Carbohydrates 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 2 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). 3 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 4 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 Gal Right Left Right Right 5 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: 6 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 7 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 8 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 9 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. 10 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 11 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) ~56% 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? 12 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) 13