Chapter 8 Carbohydrates 8.1 monosaccharides 8.2 (A-C section) Polysaccharides

carbohydrates: aldehydes/ketones with at least 2 hydroxyl groups or substances that yield such compounds on hydrolysis

Cn(H2O)n - hydrated carbon

  • range from as small as glyceraldhdye (90 g/mol) to as large as amylopectin (2e8 g/mol)

  • energy source & storage, structural component of cell walls and exoskeletons, informational molcules in cell-cell signaling

  • covalently linked with proteins to form glycoproteins and proteoglycans or lipids to form glycolipids

  1. monosaccharides = simple sugars, with 1 polyhydroxy aldehyde or ketone unit (D-glucose)

= backbones:

- unbranched carbon chains with single bonds linking all carbon atoms

- one of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group

- other carbon atoms are bounded to a hydroxyl group

  • aldose = carbonyl group is at an end of te carbon chain (in aldehyde group)

  • ketose = carbonyl group is at any other position (in ketone group)

trioses = simplest monosaccharides, 3 carbon backbone

: monosaccharides have asymmetic centers

  • all monosaccharides (except dihydroxyacetone) contain 1+ chiral carbon atom

  • molecule with n chiral centers can have 2n steroisomers

  • enantiomers - 2 different isomers that are mirror images - differ in every chiral carbon 

  1. oligosaccharides = short chains of monosaccharide units/residues joined by glycosidic bonds

epimers = 2 sugars that differ only in the configuration around one carbon atom (ex glucose epmiers: galactose - epimer of glucose at C4, mannose: epimer of glucose at C2)

simple carbohydrates: numbering carbons of a sugar - carbons are numbered beginning at the end of the chain near the carbonyl group

ribose - 5C sugar

glucose - 6C sugar

galactose - epimer of glucose

fructose - ketose form of glucose

  • know open chain (fisher projection) and cyclic (hayworth projection)

fischer projection formulas - used to represent 3D sugar structures on paper

  • bonds drawn horizontally indicate bonds that project out of the plane of the paper

  • bonds drawn vertically project behind the plane of the paper

cyclic sugars: haworth projections / haworth perspective formulas - more accurate representation of cyclic sugar structure than fischer projections

  • 6 membered ring is titled to make its plane almost perpendicular to that of the paper

  • bonds closest to the reader are drawn thicker than those farther away

  • 5 C ring = furanose, 6 C ring = pyranose, these rings are favored

    • which -OH attacks determines whether the pyranose or furanose form of the sugar is made

      • glucopyranose formed through attack of -OH from carbon 5 on carbonyl

      • glucouranose formed through attack of -OH from carbon 4 on carbonyl

sugar stereroisomers arise because many of the carbon atoms to which the hydroxyl groups are attached are chiral centers

  • enezymes that act on sugars are stereospecific

common monosaccharides have cyclic structures

— aldotetrose and all monosaccharides with 5+ backbone carbon atoms occur as cyclic structures

  • covalent bond b/w the carbonyl group and oxygen of hydroxyl group

formation of cyclic sugars - reaction of alcohol with aldehyde or ketone

  • cyclization happens when aldehyde/ketone and the alcoghol group

  • =O becomes OH

  • new chiral carbon is formed

  • 5/6 membrered rings forms if the -OH and carbonyl groups are on the same molecule

  • alcohol + aldehyde → hemiacetal

  • alcohol + ketone → hemiketal

alpha & beta anomers can interconvert 

  • cyclization reaction transforms the carbonyl unit to an alcohol

  • this alcohol can be oriented in one of two ways produce either alpha or beta configuration

  • the steroisomer with the alcohol on the same side of the ring as the CH2OH unit is the beta anomer

  • the steroisomer with the alcohol on the opposite side of the CH2OH unite is called the a anomer

  • mutarotatin - interconverison of alpha and beta anomers

different chair conformatiions of cyclic sugar

  • 6 membered cyclic sugar is not planar

  • chair form - engages in axial/equatorial equilibrium

  • bulk of R groups will affect equilibrium constant; large R groups prefer equatorial positions

anomeric carbon - the new chiral carbon that is created upon formation of the hemiacetal or hemiketal 

  • in making ring structure a new chiral center is formed (carbonyl carbon)

  • anomeric carbon (Ano. C) will always be next to the oxygen in the ring

  • aldose: carbon 1 becomes anomeric

    • ano. C in aldose is bonded to -OH (usesd to be carbonyl) and -H functional groups

  • ketoses: which carbon becomes ano. C depends on where ketone is (usually C2)

    • ano. C in ketoses is bonded to -OH (used to be carbonyl) & -CH2OH functional groups

sugar modifications

  • sugars can be oxidized

  • oxidation of aldehyde yields aldonic acids

  • oxidation of primary alcohol yields uronic acids

  • sugars can be reduced under mild conditions to produce alditols

  • sugars can be reduced to produce polyhydroxy alcohols known as alditols

    • ribitol is a component of flavin coeznymes

    • glycerol and the cyclic polyhydroxy alcohol myo-inositol are important lipid components

    • xylitol is a sweetner that is used in sugarless foods

sugar modifications: reduction of alcohol yields deoxy sugars

  • alcohol groups on sugars can be converted to hydrogen atoms in deoxy sugars

  • deoxygenation reaction is formally a reduction

  • requires a radical producing enzyme mediated reaction

  • deoxyribose

  • fucose important in glycolipids and proteins as well as polysaccharides

sugar modifications: amine substitution of alcohol yields amino sugars

  • hydroxy groups can be replaced with primary amines

  • frequently used as precursors to acylated sugars

  • often found in glycosylated proteins and lipids (proteins and lipdis with covalently attached carbohydrates)

sugar modifications: sialic acids

O-glycosides formation

  • glycosidic bonds = covalent linkage joining 2 monosaccharides

  • formed when a hydroxyl group of one sugar molecule reacts with the anomeric carbon of the other

  • condensation reaction catalyzed by acid

N-glycosides

  • N-glycosidic bonds form between the anomeric carbon and an amine

  • covalent bond between nucleobases & sugars in nucleosides occurs via the formation of glycosidic bonds

  • very slow to hydrolyze in the absence of enzymes

reducing sugars = undergo a characteristic redox reaction where free aldehyde groups react with Cu2+ under alkaline condition

  • identificiation of a sugar as nonreducing is evidence that it is glycoside

polysaccharides (glycans)

  • just as in amino acids and nucleotides, monosaccharides can polymerize by condensation

  • loss of water forming an ether linkage

  • small oligomeric sugars are important metabolites

  • larger polymeric sugars play key roles ranging from energy storage to strcuture elements

homopolysaccharides - polysaccharides made up of one type of monosaccharide

  • serve as storage forms and structural elements

heteropolysaccharides - made up of different sugar monomers

  • provide extracellular support

disaccharides - sugars composed of 2 monosaccharide units

  • important metabolites and sources of energy in foods

  • vary by composition and alpha/beta linkages

    • surcose = glucose + fructose

      • has a to B linkage between the monomer units

      • cane sugar

    • lactose = glucose + galactose

      • only found in mammalian milk

      • most mammals can only digest when infants

      • lactose intolerance

    • maltose = glucose + glucose

      • glucose dimer with similar linkage as sucrose

      • component of malt sugar

      • less sweet than sucrose & fructose

      • produced in germinating grains (starch breakdown product)

artificial sweeteners

  • many compounds have been developed as artificial sweeteners

    • sweet taste but neither not a carbohydrate or not easily metabolized

  • important part of modern diet

    • reducing caloric intake/weight loss

    • diabetes

    • avoiding tooth decay

  • different uses

    • candy vs cooking ingredient

  • sweet tasting compounds are not always good for you

    • sugar of lead: Pb(OAc)2

compound                sweetness relative to sucrose

acesulfame               200

alitame                      2000

aspartame                 180

saccharin                   350

suralose                     600

beyond oligomeric saccharides

  • polymers comprised of individual monosaccharides can be very large macromolecular system

  • larger systems play several roles

    • structural elements (cellulose, chitin)

    • energy storage (starch)

    • bacterial cell protection (mucous-like layers)

  • these polymers require enzymes to break down

  • not all organisms can break down all polysaccharides (cellulose and chitin)

cellulose fibers

  • polysaccharide cellulose forms through fibers that make up plant cell walls

  • accounts for over half of the carbon of the biosphere

  • insolube in water despite presence of multiple polar groups in the polymer

cellulose: B(1→4)-linked D-glucose

  • cellulose is a glucose polymer

  • linked by B(1-4) glycosidic bonds

  • up to 15000 glucose units per polymer strand

cellulose: tightly packed, fully extended conformation

  • cellulose strands form sheets linked in the sheets by strong hydrogen bonds

  • the sheets themselves are also bound together by strong hydrogen bonding

  • distributes hydrogen bonding, resulting in a highly strong, water insolube material

chitin - structural polysaccharide found in insects and crustaceans

  • also found in some fungi and mollusks (cephalopod beaks)

  • has high nitrogen concentration

  • derived from a modified carbohydrate

chitin: B(1→4)-linked N-acetyl-D-glucosamine

  • the repeating unit in chitin is an N-acylated aminosaccharide, N-acetylglucosamine

  • formed an insolube polymer

  • Removal of the N-acetyl groups results in chitosan - soluble form of chitin

energy storage polysaccharides 

  • animals and plants store energy over the long term in the form of either fats or starches

  • unlike the structural polysaccharides, these macromolecules are soluble

    • however large macromolecules form highly viscous solutions

  • can be broken down enzymatically as needed to provide energy

strach = contains 2 types of glucose polymer (amylose & amylopectin)

  • amylose = long, unbranched chains of D-glucose 

    • residues connected by (a1→4) linkages

  • amylopectin = larger than amylose with (a1→4) linkages

    • between glucose residues and highly branded due to (a1→6) linkages

a-Amylose: a(1→4)-linked D-glucose

  • starch is produced by plants and found as granules in chloroplast

  • mixture of a-amylose and amylopectin

  • a-amylose is a polysaccharide of glucose with a(1-4) glycosidic bonds

a-Amylose: irregularly aggregating helically coiled conformation

  • unlike cellulose, a-amylose forms a left handed helical structure

  • hydrogen bonding within the helix

  • irregularly aggregates, but not insoluble

amylopectin: a(1→6)-branches

  • cross-linked version of a-amylose

  • has alpha (1-6) branching at every 15-30 glucose units of the chain

glycogen = polymer of (a1→4)- linked glucose subunits iwth (a1→6)-linked branches

  • more entensively branched

  • more compact than starch