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7.1, Principles, 7.4 Carbohydrates as Informational Molecules: the Sugar…

- - - - anomers: isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom
      - anomeric carbon: the carbonyl carbon atom
      - ex: formation of the two cyclic forms of D-glucose
        
        reaction between the aldehyde group at C-1 and the hydroxyl group at C-5 forms a hemiacetal linkage
        
        mutarotation: the interconversion of alpha and beta anomers
    - - tend to assume either of two chair conformations
        
        interconvertible without breaking bonds
        
        requires energy input
    - - six-membered ring is tilted to make its plane almost perpendicular to that of the paper
      - bonds closest to the reader are drawn thicker than those farther away
      - converting D-hexose Fischer Projections to Haworth Perspective Formulas
        
        draw the six-membered ring (five carbons, and one oxygen at the upper right)
        
        number the carbons in a clockwise direction beginning with the anomeric carbon
        
        place the hydroxyl groups, with those on the right in the Fischer projection placed pointing down and those on the left pointing up
        
        place the terminal -CH2OH group, projecting upward for the D-enantiomer and downward for the L enantiomer
        
        place the anomeric hydroxyl group, where the hydroxyl group is placed on the same side of the ring as C-6 for a beta structure and placed on the opposite side for an alpha structure
- - - - definition: simple sugars consisting of a single polyhydroxy aldehyde or ketone unit
      - ex: D-glucose
    - - definition: short chains of monosaccharide units, or residues, joined by glycosidic bonds
    - - definition: oligosaccharides with two monosaccharide units
      - ex: sucrose (composed of D-glucose and D-fructose)
    - - definition: sugar polymers with 2+ monosaccharide units
      - ex: cellulose (linear), glycogen (branched)
- - - - definition: chiral center most distant from the carbonyl carbon
    - - D-isomers
        
        definition: configuration at reference carbon is the same as D-glyceraldehyde, on the right (dextro) side in a projection formula
        
        includes most hexoses of living organisms
      - L-isomers
        
        definition: configuration at reference carbon is the same as L-glyceraldehyde, on the left (levo) in a projection formula
  - - - term: aldose
        
        definition: monosaccharide whose carbonyl group is at an end of the carbon chain (aka, in an aldehyde group)
      - term: ketose
        
        definition: monosaccharide whose carbonyl group is at any other position than the end of the carbon chain (aka, in a ketone group)
    - - 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 bonded to a hydroxyl group
    - - trioses: have a three carbon backbone
        
        simplest monosaccharides
        
        ex: D-glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose)
      - tetroses: have a four carbon backbone
      - pentoses: have a five carbon backbone
        
        ex: D-ribose, an aldopentose
        
        ex: 2-Deoxy-D-ribose, an aldopentose
      - hexoses: have a six carbon backbone
        
        ex: D-glucose, an aldohexose
        
        ex: D-fructose, a ketohexose
      - heptoses: have a seven carbon backbone
    - - all monosaccharides (except dihydroxyacetone) contain 1+ chiral carbon atom, causing them to occur in optically active isomeric forms
      - term: enantiomers
      - in general, a molecule with n chiral centers can have 2^n stereoisomers
- - - - exists as starch in plant cells and glycogen in animal cells
        
        heavily hydrated because they have many exposed hydroxyl groups available to hydrogen bond
        
        starch: contains two types of glucose polymer, amylose and amylopection
        
        amylose: long, unbranched chains of D-glucose residues connected by (alpha1->4) linkages
        
        amylopectin: larger than amylose with (alpha1->4) linkages between glucose residues and highly branched due to (alpha1->6) linkages
        
        glycogen: polymer of (alpha1->4) linked glucose subunits, with (alpha1->6) linked branches
        
        more extensively branched
        
        more compact than starch
      - storage of glucose as polymers avoids high osmolarity
    - - cellulose: tough, fibrous, water-insoluble substance
        
        linear, unbranched homopolysaccharide, consisting of 10,000 to 15,000 D-glucose units
        
        glucose residues have the beta configuration
        
        linked by (beta1->4) glycosidic bonds
        
        not digestible by animals
      - chitin: linear homopolysaccharide composed of N-acetylglucosamine residues in (beta1->4) linkage
        
        acetylated amino group makes chitin more hydrophobic and water-resistant than cellulose
  - - - extracellular matrix (ECM): gel-like material in the extracellular space of tissues that holds cells together and provides a porous pathway for nutrient and O2 diffusion; composed of an interlocking meshwork of heteropolysaccharides and fibrous proteins
      - glycosaminoglycans: heteropolysaccharides in the ECM
        
        linear polymers composed of repeating disaccharide units
        
        one monosaccharide is always N-acetylglucosamine or N-acetylgalactosamine and the other is usually a uronic acid
        
        unique to animals and bacteria
        
        some contain esterified sulfate groups
        
        types:
        
        hyaluronan
        
        differs from other major types due to much longer polymers, lack of covalent linkage to proteins, and differing monomer units
        
        alternating residues of D-glucuronc acid and N-acetylglucosamine
        
        heparan sulfate
        
        contains variable, nonrandom arrangements of sulfated and nonsulfated sugars
        
        sulfated residues gives the molecule the ability to interact specifically with proteins
        
        heparan
        
        highly sulfated, intracellular form of heparan sulfate produced primarily by mast cells
        
        used as a therapeutic agent to inhibit coagulation of blood through its capacity to bind the protease inhibitor antithrombin
        
        provide viscosity, adhesiveness, and tensile strength to the extracellular matrix
- - - - aggrecan interacts strongly with collagen in the ECM of cartilage
  - - - fibronectin: has separate domains to bind fibrin, heparan sulfate, and collagen
        
        contain the conserved RGD sequence (Arg-Gly-Asp) to bind integrins
        
        integrins: mediate signaling between cell interior and ECM molecules
    - - O-linked: a glycoside bond joins the anomeric carbon of a carbohydrate to the -OH of a Ser or Thr residue
      - N-linked: an N-glycosyl bond joins the anomeric carbon of a sugar to the amide nitrogen of an Asn residue
    - - influences the folding and stability of the proteins
      - provides critical information about the targeting of newly synthesized proteins
      - allows specific recognition by other protiens
  - - - found richly within neurons
      - play a role in signal transduction