Chapter 8 Notes: Carbohydrates PDF

Chapter 8 notes: (KNOW HOW TO DRAW STUCTURES: LINEAR, curved, and final Haworth projection) Carbohydrates: Carbohydrates or saccharides (sugars) are the most abundant biological molecules. Act as a source of energy (glucose, starch) in the body, make structural materials (cellulose) and when pres...

Chapter 8 notes: (KNOW HOW TO DRAW STUCTURES: LINEAR, curved, and final Haworth projection) Carbohydrates: Carbohydrates or saccharides (sugars) are the most abundant biological molecules. Act as a source of energy (glucose, starch) in the body, make structural materials (cellulose) and when present on protein and cell surface, paly role in recognition events They contain three elements carbon, hydrogen and Oxygen, combined according to the formula (CH~2~O)~n~, where n≥3. The basic carbohydrate unit is called monosaccharides. - Also called Saccharides and sugars - Chemical formula: (CH2O)n , n\>=3 - Basic structural unit is monosaccharides. Monosaccharides are aldehyde or ketone derivatives of a straight chain polyhydroxyalchohols containing at least three carbon atom. They are classified according to the chemical nature of their carbonyl group and no. of their C atoms. If carbonyl group is aldehyde, the sugar is **aldose** and if carbonyl group is ketone, the sugar is **ketose**. **D-aldoses with 3-6 carbon atom (stereochemistry and nomenclature).** The arrows indicate the stereochemical relationships (arrangement of atoms). The configuration around C2 (red) distinguishes the member of each pair of monosaccrides. The L counterparts of these 15 sugars are their mirror images. The biologically most common aldoses are boxed. **\[Aldehyde**: An organic compound with a carbonyl group (C=O) bonded to a hydrogen atom and an R group (alkyl or aryl).**Ketone**: An organic compound with a carbonyl group (C=O) bonded to two R groups (alkyl or aryl). **Alcohol**: An organic compound with a hydroxyl group (-OH) attached to a saturated carbon atom (R-OH).**\]** Aldehyde or ketone derivatives of polyhdroxyalchohols: - 3-6 carbons - 3 is Aldotriose - 4 aldotetroses - 5 aldopentose - 6 aldohexoses - 6 carbon sugar - 4 chiral centers - 16 stereoisomers (2\^4=16) - Sugars that differ only by the configuration around one C atom are known as epimers of one another. - D-glucose and D-mannose are epimers with respect to C2. - Similarly, D-glucose and D-Galactose are epimers with respect to C4. - The most common aldoses include the 6 carbon sugars glucose, mannose and galactose. Epimers: Only different in one carbon Like Glu and Man only differ at C2 so are epimers of each other. Ketoses with 3-6 carbons: - Ketone group on Carbon 2 - 8 total - Most evident ones: Ribulose, Dihydroxyacetone, [Fructose]. - Here the D-ketoses with 3-6 carbon atoms are shown. The configuration around C3 (red) distinguishes the members of each pair. The most ketoses are those with their ketone function at C2 , the ones shown here. Due to less asymmetric center ketohexose has only 2^3^=8 possible stereoisomers (4 D sugar and 4 L sugars). The most common ketoses are dihydroxyacetone, ribulose and fructose. Aldehydes and ketones form Hemiacetals and Hemiketals: Alcohols react with the carbonyl groups of aldehydes and ketones to form hemiacetals and hemiketals. The hydroxy and either the aldehyde or the ketone functions of monosaccharides can likewise react intramolecularly to form cyclic hemiacetals and hemiketals. Alcohol + Aldehyde = hemiacetal (RO-CHOH-R) Alcohol + Ketone = hemiketal [Cyclization of glucose and fructose:] Shown here is cyclization of glucose and fructose. A) the linear form of D-glucose yielding the cyclic hemiacetal β-D-glucopyranose( sugar with 6 membered ring called pyranose). B) the linear form of D-fructose yielding the cyclic hemiketal β-D-fructofuranose (sugar with 5 membered rings called furanoses). The cyclic sugars are shown both as Haworth projections (in which heavier ring bonds project in front of the plane of the paper and the lighter ring bonds project behind it) and in stick form embedded in their semitransparent space-filling models with C green, H white, and O red. Haworth projections: Things on right go down, things on left go upside. When monosaccharides cyclizes, the carbonyl carbon , called anomeric carbon, becomes a chiral center with two possible configuration. So, the pair of stereoisomers that differ in configuration at the anomeric carbon are called anomers. In α anomer, the OH substituent of the anomeric carbon is on the opposite side of the sugar ring from CH~2~OH at the chiral center that designates the D or L configuration (C5 in hexoses). The other form is known as β anomer. Carbon 2 and 5 combine, C2 dropping the double bond O, and bonding to C5 with -OH up. Alpha and beta [Anomers] can be interconvert (flipped). - Alpha is hydroxyl group (-OH) down. - Beta is Hydroxyl group (-OH) up. Called [Mutarotation.] For glucose: \- 63.6% Beta anomer \- 36.4% Alpha anomer [ ] Chair conformations of B-Glucose: (powerpoint) Sugar modifications: (memorize) Oxidation of aldehyde yields aldonic acids: Sugars can be modified and covalently linked. (undergo reactions typical of aldehyde and ketones). Oxidation of an aldose converts its aldehyde group to a carboxylic acid group (here you can see at carbon 1), thereby yielding an [aldonic acid] such as **Gluconic acid**. - CHO -\> COOH (head) - Becomes Aldonic acid. - Ex: Gluconic acid Oxidation of primary alcohol yields uronic acids: Oxidation of the primary alcohol group of aldoses yield [uronic acids] ((here at carbon 6 you see) ), like **Glucuronic acid**. Uronic acids can assume the pyranose, furanose, and linear forms. - CH2OH-\> COOH (bottom) - Becomes Uronic Acids Reduction of Aldehyde yields Alditols: Aldoses and ketoses can be reduced under mild conditions, like treatment with NaBH~4~ to yield polyhydroxy alcohols known as alditols. Ribitol is a component of flavin coenzyme (vit B2 derived non protein enzyme helper). Glycerol and cyclic polyhydroxy alcohol myo-inositol are important lipid component. Xylitol is a sweetener that is used in sugarless gum and candies. - CHO-\> CH2OH - Using NaBH4 as reductor - Ex: Ribitol (Vit B2/ flavin coenzymes), Xylitol (sweetener) Reduction of Alcohol yields Deoxy Sugars: Monosaccharide units in which an O H group is replaced by H are known as deoxy sugars. The biologically most important of these are β-D---2-deoxyribose, the sugar component of DNA's sugar-phosphate backbone. L-Fucose is one of the few L sugar components of polysaccharides. - OH-\> H or CH2-\> CH3 - Adds a H - DNA Amine Substitution of alcohol yields amino sugars: In amino sugars one more OH groups have been replaced by an amino group, which is often acetylated. D-Glucosamine and D-galactosamine are the most common one. - Hydroxy replaced with amine. - -OH -\> NH2 - Ex: Glucosamine, Galactosamine Sialic Acids: N-acetylneuraminic acid, which is derived from N-acetyl-mannosamine and pyruvic acid is an important constituent of glycoprotein and glucolipids (proteins and lipids with covalently attached carbohydrate). N-acetylneuraminic acid and its derivatives are often referred to as sialic acids. In the cyclic from of the shown nine-carbon monosaccharide (N-acetylneuraminic acid) the pyranose ring incorporates the pyruvic acid residue (blue) and part of mannose moiety. - OH -\> CH3-CO-NH - Also Pyruvic acid residue Formation of O-Glycosides: The anomeric group of a sugar can condense with an alcohol to form α and β glycosides. (Greek word glykys means sweet). Figure shows formation of glycosides. The acid catalyzed condensation of α-D-glucose with methanol yields an anomeric pair of methyl-D-glucoside. The bond connecting the anomeric carbon to the alcohol oxygen is termed as **glycosidic bond**. - Add CH3OH, OH-\> OCH3 - This forms a glycosidic bond with another N in other compound. **N-Glycosides:**