chapter 7

Carbohydrates

•aldehydes or ketones with at least two hydroxyl groups, or substances that yield such compounds on hydrolysis

•

•many carbohydrates have the empirical formula (CH2O)n

Monosaccharides

•simple sugars, consist of a single polyhydroxy aldehyde or ketone unit

–example: D-glucose  

Oligosacchraides

•short chains of monosaccharide units, or residues, joined by glycosidic bonds

Disaccharides

•oligosaccharides with two monosaccharide units

example: sucrose (D-glucose and D-fructose)

Polysaccharides

•sugar polymers with 10+ monosaccharide units

–examples: cellulose (linear), glycogen (branched)

Backbones of monosaccharides

• 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 hydroxyl group

Steroisomerism in sugars

Suagr stereoisomers arise because many of the carbon atoms to which the hydroxyl groups are attached to chiral centers

• enzymes that act on sugars are stereospecific

Aldose

Carbonyl group is at he need of the carbon chain ( in an aldehyde group)

Ketone

Carbonyl group is at any other position ( in a ketone group)

Trioses

Simple set monosaccharides,three carbon backbone

Tetroses

Four carbon backbone

Pentoses

Five carbon backbone

Hexoses

Six carbon backbone

Heptoses

Seven carbon backbone

What makes sugar sweet?

•TAS1R2 and TAS1R3 encode sweet-taste receptors

•

•binding of a compatible molecule generates a “sweet” electrical signal in the brain

–requires a steric match

Fischer projection formulas

used to represent three-dimensional 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

D isomers

configuration at reference carbon is the same as D-glyceraldehyde

on the right (dextro) in a projection formula

most hexoses of living organisms

L isomer

configuration at reference carbon is the same as L-glyceraldehyde

on the left (levo) in a projection formula

Numbering carbons of sugar

Carbons are numbers beginningat the end of the chain near the carbonyl group

Epimers

Two sugars that differ only in the configuration around one atom

Hemiacetlas or hemiketals

derivatives formed by a  general reaction between alcohols and aldehydes or ketones

product of the first alcohol molecule addition

a five- or six-membered ring forms if the —OH and carbonyl groups are on the same molecule

Acetal or ketal

Product of the second alcohol molecule addition

• forms a glycosidic bond

Anomers

isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom

Formation of hte 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

Pyranoses

six-membered ring compounds

form when the hydroxyl group at C-6 reacts with the keto group at C-2

furanoses

• five-membered ring compounds

• form when the hydroxyl group at C-5 reacts with the keto group at C-2

Haworth perspective formulas

more accurate representation of cyclic sugar structure than Fischer projections

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 prespective formulas

step 1: draw the six­-membered ring (five carbons, and one oxygen at the upper right)

step 2: number the carbons in a clockwise direction beginning with the anomeric carbon

step 3: place the hydroxyl groups

hydroxyl groups on the right in a Fischer projection are placed pointing down and those on the left are placed pointing

step 4: place the terminal —CH2OH group

projects upward for the D enantiomer, downward for the L enantiomer

step 5: place the anomeric hydroxyl group

for a β structure, the hydroxyl group is placed on the same side of the ring as C-­6

for an α structure, it is placed on the opposite side

Aldonic acids

Form following the oxidation of the carbonyl carbon of aldoses

Form stable intramolecular esters called lactones

Uronic acids

Form following oxidation at C-6

Form stable intramolecluar ester called lactones

Reducing sugars

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

reduction of Cu2+ to Cu+ forms a brick-red precipitate

ketoses that can tautomerize to form aldehydes are also reducing sugars

O -glycosidic bond

covalent linkage joining two monosaccharides

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

readily hydrolyzed by acid

Reducing end

The end of the disaccharide or polysaccharide chain with a free anomeric carbon

Name reducing oligosaccharides

step 1: with the nonreducing end on the left, give the configuration (α or β) at the anomeric carbon joining the first unit to the second

step 2: name the nonreducing residue using “furano” or “pyrano”

step 3: indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers

step 4: name the second residue and repeat for additional residues

Three common disaccharides

Lactose

Sucrose

Trehalose

Polysaccharides

most carbohydrates in nature occur as polysaccharides (Mr > 20,000)

also called glycans

Homopolysaccharides

contain only a single monomeric sugar species

serve as storage forms and structural elements

Heterpolysaccharides

contain 2+ kinds of monomers

provide extracellular support

Distinction between proteins and polysaccharides

The distinction between These two is the consequence of the mechanism of assembly

There is no template for polysaccharide synthesis

The program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of monomer units

Storage polysaccharides

Starch in plant cells and glycogen in animal molecules

• starch and glycogen molecules are heavily hydrated because they have many exposed hydroxyl groups available to hydrogen

Starch

contains two types of glucose polymer, amylose and amylopectin

amylose = long, unbranched chains of D-glucose residues connected by (α1→4) linkages

amylopectin = larger than amylose with (α1→4) linkages between glucose residues and highly branched due to (α1→6) linkages

Glycogen

polymer of (α1→4)-linked glucose subunits, with (α1→6)-linked branches

more extensively branched

more compact than starch

Cellulose

tough, fibrous, water-insoluble substance

• Linear, unbranched homopolysaccharide, - -consisting of 10,000 to 15,000 D-glucose units

• glucose residues have the β configuration

• Linked by (β1→4) glycosidic bonds

• animals do not have the enzyme to hydrolyze  (β1→4) glycosidic bonds

Chitin

Linear homopolysaccharide composed of N- acetylglucosamine residues in ( beta 1 —4) linkage

-Acetylated amino group makes chitin more hydrophobic and water resistant then cellulose

Helical structure of starch and glycogen

most stable three-dimensional structure for the (α1→4)-linked chains of starch and glycogen

six residues/turn

Linear structure of cellulose

most stable conformation is a straight, extended chain

each chair is turned 180° relative to its neighbors

Peptiodolglycan

• rigid component of bacterial cell walls

• heteropolymer of alternating (β1→4)-linked N-acetylglucosamine and N-acetylmuramic acid residues

  cross-linked by short peptides

Extra cellular matrix (ECM)

• gel like material in the extracellular space of tissues that hold cells together and provides a porous pathway for nutrient and O2 diffusion

• Composed of interlocking mesh work of heteropolysaccharides(ground substance) and fibrous proteins

• -basement membrane(specialized ECM) also contain heteropolysaccharides

Glycosaminoglycans

heteropolysaccharides in ECM

linear polymers composed of repeating disaccharide units

one monosaccharide is always either N-acetylglucosamine or N-acetylgalactosamine and the other is usually a uronic acid

unique to animals and bacteria

some contain esterified sulfate groups

Hyaluronan(hyaluronic acid)-

alternating residues of D-glucuronic acid and N-acetylglucosamine

Heparin 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

Glycoconjugate

Biologically active molecule consisting of an informational carbohydrate joined to a protein or lipid

Proteoglycans

• Macromolecules of the cell surface or ECM consisting of 1 + sufated glycosaminoglycan chain(s) joined covalently to a membrane or protein or secreted protein

• major component of extracellular matrices

Glycoproteins

• have one or several oligosaccharides joined covalently to a protein

• - found ont he outer face of plasma membrane in ECM, in blood and in organelles( golgi complexes, secretory granules and lysosomes )

• Oligosaccharides portions are heterogenous and rich in information

Glycolipids

Plasma membrane components in which the hydrophilic head groups are oligosaccharides

Gylcosphingolipids

Class of glycolipids with specific backbone structure

• neurons are rich in glycosphingolipids

• Play a role in transduction

Proteoglycan unit

“Core protein” with covalently attached glycosaminoglycan(s)

Tetrasaccharide linker

Connects to glycosaminoglycan to Ser residue of the protein

Syndecans

Single trans membrane domain and extracellular domain bearing 3-5 chains of heparan sulfate and chondroitin

Glypicans

attached to the membrane by a GPI anchor (a glycosylated derivative of the membrane lipid phosphatidylinositol)

NS Domian

highly sulfated domains that alternate with domains having unmodified GlcNAc and GlcA residues

Poroteoglycan aggregates

supramolecular assemblies of many core proteins all bound to a single molecule of hyaluronan

aggrecan interacts strongly with collagen in the ECM of cartilage

Thrombin and Antithrombin

antithrombin binds to and inhibits the protease thrombin only in the presence of heparan sulfate

both proteins are rich in Arg and Lys residues

interact electrostatically with the sulfates of the glycosaminoglycans

How does chondroitin sulfate, dermatan sulfate, keratan sulfate, and heparan sulfate differ from hyaluronan in three respects:

generally much shorter polymers

covalently linked to specific proteins (proteoglycans)

one or both monomer units differ from hyaluronan

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

What is the purpose of interactions between cells and the ECM?

• anchor cells tot he ECM providing strength and elasticity of skin and joints

• Provide paths that direct the migration of cell interior developing tissue

• Convey information in both directions across the plasma membrane

O linked -(type of attachment)

 a glycoside bond joins the anomeric carbon of a carbohydrate to the —OH of a Ser or Thr residue

N-LInked ( type of attachment )

an N-glycosyl bond joins the anomeric carbon of a sugar to the amide nitrogen of an Asn residue

Mucins( ex of glycoproteins)

secreted or membrane glycoproteins

can contain large numbers of O-linked oligosaccharide chains

present in most secretions

Ex of glycoprotiens( protiens of blood )

examples: immunoglobulins (antibodies), follicle-stimulating hormone, luteinizing hormone, and thyroid-stimulating hormone

Ex of glycoproteins- milk protiens

example: major whey protein α-lactalbumin

Glycomics

= the systematic characterization of all carbohydrate components of a given cell or tissue, including those attached to proteins and to lipids

What are the advantages of adding oligosaccharides to proteins?

covalently attached oligosaccharides:

influence the folding and stability of the proteins

provide critical information about the targeting of newly synthesized proteins

allow specific recognition by other proteins

Gangliosides

membrane lipids of eukaryotic cells in which the polar head group is a complex oligosaccharide containing a sialic acid and other monosaccharide residues

Lipopolysaccharides

dominant surface feature of the outer membrane of gram-negative bacteria

Glycobiology

the study of the structure and function of the challenge is to understand how cells use specific oligosaccharides to encode information about:

intracellular targeting of proteins

cell-cell interactions

cell differentiation and tissue development

extracellular signals

Lectins

• bind carbohydrates with high specificity and with moderate to high affinity

• -function :cell-cel recondition, signaling,adhesion ,intracellular targeting of newly synthesized proteins

Selections

family of plasma membrane lectins that mediate cell-cell recognition and adhesion in a wide range of cellular processes

move immune cells through the capillary wall

mediate inflammatory responses

mediate the rejection of transplanted organs 

Methods of carbohydrate analysis

How can you determine oligosaccharides and polysaccharide structures?

more complex than protein and nucleic acid analysis

can employ a variety of methods to determine sequence, configuration at anomeric and other carbons, and positions of glycosidic bonds:

traditional chemical and enzymatic approaches

mass spectrometry

high-resolution NMR spectroscopy

Solid phase synthetic methods

carbohydrate chemists can synthesize short segments of almost any glycosaminoglycan

solid-phase oligosaccharide synthesis:

based on the same principles as peptide synthesis

yields defined oligosaccharides

useful in exploring lectin-oligosaccharide interactions