2O)n | Fraction glucidique | | | 1 fonction carbonyle | + | | | n atomes de carbone | Fraction non glucidique | | | | aglycone | | | Aldoses | | | | 1 fonction aldéhyde sur C1 | | | | Cetoses | | | | 1 fonction cétone sur C2 | | | | X-2 (diholoside) | | | | X-3 (tiholoside) | | | | X= 4 (tetraholoside) | | | | Etc..... | | ## 3. The Oses These are polyols which carry at least 2 alcohol functions, at least one of which is a primary alcohol function, and a carbonylated reducing function, i.e.: - Aldehyde (-CHO), in which case the ose is an aldose. - Ketone (>C=O), in which case the ose is a ketose. **The classification of oses is based on two criteria:** - Number of carbon atoms in the ose (the first element has 3C) - Nature of the carbonyl or reducing function. The combination of these two criteria can be used to characterize an ose. | **C:** | **Triose** | **Tetrose** | **Pentose** | **Hexose** | **Heptoses** | |---|---|---|---|---|---| | **Aldose** | aldotriosis | aldotetrose | aldopentosis | aldohexose | aldoheptosis | | **Ketosis** | ketotriosis | ketotetrolysis | ketopentosis | ketohexosis | ketoheptosis | ## 4. The Osides Osides are polymers of oses linked by osidic bonds. They include heterosides, hydrolysis of which releases oses and non-carbohydrate compounds (aglycone), holosides, hydrolysis of which releases only oses, and oligosides and polyosides, the difference between which lies in the number of monomers forming the polymer. ### *The osidic or glycosidic bond:* An osidic bond is formed by condensation between the reducing hydroxyl (hemiacetal function) of an ose carried by the anomeric carbon (*C*₁ for aldoses and *C*₂ for ketoses), a semi-acetal OH in the α or β position, with an -OH (or -NH₂ or -SH) hydroxyl of another ose. **Three types of bond can be formed:** - OHsemi-acetal + OH primary alcohol (reducing diholoside, free 1OH semi-acetal) - OHsemi-acetal + OH secondary alcohol (reducing diholoside: idem) - OHsemi-acetal + OH semi-acetal (diholosidenon reducing not of free semi-acetal OH) **Example:** D-glucose and D-galactose ## *Nomenclature and convention:* The osidic bond is defined not only by the oses, but also by the anomer of the ose engaging its semi-acetalic function, and by the number of the atom of the other ose. The generic name will be: *α/β,D/L-X...osyl or osido(1-n)α/β,D/L-Y...ose/osie* - X: name of carbohydrate 1 - Y: name of carbohydrate 2 - n: carbon number involved in the osidic bond - Osyl/osido: this means that the hemiacetal function of the first ose is involved in the osidic linkage - Ose: this means that the hemiacetal function of the last ose is free - Oside: this means that the hemiacetal function of the last ose is involved in the osidic bond. ### Holosides ### *Oligosides:* Three diholosides exist in the free state, with a gross formula of *C₁₂H₂₂O₁₁*: lactose (animal milk), sucrose (plant) and threose (insect haemolymph, fungi). The others come from the hydrolysis of polyosides. ### *Reductive diholosides:* **Lactose:** This is the sugar found in mammalian milk at a concentration of around 50g/L. **Maltose:** This is a product of the breakdown of starch and glycogen. Hydrolysis produces 2 molecules of glucose. **Isomaltose:** This is a breakdown product of starch and glycogen. It is formed from two glucoses linked by a bond of the type (α1-6). **Cellobiose:** This is a breakdown product of cellulose. Hydrolysis produces 2glucose molecules. It is made up of two glucoses linked by a (β1-4) bond. ### *Non-reducing diholosides:* **Sucrose:** This is a diholoside found in plants. An intermediate product of photosynthesis, it is the carbohydrate carrier in plants. It is stored in the stalks of sugar cane and in the roots of beetroot. **Trehalose:** This is a diholoside found in fungi, bacteria and insect haemolymph. Many organisms accumulate it in response to thermal shock (cold) or desiccation. ### *Other oligosides:* **Raffinose:** This is a triholoside present in beetroot which is eliminated during sugar refining. ### *Polyosides:* They are made up of a chain of several dozen to several thousand oses. They are also known as polysaccharides. They have two main functions in nature: - Energy reserve (starch, glycogen,... whose oses are linked by osidic bonds in the α position). - Structural role in certain cells (cellulose, chitin,... whose oses are linked by osidic bonds in the β position). ### *Reserve polyosides:* **Starch:** This is a high polymer that is insoluble in cold water, although hydrophilic. It is in this condensed form that plants accumulate photosynthesised carbohydrates. Two homogenous fractions can be extracted: - **Amylose:** which accounts for 20% of starch (300 to 1000 D-glucose residues, linked by a glycosidic bond (α1-4), is soluble in warm water and crystallises on cooling. - **Amylopectin:** which accounts for 80% of starch, gives a viscous paste when heated. Amylopectin is distinguished by a higher number of glucose and a branched structure. On the main chain (α1-4) branching points, repeating approximately every 20 to 30 residues, are formed by a bond (α1-6). **Glycogen:** This is a polyglucose that animals store in the cytosol of hepatocytes and in muscles. It has the same structure as amylopectin but is more compact, with the following differences: - The branches occur every 8 to 12 residues and even 3 to 5 in the centre of the molecule. - The average length of branched chains is shorter. **Inulin:** This is a polymer of β-D-fructofuranose with 30 to 100 units linked by β2-1 bonds, found in certain plants such as dahlias, artichokes and Jerusalem artichokes. **Dextrans:** Bacterial and yeast reserves, these are polymers of α-D-glucose linked by bonds (α1-6), with occasional branches on C3 or C4. They are a component of dental plaque, the product of oral bacterial proliferation. They are used: - As plasma substitutes in therapeutics. - As a phase for low-pressure liquid chromatography, by grafting ionised functional groups for ion exchangers. ### *Structural polyosides:* **Cellulose:** Present in certain bacteria, it is the major constituent of plant wall fibres. It is a linear polymer with a glycosidic bond of the type: (β1-4). Cellulose represents half of the carbon available on earth, but is not a source of glucose, except for ruminants. **Chitin:** It differs from cellulose only in the C2 of glucose: its hydroxyl is replaced by the acetylamine group. This GlcNac (β1-4) polymer has the same structure as cellulose. It is found in the external skeleton of invertebrates (crustaceans, molluscs, insects). ### *Heterosides:* Glycoconjugates are molecules resulting from the covalent association of carbohydrates with other types of molecules, and are often referred to as glycoconjugates: - **Glycolipids:** association of oligo or polyosides with the lipids in the membranes of animal or bacterial cells. - **Proteoglycans (PG):** polyosides that are often very long (glycosaminoglycans or GAGs) are associated with a protein and make up the majority (>90%) of the protein. - **Glycoproteins (GP):** these are proteins on which short carbohydrate chains have been grafted, the proportion of which generally varies from 1 to 20%. - **Peptidoglycans:** a network of polysides linked by numerous small peptides. - **Glycatedproteins:** products of the chemical binding of one unit of glucose. The hyperglycaemia of insulin-induced diabetes favours the binding of this ose to plasma proteins (a marker of diabetes).