Carbohydrate chemistry - PowerPoint Presentation

Carbohydrate chemistry Dr. Yasser Elghobashy Medical Biochemistry and Molecular Biology

Definition and classificationDefinition: CHO are aldehyde or ketone derivatives of polyhydric alcohols or compounds that can be hydrolyzed to them. Classification: according to hydrolytic products Monosaccharides : are simple sugars that can't be hydrolyzed into smaller molecules. Disaccharides: 2 monosaccharides Oligosaccharides: 3 – 10 monosaccharides Polysaccharides: more than 10 monosaccharides

MonosaccharidesMonosaccharides can be classified according to: The number of carbon atoms into: Trioses: 3 carbons Tetroses : 4 carbons Pentoses : 5 carbons Hexoses: 6 carbons Heptoses : 7 carbons The reactive group into: Aldoses: contain aldhyde group Ketoses: contain ketone group Nomenclature can be combined e.g glucose is an aldohexose while fructose is an ketohexose.

Structure of monosaccarides Trioses: Glyceraldhyde Dihydroxyacetone Tetroses: Pentoses: Erthrose Erythrulose Ribose Ribulose xylose xylulose

Hexoses Glucose Mannose Galactose Fructose ( Aldo hexose) (Aldohexose) (Aldohexose) (Ketohexose)

Cyclic forms of sugarsHexoses and pentoses are present in a cyclic structure called Hawarth forms. Glucose is present in a six-numbered ring called pyran ring forming glucopyranose.While fructose and ribose are present in a five-numbered ring called furan ring forming fructofuranose and ribofuranose.The hemiacetal bond present in glucose and ribose is formed between an alcohol and aldhyde group.While the hemiketal bond present in fructose is formed between an alcohol and ketone group.In glucose the –OH group on C5 react with the carbonyl group on C1 to form stable, cyclic hemiacetal.

Cyclic structure of glucose:

IsomerismIsomers: These are compounds having the same molecular formula but differ in spacial configuration. It is due to the presence of asymmetric carbon atom . Asymmetric carbon atom: is a carbon atom attached to 4 different groups or atoms.

Types of isomersD and L isomers: The orientation of the H and OH groups around the carbon atom adjacent to the terminal primary alcohol carbon (carbon 5 in glucose) determines whether the sugar belongs to the D or L series. D form: when the –OH group is on the right side. L from: when the –OH group is on the left side.This in relation to the smallest monosaccharide (glyceraldhyde) which is known as a parent carbohydrate.Most monosaccharides occurring in mammals are D sugars

2. Anomers: These are isomers differing in the orientation of the –H and –OH groups around the anomeric C atom. Anomeric carbon atom: Is a new asymmetric carbon atom that is created by cyclization at the carbon bound to oxygen in hemiacetal formation e.g C1 in glucose.α form if the -OH is on the right of the anomeric C.β form if the -OH is on the left of the anomeric C.In solution, α and β sugars change slowly to an equilibrium mixture of both, a process kown as mutarotation.

3. Epimers:These are isomers which differ in the orientation of –H and –OH groups around C2, C3 and C4 of glucose.The most important epimers of glucose are mannose at C2 and galactose at C4.

4. aldo-keto isomers: Fructose has the same molecular formula as glucose but differs in its structural formula , since there is a keto group in position 2, which is the anomeric carbon of fructose, whereas there is an aldehyde group in position 1, the anomeric carbon of glucose.5- Optical activity:It is the ability of sugars to rotate the plane polarized light (PPL).Specific rotation: If the PPl is rotated to the right (clock wise) the compound is dextrorotatory (indicated by d or +).If the PPl is rotated to the left (anticlock wise) the compound is levorotatory (indicated by l or -).Glucose is dextrorotatory, fructose is levorotatory.

Derived Monosaccarides Sugar acids: These are acids resulting from oxidation of C1 and or C6 of aldoses Example : acids of glucose are formed by oxidation of; Aldhyde carbon (C1)→ aldonic acid (gluconic)Hydroxyl carbon (C6)→uronic acid (glucuronic)Both → Aldaric acid (glucaric or saccharic)

2. Amino sugars:In which the hydroxyl group at C2 is replaced by an amino group.Examples: Glucosamine: is a constituent of hyaluronic acid. Galactosamine is found in polysaccharide of cartilage, chondroitin sulfate. It may occur in acetylated form e,g glycoproteins.

3. Deoxy sugars: Obtained by removal of oxygen atom.Example : 2-deoxyribose found in DNA 4. Sugar alcohols : Formed by reduction of monosaccharide at the carbonyl carbon Examples:

D-glucose → D-sorbitol.D-mannose → D-mannitol.D-galactose → D-dulcitol . D-fructose → D- mannitol and D-sorbitol. D-ribose → D- ribitol.D-ribitol is a constituent of riboflavin (vitamin B2) and in hydrogen carriers FAD and FMN.

DisaccharidesMaltose Maltose is called malt sugar It is formed of 2 units of α -glucose linked together by α 1-4 glucosidic bond.It is a reducing sugar as it has a free carbonyl group

B. LactoseLactose is the milk sugarIt is formed of α -glucose and β - galactose linked together by β 1-4 galactosidic bondIt is a reducing sugar as it has a free carbonyl group.

C. SucroseIt is called cane sugar. It is formed of α -glucose and β -fructose linked by α 1-β2 glycosidic bond (C1 of glucose and C2 of fructose).It is not reducing as it has no free aldhyde or ketone group so it will not reduce an alkaline copper reagent such as Fehling’s solution.

Invert sugar:It is a mixture of glucose and fructose which result from the hydrolysis of sucrose.It is called “invert sugar” because the strongly levorotatory fructose changes (inverts) the previous dextrorotatory action of sucrose. N.B. A sugar can react with an alcohol to form an acetal known as glycoside e.gCardiac glycosides: which contain steroids (as alcohols). These include derivatives of digitalis used in the treatment of heart failure.Ouabain: an inhibitor of the Na-K ATPase of cell membranes.Streptomycin: is an antibiotic.

Polysaccharides Homogeneous or homo-polysaccharides Starch: Is a homopolymer of glucose It forms α-glucosidic chain called a glucosan.It is the storage form of glucose in plants.It is a mixture of amylose and amylopectin.Amylose: Linear unbranched polymer of α-D-glucose linked by α1-4 glycosidic bond.Amylopectin: Branched polymer of α -D-glucose linked by α1-4 glycosidic bond and α1-6 at the branch point which contain 25-30 glucose unit.

2. Glycogen:It is the storage form of CHO in animals.Found mainly in liver and muscle. It is a highly branched form of amylopectin (containing both α 1-4 and α 1-6 linkages). The branch contains 8 – 10 glucose units.3. Cellulose:It is formed of β-glucose units linked by β 1-4 glycosidic bond.It is unbranched.Is a structural polysaccharide in plant cells.It forms part of the human diet (e.g in vegetables and fruits), but not hydrolyzed by human enzyme systems.

4. Inulin:It is a polysaccharide formed of fructose (fructosan) . Found in artichokes, tubers and roots of dahlias. It is used to determine the glomerular filtration rate. 5. Dextrins : These are intermediates in the hydrolysis of starch.6. Dextrans.

II. Glycosaminoglycans (mucopolysaccharides) These are (complex) heteropolysaccharides . Characterized by high content of amino sugars and uronic acids.It is called proteoglycan when attached to a protein molecule.Proteoglycans provide the ground substance of connective tissues.Examples:Hyaluronic acid: It is formed of β-glucuronic acid and N-acetyl glucosamine.Found in synovial fluid, vitreous body of the eye, cartilage and loose connective tissue.

2. Chondroitin sulfate:It is formed of β-glucuronic acid and N-acetyl galactosamine . It is a prominent component of cartilage. 3. Heparin: It is formed of sulfated glucosamine (few are acetylated) and sulfated iduronic acid.It is an important natural anticoagulant.Secreted by the mast cells.It activates lipoprotein lipase enzyme.

Glycoproteins (Mucoproteins)They are proteins containing oligosaccharides. Present in many fluids and tissues including the cell membranes. It contains sialic acid .