Carbohydrates Chemistry Lecture Note

Carbohydrates Carbohydrates Definition • Carbo-Hydrate: have formula Cn(H2O)n (for n ≥ 3) • The precursor-macromolecule relationship is: – Monosaccharide–polysaccharide (or oligosaccharide) • Carbohydrates are everywhere (ubiquitous) and versatile in function; fulfill a variety of functions. – Can be covalently linked with proteins and lipids; are intimately involved in nucleic acids ROLES Monosaccharide Polysaccharide 1. Energy source/storage glucose, fructose, etc. 2. Structure glucose, glycerol Cellulose, chitin, lipids & membranes 3. Information 4. Recognition Nucleic acids Glycolipids & glycoproteins ribose (nucleotides) many Starch, glycogen Carbohydrates The 4 S’s Size Shape Solubility Stability Range from as small as glyceraldehyde (Mw = 90 g/mol) to as large as amylopectin (Mw > 200,000,000 g/mol) Depends on size and glycosidic bond Very polar, very soluble, until large polymers Stable due to glycosidic bond Carbohydrates Monosaccharides C3(H2O)3 • Basic nomenclature: triose, tetrose, pentose, hexose – Use the suffix “-ose” – Aldehyde & ketone functions solve problem of a “carbohydrate” H–C O = ld na a C1 de ehy H–C–OH H2O–CH Make H H–C–OH H2O–CH Make C 2a ket H–C–OH H2O–C one OK, we know enough chemistry to drawwith a What is wrong carbon-hydrate! this structure? H–C–OH H–C–OH H H H–C–OH C=O H–C–OH H = aldoses aldotriose (triose), aldotetrose(tetrose), aldopentose, aldohexose = ketose Ketotriose (triulose), ketotetrose (tetrulose)… (pentulose, hexulose). Carbohydrates Monosaccharides • • An aldose is a carbohydrate with aldehyde functionality. A ketose is a carbohydrate with ketone functionality. Are there any chiral carbons? Carbohydrates Monosaccharides Glyceraldehyde L-Glyceraldehyde • Chemical Features: D-Glyceraldehyde – Chirality • One or more asymmetric carbons • Linear and ring forms – Derivatives: the chemistry of carbohydrates – Polymerization • The Glycosidic Bond • Non-covalent bonds in macro-molecular structure Carbohydrates Monosaccharides Fischer projections • Vertical bonds are between carbons, with highest oxidation state at the top, AND project away from you. • Horizontal bonds are pointing toward you. • If hydroxyl is on the left; its L • If hydroxyl is on the right; its D • Here D=R & L=S It turns out that the L form of glyceraldehyde is called L because it is “levorotary,” meaning it will rotate plane-polarized light to the left, or counter-clockwise. D rotates “dextrorotary.” Carbohydrates Nearly all sugars in biology are “D” GREEN =functional group RED=highest numbered chiral carbon Right (sort of like most amino acids are “L”). As you go to tetrose, pentose, etc., you are adding more chiral carbons. A D- or L-sugar is defined by the chirality of the highest numbered carbon. Another complete set of 15 L-aldoses exist, and are enantiomers of their D-aldose relatives shown here. Rib Right Right Right Glc Man Right Left Right Right Gal Carbohydrates D-ketoses The first chiral sugar is a tetrulose. Rul There is also another complete set of 7 L-ketoses exist, and are enantiomers of their D-ketose relatives shown here. Fru Fructose is the ketose form of glucose Right Ribulose is the ketose form of ribose Right Left Right Carbohydrates Monosaccharides: Stereoisomer Nomenclature • Enantiomers – stereoisomers that are nonsuperimposable complete mirror images – Example: D-sugars & L-sugars • Diastereomers – stereoisomers that are not complete mirror images – Diastereomers have different physical properties (e.g., water solubility) – Example: Carbohydrates Monosaccharides: Stereoisomer Nomenclature • Epimers – Epimers are stereoisomers that differ at only one chiral center – Epimers are diastereomers; diastereomers have different physical properties (i.e., water solubility, melting temp) – Example: – Glc & Man (2 epimer); Glc & Gal (4 epimer); Rib & Ara; Rib & Xyl • Anomers – Anomers have different chirality at carbon involved in ring formation Carbohydrates Monosaccharides: Stereoisomer Nomenclature • D-Mannose and D-galactose are both epimers of D-glucose. • D-Mannose and D-galactose vary at more than one chiral center and are diastereomers, but not epimers. Left Right Right Left Left Left Right Right Left Right Right Right Carbohydrates Monosaccharides: The most important sugars • Glyceraldehyde and dihydroxyacetone are the simplest (3 carbon) aldose and ketose, respectively. • Ribose (Rib) is the standard five-carbon sugar. • Glucose (Glc) is the standard six-carbon sugar. • Galactose (Gal) is an C4-epimer of glucose. • Mannose (Man) is an C2-epimer of glucose. • Fructose (Fru) is the ketose form of glucose. • Ribulose (Rul) is the ketose form of ribose. Need to know, recognize, draw Fisher Projection, name, abbreviate Carbohydrates • Enantiomers Monosaccharides: Stereoisomer Nomenclature – stereoisomers that are nonsuperimposable complete mirror images – Example: D-sugars & L-sugars • Diastereomers – stereoisomers that are not complete mirror images – Diastereomers have different physical properties (e.g., water solubility) – Example: ribose & lyxose • Epimers – Epimers are stereoisomers that differ at only one chiral center – Epimers are diastereomers; diastereomers have different physical properties (i.e., water solubility, melting temp) – Example: – Glc & Man (2 epimer); Glc & Gal (4 epimer); Rib & Ara; Rib & Xyl • Anomers – Anomers have different chirality at carbon involved in ring formation Carbohydrates Monosaccharides: What are these “ring” forms? • SUGARS WITH ≥5 CARBONS RAPIDLY AND STABLY FORM RINGS THROUGH HEMIACETAL (ALDOSES) AND HEMIKETAL (KETOSES) BONDS. Hemiacetals – : Chiral Not chiral • If the aldehyde (R1) and the alcohol (R2) are on the same molecule, you have a RING! • Due to the oxygen, the ring is heterocyclic Carbohydrates Monosaccharides: What are these “ring” forms? Attack sp2 carbon from top Attack sp2 carbon from bottom •Anomers –Anomers are different chirality at carbon involved in ring formation –These are designated a (OH down) or b (OH up) a The carbon (with 2 bonds to oxygen) in now chiral; its called the “anomeric” carbon. b *for D-sugars Carbohydrates Monosaccharides: What are these “ring” forms? – : Hemiketals Attack sp2 carbon from top a (OH down) or b (OH up) Besides the a & b, what is this pyran and furan? Its all about which alcohols are used in these reactions. Carbohydrates Monosaccharides: What are these “ring” forms? Hemiketals a (OH down) or b (OH up) Besides the a & b, what is this pyran and furan? Its all about which alcohols are used in these reactions. Carbohydrates Monosaccharides: What are these “ring” forms? • Six-membered oxygencontaining rings are called pyranoses after the pyran ring structure. • Five-membered oxygencontaining rings are called furanoses after the furan ring structure. a (OH down) b (OH up) The way we are drawing these sugars is called a Haworth projection….. Carbohydrates Haworth projections right • Highest carbon with hemiacetal/hemiketal alcohol in back; oxygen to right. • If chiral (as for D-sugars), next highest carbon up if D, or down if L. • The anomeric carbon is usually drawn on the right side. • Other chiral carbons are down if right in Fisher projection and up if left in Fisher • b-D-Glc pyranose goes up, down, up down, up as you go from C5 to C1 • b-D-Rib furanose goes up, down, down, up as you go from C4 to C1 left right right down left up D-Glucose right down b-D-Glucopyranose right right right right Carbohydrates • Pentoses and hexoses readily undergo intramolecular cyclization. • The former carbonyl carbon becomes a new chiral center, called the anomeric carbon. • ALL forms are in equilibrium in solution Process is called “mutarotation” – When the former carbonyl oxygen becomes a hydroxyl group, the position of this group determines if the anomer is a or b. – If the hydroxyl group is on the opposite side (trans) of the ring as the CH2OH moiety, the configuration is a. – If the hydroxyl group is on the same side (cis) of the ring as the CH2OH moiety, the configuration is b. Just remember, for D-sugars, a (OH down) or b (OH up) Carbohydrates Why is the b-Glc more stable than a-Glc, and visa versa for Man? to answer this, we need to look at ACTUAL structures. These Haworth Projections are not the actual conformation…. What is? ~56% Carbohydrates Actual Conformations of Cyclized Monosaccharides • Cyclohexane rings have “chair” or “boat” conformations. • • Pyranose rings favor “chair” conformations. Multiple “chair” conformations are possible but require energy for interconversion (~46 kJ/mole). Furanose rings are envelopes • OH H H a OH We can understand why there is a 2:1 ratio of b:a glucose….. steric hindrance AND why a-mannose is favored (2:1 ratio of a:b) Carbohydrates Monosaccharides: Chemistry • Chemical Features: – Chirality • One or more asymmetric carbons • Linear and ring forms – Derivatives: the chemistry of carbohydrates ① • Oxidation – C1 – C6 ② ③ ④ • Reduction – C1/C2 – Other carbons • Ester formation • Amino sugars – Polymerization • The Glycosidic Bond • Non-covalent bonds in macro-molecular structure 1 Carbohydrates ① Monosaccharides: Derivatives Oxidation: These make “sugar acids” or “acid sugars” • Oxidation of aldehyde/ketone to acid (2e– loss); reaction from the C1 of aldoses – named as “onic” acids (“onate” for conjugate base) – these sugars can reduce Cu2+ to Cu+ (Fehling’s/Benedict’s test) or Ag+ to Ag0 (Tollens’ test) – Example: gluconic acid Blue This reaction is called Benedict’s test H2O Red (ppt) The copper is reduced and the sugar is oxidized. So, the sugar is called a Reducing Sugar. Reducing sugars have a free anomeric carbon. Carbohydrates ① Monosaccharides: Derivatives Oxidation: These make “sugar acids” or “acid sugars” Colorimetric Glucose Analysis d-D-Gluconolactone b-D-Glucose CH2OH CH2OH O OH OH HO OH O Glucose oxidase OH HO O2 O OH H2O2 NH NH2 OCH3 OCH3 2 H2 O Peroxidase OCH3 OCH3 NH NH2 Oxidized o-dianisidine (bright orange) Reduced o-dianisidine (faint orange) • Enzymatic methods are used to quantify reducing sugars such as glucose. – The enzyme glucose oxidase catalyzes the conversion of glucose to glucono-d-lactone and hydrogen peroxide. – Hydrogen peroxide oxidizes organic molecules into highly colored compounds. – Concentrations of such compounds is measured colorimetrically. • Electrochemical detection is used in portable glucose sensors. Carbohydrates ① Monosaccharides: Derivatives Oxidation: These make “sugar acids” or “acid sugars” Oxidation of alcohol to acid (4e– loss) – Reaction for C6 groups; like C1 oxidation, normally on aldoses – Named as uronic acids (“uronate” for conjugate base) – Many L-sugars are Uronic acids – Examples: D-glucuronic acid, L-iduronic acid CHO CH2OH D-Idose L-Idose L-Iduronic acid Carbohydrates ② Monosaccharides: Derivatives Reduction: These make “sugar sugar alcohols alcohols” or “deoxysugars” deoxysugars Reduction of aldehyde/ketone to alcohol (2e– gain) – Only carbon not already an alcohol is the anomeric carbon (C1/C2) – Named as sugar “ol” or “itol” – Examples: glycerol, mannitol, glucitol (sorbitol) Reduction of alcohol carbon to methyl/methylene (2e– gain) – Can react at any except the anomeric carbon – Named as “x-deoxy” sugar with x being the reduced carbon – Many are L-sugars, and have specific trivial names – Examples: 2-deoxyribose, L-Fucose (Fuc) and L-Rhamnose (Rha) Fuc is also 6-deoxy L-Gal Rha is also 6-deoxy L-Man OH OH b-D-Gal b-D-Man Carbohydrates ② Monosaccharides: Derivatives sugar alcohols Reduction: These make “sugar alcohols” or “deoxysugars” deoxysugars Reduction of aldehyde/ketone to alcohol (2e– gain) – Only carbon not already an alcohol is the anomeric carbon (C1/C2) – Named as sugar “ol” or “itol” – Examples: glycerol, mannitol, glucitol (sorbitol) Reduction of alcohol carbon to methyl/methylene (2e– gain) – Can react at any except the anomeric carbon – Named as “x-deoxy” sugar with x being the reduced carbon – Many are L-sugars, and have specific trivial names – Examples: 2-deoxyribose, L-Fucose (Fuc) and L-Rhamnose (Rha) Fuc is also 6-deoxy L-Gal Rha is also 6-deoxy L-Man OH OH b-D-Gal Notice that for L-sugars the b-anomer is down and a-anomer is up b-D-Man Carbohydrates Monosaccharides: Chemistry • Chemical Features: – Chirality • One or more asymmetric carbons • Linear and ring forms – Derivatives: the chemistry of carbohydrates ① • Oxidation – C1 – C6 ② -onic -uronic • Reduction -tol – C1/C2 – Other carbons deoxy- ③ ④ • Ester formation • Amino sugars – Polymerization • The Glycosidic Bond • Non-covalent bonds in macro-molecular structure Carbohydrates ③ Monosaccharides: Derivatives Esters: condensation of alcohol (sugar) and an acid • Most important sugar esters use a phosphoric acid • These are called phospho-sugars or sugar phosphates • Examples: nucleotides, Glc 6-P, Fru 1,6-P2, glycerol 3-phosphate D- a-D-Glucose 6phosphate b-D-Fructose 1,6-bisphosphate Carbohydrates ④ Monosaccharides: Derivatives Amines: condensation of alcohol (sugar) and ammonia • • • These are called amino-sugars In biology, its ALWAYS at C2 Often, the amino group will be further modified by an acetyl group (CH3CO–) making an amide • Named as sugar“amine” and when acetylated, the “N-acetyl” comes first. When abbreviation used “NAc” it comes after. • Examples: glucosamine, N-acetyl-glucosamine (GlcNAc) (GlcNAc) Carbohydrates Monosaccharides: Derivatives Important Hexose Derivatives ③ ④ ④ ①&④ ② R=pyruvyl ①&④ ① Carbohydrates Monosaccharides: Derivatives Muramic acid =O Combined acid (C3) + amine (C2) Pyruvic acid = H3C-C-COO– ①&④ Sialic Acid (Sia) (aka NeuNAc) Combined acid (C1) + amine (C2) Carbohydrates Monosaccharides: Chemistry • Chemical Features: – Chirality • One or more asymmetric carbons • Linear and ring forms – Derivatives: the chemistry of carbohydrates • Oxidation – C1 – C6 • Reduction – C1/C2 – Other carbons • Ester formation • Amino sugars – Polymerization • The Glycosidic Bond • Non-covalent bonds in macro-molecular structure Carbohydrates The Glycosidic Bond Carbohydrates Hemiacetals and Hemiketals are reactive to alcohols in condensation reactions • Hemiacetals condense with alcohols to form Acetals. • Hemiketals condense with alcohols to form Ketals. • These reactions form the basis of the GLYCOSIDIC BOND. MeOH : – Glc • Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon (the hemiacetal/hemiketal) and a hydroxyl carbon (the other sugar). The Glycosidic Bond • The glycosidic bond between sugars is stable and does not readily hydrolyze. – The anomeric carbon involved in the glycosidic linkage is fixed in its chirality and is therefore nonreducing. – The second monomer, with its unreacted hemiacetal, is still reducing. (b-D-adenine riboside) Carbohydrates Hemiacetals and Hemiketals are reactive to alcohols in CONDENSATION reactions • Hemiacetals condense with alcohols to form Acetals. • Hemiketals condense with alcohols to form Ketals. • These reactions form the basis of the GLYCOSIDIC BOND. • Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon (the hemiacetal/hemiketal) and a hydroxyl carbon (the other sugar). The Glycosidic Bond • The glycosidic bond between sugars is stable and does not readily hydrolyze. – The anomeric carbon involved in the glycosidic linkage is fixed in its chirality and is therefore nonreducing. – The second monomer, with its unreacted hemiacetal, is still reducing. (b-D-adenineriboside) Carbohydrates Disaccharides Carbohydrates Disaccharides: • Disaccharides can be named by the organization and linkage or a common name. – The disaccharide formed upon condensation of two glucose molecules via a 1 à 4 bond is described as α-D-glucopyranosyl-(1à4)-D-glucopyranose. – The common name for this disaccharide is maltose. Still undergoing If you only know the sugar and the linkage mutarotation Non-reducing à reducing If you know the exact structure of the sugar As we make sugar-polymers, the convention is to have the non-reducing sugar to the LEFT and the reducing end at the RIGHT. Carbohydrates Disaccharides: Here is likely the first disaccharide you encountered in your life: Lactose. Non-reducing à reducing Still undergoing mutarotation Carbohydrates Disaccharides: Here is likely the disaccharide you ingest the most: Sucrose. b Notice that these are drawn upside down Trehalose • Nonreducing Disaccharides • • Review of Glycosidic Bonds Two sugar molecules can be also joined in a glycosidic bond between two anomeric carbons. The product has two acetal groups and no hemiacetals or hemiketals. There are no reducing ends; this is a nonreducing sugar. Carbohydrates Polysaccharides • The majority of natural carbohydrates are usually found as large polymers. • These polysaccharides can be: – homopolysaccharides (one monomer unit) – heteropolysaccharides (multiple monomer units) – linear (one type of glycosidic bond) – branched (multiple types of glycosidic bonds) • Polysaccharides do not have a defined molecular weight. – This is in contrast to proteins because, unlike proteins, no template is used to make polysaccharides. – Polysaccharides are often in a state of flux; monomer units are added and removed as needed by the organism. Carbohydrates Polysaccharides: Polymers of Glucose Homopolymers of Glucose: - Starch - Glycogen - Cellulose - Chitin • Glycogen and Starch are the main storage polysaccharides for energy. – Glycogen and starch are insoluble due to their high molecular weight and often form granules in cells. – Molecular weight reaches several millions (~200 x 106)(can see in microscope). • Glycogen is a branched homopolysaccharide of glucose. – Glucose monomers form a(1 ® 4) linked chains. – There are branch points with a(1 ® 6) linkers every 8–12 residues. • Starch is a mixture of two homopolysaccharides of glucose. – Amylopectin is like glycogen, but the branch points (a(1 ® 6) linkages) occur every 24–30 residues. – Amylose is an unbranched polymer of a(1 ® 4) linked residues. Carbohydrates Polysaccharides: Glycogen & Starch Glycosidic Linkages in Glycogen and Starch (Glc-a(1à4)Glc)n with a(1à6) branching, where n≈106 Non-reducing à reducing Haworth projections are not good to show the actual shape, which is helical! Carbohydrates Polysaccharides: Glycogen & Starch n≈150 I2 2 3 5 1 4 6 Axial = a-OH 4 6 3 5 2 1 • Granules contain enzymes that synthesize and degrade these polymers. • Glycogen and amylopectin have one reducing end but many nonreducing ends. • Enzymatic processing occurs simultaneously in many nonreducing ends. Carbohydrates Polysaccharides: Glycogen & Starch n≈150 I2 2 3 5 1 4 6 Axial = a-OH 4 6 3 5 2 1 Carbohydrates Polysaccharides: Glycogen & Starch Mixture of Amylose and Amylopectin in Starch Carbohydrates Polysaccharides: Homopolymers of Glucose: - Starch - Glycogen • Cellulose is a linear homopolysaccharide of glucose. – Glucose monomers form b(1 ® 4) linked chains. - Cellulose – Hydrogen bonds form between adjacent monomers. – There are additional H-bonds between chains. - Chitin – Structure is now tough and water insoluble. – It makes up almost 50% of plant mass; in cell walls. – Cotton is nearly pure fibrous cellulose. • Chitin is a linear homopolysaccharide of Nacetylglucosamine (GlcNAc). – N-acetylglucosamine monomers form b(1 ® 4)linked chains. – forms extended fibers that are similar to those of cellulose – hard, insoluble, cannot be digested by vertebrates – structure is tough but flexible, and water insoluble – found in cell walls in mushrooms and in exoskeletons of insects, spiders, crabs, and other arthropods Carbohydrates Polysaccharides: Cellulose & Chitin Glc b 1 OH 4 Glc OH Glycosidic Linkages in Cellulose & Chitin (Glc-b(1à4)Glc)n with NO branching, n ≈ 104 Notice that each chair is alternatively flipped 3 Equitorial = b-OH 4 THE most abundant biological molecule in the world: ~300 trillion kg 2 5 6 1 Carbohydrates Polysaccharides: Cellulose & Chitin • The fibrous structure and water insolubility make cellulose a difficult substrate to act upon. • Most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyze b(1®4) linkages (b-Amylase or cellulase). • Fungi, bacteria, and protozoa secrete cellulase, which allows them to use wood as source of glucose. • Ruminants and termites live symbiotically with microorganisms that produce cellulase and are able to absorb the freed glucose into their bloodstreams. • Cellulases hold promise in the fermentation of biomass into biofuels. Carbohydrates Polysaccharides: Cellulose & Chitin b b 4 1 4 (GlcNAc-b(1à4)-GlcNAc)n