Carbohydrates

 

A.  What are carbohydrates

            -the single most abundant class of organic molecules found in nature.

            -"hydrates of carbon" ( C+H2O);  thus the formula CH2O

 

1. Carbohydrates function in living systems as:

a. energy reservoir in biosphere

b. energy storage and distribution within organisms

c. biosynthetic precursors to amino acids & nucleic acids

d. units within glycoproteins which are used for cellular communication*

e. antigentic properties (ABO blood in humans) which identifies "self" from "non-self".

f. structural and mechanical components: cell walls in plants & bacteria and cartilage in animals

 

   2. Chemical characteristics of carbohydrates

    a. existence of at least one asymmetric carbons (chiral)

    b. ability to exist in either a linear or ring structure

    c. the capacity to form polymeric structures via glycosidic linkages

    d. the potential for form multiple hydrogen bonds with water and other molecules

 

 

B. Carbohydrate Classification

 

1. Monosaccharides- Sugars that cannot be hydrolyzed into simpler sugars that consist of 3 to 7 carbons

    and exist as either:

* Aldoses- multi-hydroxylated aldehydes

* Ketoses- multi-hydroxylated ketones  

 

Image:  Aldoses.  Aldoses from IUPAC

Image:  Ketoses 

 

a. Carbohydrate size is represented by the appropriate numeric prefix

-ex. triose (3-C monosaccharide)

tetrose (4-C monosaccharide)

 

b. Functionality is represented by the appropriate prefix: Keto or aldo

-ex.  aldotriose (3-C monosaccharide with an aldehyde group)

   ketotriose (3-C monosaccharide with a ketone group)

 

Note:  sometimes the -ul- is used instead of the keto prefix.

     -ex.ketotriose = triulose

    ketotetrose = tetrulose  

 

Tutorial:  Nomenclature of Carbohydrates: IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN)

 

c.  Properties

1. Most are water-soluble

  2. Most taste sweet

3. Have an asymmetric center (chiral) thus creating enantiomers

-for each asymmetric center- 2 enantiomeric versions are created.

          -Fischer Projection models are used extensively to draw linear monosaccharides.

-Denoting D or L refers to the configuration of the highest numbered asymmetric carbon (farthest from

aldehyde or ketone groups)

 -D is used if the hydroxyl group on the highest number asymmetric carbon is on the right of the model

         -D-form monosaccharides prevail in nature (like L-amino acids)

             i. diastereomer sugars- sugars that differ at more than one chiral carbon

                ex. D-glucose and D-talose

             ii. epimer sugars- sugars that differ at one chiral carbon

                ex. D-glucose and D-mannose

 

Tutorial:  Chirality from Garrett & Grisham

 

d.     Structures  -all monosaccharides have the ability to cyclize and form ringed structures

1. aldoses cyclize to produce cyclic hemiacetals

-forming a pyranose sugar

                                 

Image:  Formation of cyclic D-Glucose

Applet: Demonstating chain cyclization

 

2. ketose cyclize to produce cyclic hemiketals  - forming a furanose sugar

             

** Each cyclization process can produce 2 isomers based on position of hydroxy of 1st carbon

     -anomers- isomers that differ by hydroxyl position of anomeric carbon (C-1)

a. a-anomer- when the hydroxyl on the anomeric carbon is the same side as the oxygen on the highest

    numbered asymmetric carbon.   (Fischer)-the hydroxyl on the anomeric is opposite the C-6 in the Haworth

 

Image:  Cyclic Aldoses & Ketoses

Images: Monosaccharides

 

b. b-anomer- when the hydroxyl on the anomeric carbon is on the opposite side as the oxygen on the

    highest numbered asymmetric carbon

   -the hydroxyl on the anomeric is on the same side as C-6 in the Haworth

 

c. Fischer to Haworth-  Right is down, left is up

 

d. Mutarotation- interconversions of the a & b forms of a sugar with the linear conformation being the intermediate

 

3. Ring stabilization

a. Chair conformation is more stable than boat

b. Bulkier substituents are more stable in equatorial and axial positions.

 

e. Derivatives

                                     

1. Reducing sugars- sugars with a free anomeric carbon (straight chain exposing the aldehyde) can reduce certain

    oxidizing agents (H2O2, ferricyanide, and certain metals)

             

            a. CuSO4  -Fehling's Solution - used to test for aldehydes -   Cu2O  is produced as a red precipitate

                -Reducing sugar (glucose) + CuSO4 + OH- --> Cu2O(s)  + sugar acid + H2O

                         

                 Aldehyde + 2 Cu+2+ 5 OH--->Carboxylate+ Cu2O+3 H2O                             

  

                The aldehyde has been oxidized to a carboxylate (sugar acid)

               

            *Diabetes mellitus test kits utilize these procedures to test for high levels of reducing sugars in the blood and urine

 

            b.  Tollen's test is also a test used for identifying reducing sugars (open aldehydes). 

                       

            c. Naming sugar acids

                1. aldonic acids: -onic acid suffix when anomeric (#1) carbon is oxidized to carboxyl

                2. uronic acids: -uronic acid suffix when non-anomeric (#6) carbon is enzymatically oxidized to carboxyl

                3. aldaric acids: -aric acid suffix when anomeric (#1) and non-anomeric (#6) are oxidized to carboxyl (dicarboxylic acid)

 

            d. Examples

                - both aldonic and uronic acids tend to esterify forming 5- and 6-membered rings (cyclization)

                - Vitamin C (L-ascorbic acid) is produced from glucose through enzymatic steps. Properties of Vitamin C

 

2. Sugar Alcohol- reduction of the aldehydes/ketones with NaBH4 producing sugar alcohols

             -also known as alditols (-itol added to the parent name)

 

             ex. sorbitol, mannitol, xylitol. -sweetners in sugarless gums and mints

                   inositol- found in certain fat molecules in cell membranes

                   ribitol- found in the flavin coenzymes (FAD, FMN)

                   glycerol- serves as the backbone in triglyceride biosynthesis.

                   FAD is a coenzyme to many biological enzyme.

 

                IUPAC Nomenclature for alditols

 

3. Deoxy sugars -removal of hydroxyls from monosaccharides

               ex. 2-Deoxy-D-ribose (b-D-2-deoxyribose) - found in DNA

               ex. 6-deoxy-D-rhamnose - found in ouabain (cardiac glycoside) poison arrows, inhibits Na+/K+ ATPase

                                                

        4. Sugar Esters -sugars that form ester linkages with other molecules

               ex. Phosphate esters of sugars. Found in many biomolecules (DNA, ATP, GTP, and many

                metabolic intermediates)

                       

        5. Amino Sugars -sugars that incorporate an amino group instead of a hydroxyl group

                ex. D-glucosamine & D-galactosamine (amino groups at C-2)

                    -found in peptidoglycan (part of cartilage) and chitin in many arthropods

 

                Muramic acid and Neuraminic acid- found in cells walls of most bacteria

                    Naming of amino sugars

 

Review:  Monosaccharides & their derivatives

 

2.  Disaccharides- composed of 2 monosaccharides

-glycosidic linkage- ester linkage between an anomeric carbon of one monosaccharide and a hydroxyl

moiety of a second monosaccharide

  -the glycosidic linkage produced from a dehydration reaction produces a mixed acetal

  -glycosidic linkages are catalyzed by glycosyltransferases

  -the hydrolysis of glycosidic bonds (breaking by adding water) is catalyzed by glycosidases (glycoside hydrolases)

   -classification of glycosidases

  -each disaccharide contains a reducing end (free anomeric carbon) and a non-reducing end where the

terminus is the hydroxyl group of the first residue

 

a. Homodisaccharides

 

1. Maltose - (glucose-a-1-->4-glucose)- result of amylase enzymes (diastase) working on starch.

Found in malted milk, used as a precursor for brewing of beer (yeast)

 

2. Isomaltose- (glucose-a-1-->6-glucose)- result as the product of hydrolysis of some polysaccharides (dextran)

 

3. Cellobiose- (glucose-b-1-->4-glucose)- constituent in cellulose

 

b. Heterodisaccharides

 

 1. Lactose (galactose-b-1-->4-glucose)- found in milk

 

Definition:  Lactose Intolerance.  U.S. National Library of Medicine (NIH)

 

2. Sucrose (glucose-a-1-->2-b-fructose)- known as table sugar, exists as a nonreducing sugar

                (no free anomeric sugar)

 

3. Trehalose- (glucose-a-1-->1-a-glucose)- found as "blood sugar" in insects.  Believed to function also as a

    natural cryoprotectant.

 

4. Glycosaminoglycan-  Chrondroitin-4-sulfate, Chrondroitn-6-sulfate, hyaluronic acid, heparin, dermatan sulfate.

            - these are abundant in extracellular spaces, particularly in connective tissue, such as cartilage, tendon,

              skin and blood vessel walls.

 

Nomenclature:  Disaccharides 

 

3.  Polysaccharides- long chains of monosaccharides

 

a. Homopolysaccharides (homoglycans) -typically named for the monosaccharides they contain

             ex. Mannans or glucans

           1. Amylose- long chains of (glc-a-1-->4-glc) - One component of starch -dyes blue with iodine

 

2. Amylopectin-  long chains of (glc-a-1-->4-glc) but with (glc-a-1-->6-glc) side branches about every 12-20 residues

              -dyes red-violet with iodine

              -most corn starch in the U.S. is about 25% amylose and 75% amylopectin

 

           3. Dextran- long chains of (glc-a-1-->6-glc).  Found in bacteria and yeast.  Major  contributor in dental plaque.

                Used in column chromatrography (Sephadex and Bio-gel)

 

4. Glycogen- long chains of (glc-a-1-->4-glc) but with (glc-a-1-->6-glc) side branches about every 8-12residues. 

   Major storage for glucose in mammals.  Found in the liver.

 

5. Cellulose- long chains of (glc-b-1-->4-glc). Found in the cell walls of nearly every living plant species.

-Most abundant biomolecule in the world

                    

       6. Chitin- long chains of N-acetyl-D-glucosamine in b(1-->4) arrangement

            -found in the exoskeletons of most arthropods.

            -second most abundant biomolecule in the world.

 

    b. Heteropolysaccharides.  Less common. Made from different monosaccharides

Additional Links: 

    Carbohydrates.  From the Virtual Textbook of Organic Chemistry. Michigan State University

 

    Biological Science 302:  Carbohydrate Chemistry.  Rice University.  This is a pdf format page.