Chem 1152: Ch. 17 Carbohydrates

Introduction Biomolecules: Organic compounds produced by living organisms Carbohydrates Lipids Proteins Nucleic acids Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011 Functions of carbohydrates 1.Provide energy through their oxidation 2.Supply carbon for synthesis of cell components 3.Serve as stored form of chemical energy 4.Form structural elements of some cells and tissues

Classes of Carbohydrates (saccharides) Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011; Hydrates of carbon C m (H 2 O) n characterized by having multiple functional groups Hydroxyl (alcohols) Carbonyl (aldehydes or ketones) Mono- and disaccharides are simple sugars glucose fructose galactose ribose deoxyribose sucrose lactose Fructo-oligosaccharides galactooligosaccharides starch glycogen cellulose chitin

Stereochemistry of Carbohydrates Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011; Stereoisomerism: Isomeric molecules can have the same molecular formula and sequence of bonded atoms (constitution), but that differ in the three- dimensional orientations of their atoms in space. Enantiomers: Stereoisomers that are mirror images of each other. Chirality: “Handedness”. Refers to compounds that cannot be superimposed on mirror image. -Defined relative to central, chiral atom (carbon) enantiomers

Stereochemistry of Carbohydrates Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011 Chiral Carbon: Carbon in organic compound that has four different groups attached to it. Chirality: “Handedness”. Refers to compounds that cannot be superimposed on mirror image. enantiomers OHC H C OH CH 2 OH CHO H C HO CH 2 OH OHC C OH CH 2 OH CHO C HO CH 2 OH Chiral carbon CHO indicates aldehyde

Identify Chiral Carbons OHC H C OH CH 2 OH OHCCHOH CH 2 OH CH 3 CCH 2 CH 3 O * CH 3 CHCH 3 OH CH 3 CHCH 2 CH 3 OH

Identify chiral carbons in glucose glucose C C C C CH 2 C OH HOH H H H H O

p. 528 chiral carbons in glucose Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011 Max # of possible stereoisomers = 2 n where n = number of chiral carbon atoms

Fischer Projections Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Chiral Carbons represented by intersection of two lines  For fischer projections the prefixes (L-) and (D-) do not indicate Levorotatory (L-) and Dextrorotatory (D-) related to rotation of polarized light, but represent orientation of functional groups when compared to glyceraldehyde

Fischer Projections Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  When carbonyl is up:  D-family: OH (or NH 2 ) group of chiral C most distant from anomeric center projects to right  L-family: OH (or NH 2 ) group of chiral C most distant from anomeric center projects to left  Anomeric center: Carbonyl (aldehyde or ketone) carbon  Biological systems can only utilize the D- isomers. * * * * * * * * * * * *

Monosaccharides Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Contain single polyhydroxy aldehyde or ketone unit  Further classified based on number of C atoms  Aldehydes contain prefix aldo-, ketones have prefix keto-

Monosaccharides: D- aldoses Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011 Chiral C’s? stereoisomers

Monosaccharides: Chemical Properties Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Because aldehydes and ketones react with alcohols to form hemiacetals and hemiketals, all monosaccharides with at least 5 carbon atoms exist predominantly in cyclic forms.  These are depicted using Haworth structures  Ring is drawn with oxygen to the back, and anomeric carbon to the right.  Furanose ring: A 5-member ring containing an oxygen atom.  Pyranose ring: A 6-member ring containing an oxygen atom.

Glucose (Blood Sugar) Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Primary source of energy in cells  Precursor for Vitamin C synthesis  Modified subunits can form long polymer chains  starch, cellulose, glycogen  Beta-hydroxy group: OH attached to anomeric carbon above ring.  Alpha-hydroxy group: OH attached to anomeric carbon below ring. Hemiacetal (on C1) 64% 0.02% 36%

Fructose  Fruit sugar  Absorbed directly into bloodstream during digestion (like glucose and galactose)  Anomeric carbon is C2 70% 22% Hemiketal (on C2)

Chemical Reactions of Sugars Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Open chain forms of monosaccharides (aldehydes, hydroxyketones) can be readily oxidized.  Ex. Use of Benedict’s reagent to oxidize aldehydes and ketones with hydroxy group on adjacent carbon.  At the same time, the cyclic forms are converted to open-chain forms and also react.  Reducing sugars: Monosaccharides that can be oxidized  Oxidation of carbohydrates to CO 2 and H 2 O very important at cellular level, serves as source of heat and energy. Reducing sugar + Cu(II)  oxidized cmpd + Cu 2 O Deep blue solution Red-orange precipitate

Other Important Monosaccharides Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011: Ribose and Deoxyribose  Used to synthesize RNA and DNA  Used in protein synthesis Phosphodiester bond

Other Important Monosaccharides Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011 Galactose  Synthesized in mammary gland, incorporated into milk lactose  Component of the antigens present on blood cells that determine blood type

Joining Monosaccharides Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Monosaccharide units can be joined together by glycosidic linkages.  Glycosidic linkages are same as adding alcohol to hemi- intermediates Hemiacetal intermediate acetal

Polymerization of Monosaccharide Subunits Through glycosidic linkages at the hemiacetal carbons, many monosaccharide subunits can be put together to form long, branching chains via 1,4 or 1,6 linkages. These can be between 2 α, 2 β, or between an α and a β.

Disaccharides Maltose LactoseSucrose Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011

Properties of Polysaccharides Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011

Components of plant starch built from glucose residues. Plant starches Amylose (10-20%) No branching Amylopectin (80-90%) Branching residues

Structure of glycogen Polymer built from glucose subunits. Glucose in Glycogen are connected via α-1,4 or α-1,6 linkage. α-1,4 linkage makes a linear chain, α-1,6 linkage makes a branch (~every 10 residues). The end glucose residues without open 1’-OH is called nonreducing ends. Branches provide more non-reducing ends for rapid degradation.

What is glycogen and why do we need it? Long-term energy reservoir for glucose in animal and fungal cells. Found primarily in muscle and liver cells. Liver produces glycogen for needs of organism, while muscle takes care of only itself. Glycogen not as energy rich as fatty acids, and is used differently. Controlled breakdown of glycogen and glucose release maintain blood-glucose levels. Glucose is the primary fuel used by the brain. Unlike fatty acids, glucose from glycogen is readily mobilized and can provide energy for sudden, strenuous activity.

Most important structural polysaccharide and single most abundant organic compound on earth. Provides strength and rigidity to plant cell walls. Wood is ~50% cellulose. Contains glucose subunits. Form extended straight chains that hydrogen bond with parallel chains, creating long, rigid fibers. Undigestible by humans.Cellulose

Intramolecular addition of alcohols to aldehyde  C1 is hemiacetal carbon.  Attached to it you will find: H, OH, OR and R, just like non-cyclical compounds. Intramolecular hemiacetal glucose C C C C CH 2 C OH HOH H H H H O * CH 2 OH C C C C O C OHOH H OH H H H H