Carbohydrate Structure

Key Concepts

• Carbohydrates -- variety of important functions in living systems:
  • nutritional (energy storage, fuels, metabolic intermediates)
  • structural (components of nucleotides, plant and bacterial cell walls, arthropod exoskeletons, animal connective tissue)
  • informational (cell surface of eukaryotes -- molecular recognition, cell-cell communication)
  • osmotic pressure regulation (bacteria)
• Multiple chiral C atoms --> large variety of stereoisomers possible
  • diastereoisomers, enantiomers, epimers
• Hemiacetal/hemiketal formation (internal cyclization of monomers) --> anomers
  • Mutarotation spontaneously interconverts anomers in solution IF the anomeric C is free
  • Carbohydrates with a free anomeric C (the carbonyl C of the aldehyde or ketone) can reduce various oxidants -- they're REDUCING SUGARS.
• Acetal/ketal formation (glycosidic bonds) --> connection of sugar monomers into oligomers and polymers
  • Lots of diversity possible with different connectivities in glycosidic linkages
• Many sugar derivatives are important participants in metabolism and in biologically important structures.
• Conformation of a(1-> 4) linked homopolymers of glucose leads to a tightly coiled, helical structure.
  • conducive to nutrient storage
• Conformation of b(1-> 4) linked homopolymers leads to flat, ribbon-like structures stabilized by intra- and interchain hydrogen bonds. 
  • conducive to structural role

• polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis ("polyhydroxy" = 2 or more OH groups)
• (CH₂O)_n = saccharides = sugars
• Functions:
  • nutritional (energy storage, fuels, metabolic intermediates)
  • structural
    • components of nucleotides (building blocks of nucleic acids) and of cofactors for enzymes
    • cell walls (bacteria, plants)
    • exoskeletons (arthropods)
    • animals: connective tissue, cartilage, bone, intercellular cement
  • informational (cell surface of eukaryotes -- molecular recognition, cell-cell communication)
  • osmotic pressure regulation (bacteria)
• 3 size classes
  • monosaccharides: 1 sugar unit, with n = 3-8 C atoms
  • disaccharides: a few sugar units (~2-10)
  • polysaccharides: many sugar units

Monosaccharides

• H₂O-soluble
• backbone = linear (unbranched) chain of C atoms, connected by single bonds
• 1 C atom a carbonyl, the others with -OH groups

Aldoses have an aldehyde functional group; ketoses have a ketone functional group. The simplest monosaccharides of biological interest have n=3 (trioses): glyceraldehyde and dihydroxyacetone (What enzyme catalyzes interconversion of glyceraldehyde-3-P and dihydroxyacetone-P? What kind of reaction is that?)

Glyceraldehyde has a chiral carbon at C2. 2 enantiomers (D and L): nonsuperimposable complete mirror images. All naturally-occurring sugars are derived from the D isomer. number of stereoisomers = 2n, where n = # of asymmetric C atoms

D vs. L determined by configuration of the penultimate C, the chiral C furthest from the carbonyl C. There are two D isomers for the tetroses:

Fig. 9-3a (Nelson & Cox, Lehninger Principles of Biochemistry, 3rd ed., 2000): The series of D aldoses (n = 3, 4, 5, and 6) How many asymmetric C atoms are there in a 6-carbon aldose?

Fig. 9-3b (Nelson & Cox, Lehninger Principles of Biochemistry, 3rd ed., 2000): The series of D ketoses (n = 3, 4, and 5) How many asymmetric C atoms are there in a 6-carbon ketose?

D-glucose and L-glucose are ENANTIOMERS: non-superimposable COMPLETE mirror images (differ in configuration at EVERY CHIRAL CENTER). How many chiral C atoms does glucose have? At how many of those chiral C atoms does the configuration of D-glucose differ from that of L-glucose?

Diastereoisomers (diastereomers) = isomers that are not mirror images; they are optical isomers that differ in configuration at 1 or more chiral centers, but NOT at EVERY chiral center. D-glucose and D-galactose are diastereomers about position 4; D-mannose and D-idose (see fig. above) are diastereomers at positions 2 and 3. EPIMERS = sugars that differ in configuration at ONLY 1 POSITION Examples of epimers (refer back to Fig. 9-3a above): D-glucose & D-galactose (epimeric at C4) D-glucose & D-mannose (epimeric at C2) D-idose & L-glucose (epimeric at C5)

• Condensation of aldehyde or ketone with alcohol --> hemiacetal or hemiketal; no atoms eliminated, just rearranged
  • Hemiacetal and hemiketal formation freely reversible in solution at pH 7.
• Condensation of hemiacetal or hemiketal with second alcohol yields acetal or a ketal, with elimination of H₂O.
  • When 2nd alcohol is part of another sugar molecule, acetal or ketal linkage = a GLYCOSIDIC BOND.
  • Acetal and ketal formation (glycosidic bond formation) are NOT reversible at pH 7; glycosidic bonds are stable at pH 7, and require acid catalysis (or an enzyme) to hydrolyze at any reasonable rate.
• See also Fig. 9-5 in Nelson & Cox (Lehninger Principles of Biochemistry, 3rd ed.. 2000)

• Sugars with 5 or more C atoms can form five-membered rings (furanose rings) or six-membered rings (pyranose rings) by internal hemiacetal or hemiketal formation.
• Rings can adopt either chair or boat conformation
  • chair is more stable for steric reasons (boat is only seen in derivatives with very bulky substituents)
  • the most stable conformation is the one with the most bulky substituents in the equatorial positions.
• Conformational formulas (below) are a much more accurate way to depict sugar structures, but in this class we'll mostly use Haworth projections (Haworth perspective formulas).

• Formation of ring creates new chiral center at C1; new chiral center (original carbonyl C) = anomeric C.
• Generates 2 more stereoisomers, called anomers, a and b (differing only in configuration at anomeric C). (sugar1).

MUTAROTATION: Unlike the other stereoisomeric forms, a and b anomers freely interconvert in solution via open chain form of sugar (reversible formation of internal hemiacetal or hemiketal linkage) Fig. 9-6 (Nelson & Cox, Lehninger Principles of Biochemistry, 3rd ed., 2000): Mutarotation (showing flat projections of both open and cyclic forms)

Mutarotation with conformational formulas (figure at right) Equilibrium ratio of a and b anomers depends on structure of specific sugar (higher concentration of conformation with the most bulky substituents in equatorial positions, less steric hindrance). Which anomer of D-glucose is more stable and thus would predominate?

• Rules for converting Fischer projections to Haworth projections:
  Note: Anomeric C is the C with TWO OXYGEN SUBSTITUENTS, the O in the ring PLUS an -OH (or -O-R in an acetal/glycosidic linkage)
  See Fig. 9-6 above for application of the rules below.

Derivatives of carbohydrates play important roles in biochemistry (examples on right). Some common monosaccharide abbreviations: Glc = glucose Fru = fructose Gal = galactose Man = mannose Rib = ribose Xyl = xylose GlcN = glucosamine GlcNAc = N-acetylglucosamine GalN = galactosamine GalNAc = N-acetylgalactosamine GlcA = gluconic acid GlcUA = glucuronic acid

Fig. 9-10 (Nelson & Cox, Lehninger Principles of Biochemistry, 3rd ed., 2000): Sugars as reducing agents (a) oxidation of anomeric C of glucose and other sugars (e.g. by Fehling's reaction, involving reduction of Cu2+ to Cu+ in alkaline solution) Aldehyde (or ketone, which can convert to aldehyde through an enediol intermediate) can be oxidized to carboxylic acid, so if sugar has a free anomeric C (equivalent to a free aldehyde, since hemiacetal is in equilibrium with straight chain aldehyde), it is a "REDUCING SUGAR", meaning that it can serve as a REDUCING AGENT, it can BE OXIDIZED. Reaction is more complex than shown in figure, but reduction of Cu2+ to Cu+ under alkaline conditions forms a red precipitate of cuprous oxide (Cu2O), the basis for a colorimetric test for reducing sugars. NOTE that a reducing sugar has to have a FREE ANOMERIC CARBON -- if the anomeric C is tied up in a glycosidic bond, that sugar is NOT a reducing sugar.

(b) Blood [glucose] commonly measured by measuring amount of H2O2 produced in reaction catalyzed by the enzyme glucose oxidase. (A second enzyme, peroxidase, is used to catalyze reaction of the H2O2 with a colorless compound to produce a colored product that can be measured spectrophotometrically.)

Oligosaccharides

• ~2-10 monosaccharide units joined by O-glycosidic bonds
• glycosides formed between two monosaccharides = disaccharides
• glycosidic bond = an acetal (or ketal) linkage -- NOT readily reversible in solution unless it's catalyzed (by acid or an enzyme).
• large diversity possible:
  • different monosaccharide units
  • anomeric configuration (a or b)
  • diversity through different connectivities (linkage through OH groups on different C atoms)

Figure on right: cellobiose, with b(1-> 4) linkage between 2 glucose monomers cellobiose = the building block of cellulose Cellulose: a homopolysaccharide, with D-Glc monomers in b(1-> 4) linkage

Figure on right: maltose, with a(1-> 4) linkage between 2 glucose monomers maltose = the building block of glycogen and starch Both glycogen and starch are homopolysaccharides with D-glucose monomers in a(1-> 4) linkage, except that there are also occasional branch points with a(1-> 6) linkages (see below)

Polysaccharides

• 2 types:
  • HOMOpolysaccharides (all 1 type of monomer), e.g., glycogen, starch, cellulose, chitin
  • HETEROpolysaccharides (different types of monomers), e.g., peptidoglycans, glycosaminoglycans
• Functions:
  • glucose storage (glycogen in animals & bacteria, starch in plants)
  • structure (cellulose, chitin, peptidoglycans, glycosaminoglycans
  • information (cell surface oligo- and polysaccharides, on proteins/glycoproteins and on lipids/glycolipids)
  • osmotic regulation
• Starch and glycogen
  • Function: glucose storage
  • Starch -- 2 forms:
    • amylose: linear polymer of a(1-> 4) linked glucose residues
    • amylopectin: branched polymer of a(1-> 4) linked glucose residues with a(1-> 6) linked branches
  • Glycogen:
    • branched polymer of a(1-> 4) linked glucose residues with a(1-> 6) linked branches
    • like amylopectin but even more highly branched and more compact
    • branches increase H₂O-solubility 
  • Branched structures: many nonreducing ends, but only ONE REDUCING END (only 1 free anomeric C, not tied up in glycosidic bond)

• Fig. 9-15c (Nelson & Cox) Cluster of amylose and amylopectin like that believed to occur in starch granules Strands form double-helical structures with each other NOTE: just ONE reducing end per molecule Many nonreducing ends on branched molecules (amylopectin, as also occurs in glycogen) Removing glucose from storage: enzymes remove glucose residues one at a time from the NONreducing ends (many sites, so rapid mobilization of Glc).

• Fig. 9-16 (Nelson & Cox, Lehninger Principles of Biochemistry, 3rd ed., 2000): Conformation of a(1-> 4) linked homopolymers of glucose leads to tightly coiled, helical structure (sugar2)
  • Result: dense granules of stored starch or glycogen (efficient use of space for stored nutrient)

• Cellulose and chitin
  • Function: STRUCTURAL, rigidity important
  • Cellulose:
    • homopolymer, b(1-> 4) linked glucose residues
    • cell walls of plants
  • Chitin:
    • homopolymer, b(1-> 4) linked N-acetylglucosamine residues
    • hard exoskeletons (shells) of arthropods (e.g., insects, lobsters and crabs)
      
      
• Fig. 9-17 (Nelson & Cox, Lehninger Principles of Biochemistry, 3rd ed., 2000): Conformation of b(1-> 4) linked homopolymers of glucose (or N-acetylglucosamine) leads to a flat, ribbon-like structure stabilized by intra- and interchain hydrogen bonds. 

• Keeping track of blood glucose levels

Non-enzymatic glycosylation of proteins (the Amadori reaction) occurs when glucose concentration is high.  Reaction proceeds through formation of a Schiff base (figure on right).  In diabetes mellitus, blood sugar levels are often quite high.  Under these conditions there's glycosylation of the N-terminal Val of hemoglobin.  Resulting protein, Hemoglobin A1C can be readily measured. Level of Hemoglobin A1C is another measure of variation in blood glucose levels in diabetic individuals.