Chapter 7

Carbohydrates and the Glycoconjugates of Cell Surfaces

Biochemistryby

Reginald Garrett and Charles Grisham

Essential Question

• What is the structure, chemistry, and biological function of carbohydrates?

• (CH2O)n or (C · H2O)n

• Breakdown of carbohydrates provides energ

y.

• Glycolipids and glycoproteins are glycoconju

gates involved in recognition between cell ty

pes or recognition of cellular structures by ot

her molecules.

Outlines• How Are Carbohydrates Named?• What Is the Structure and Chemistry of

Monosaccharides? • What is the Structure and Chemistry of

Oligosaccharides?• What is the Structure and Chemistry of

Polysaccharides? • What Are Glycoproteins, and How Do They

Function in Cells?• How Do Proteoglycans Modulate Processes

in Cells and Organisms?

7.1 – How Are Carbohydrates Named?

Carbohydrates are hydrates of carbon.

• Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions.

• Oligo = "a few" - usually 2 to 10 • Polysaccharides are polymers of the

simple sugars.

7.2 – What Is the Structure and Chemistry of Monsaccharides?

An organic chemistry review

• Aldoses and ketoses contain aldehyde and ketone functions, respectively.

• Triose, tetrose, etc. denotes number of carbons.

• Aldoses with 3C or more and ketoses with 4C or more are chiral.

• Review Fischer projections and D,L system.

Stereochemistry Review

Read text on p. 204-207 carefully!

• D,L designation refers to the configuration of the highest-numbered asymmetric center.

• D,L only refers the stereocenter of interest back to D- and L-glyceraldehyde!

• D,L do not specify the sign of rotation of plane-polarized light!

• All structures in Figures 7.2 and 7.3 are D.

• D-sugars predominate in nature.

More Stereochemistry

Know these definitions

•Stereoisomers that are mirror images of each other are enantiomers.

•Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers.

•Any 2 sugars in a row in Figures 7.2 and 7.3 are diastereomers.

•Two sugars that differ in configuration at only one chiral center are epimers.

Cyclic monsaccharide structures and anomeric forms

• Glucose (an aldose) can cyclize to form a cyclic hemiacetal.

• Fructose (a ketose) can cyclize to form a cyclic hemiketal.

• Cyclic form of glucose is mainly a pyranose.

• Cyclic form of fructose is mainly a furanose.

Cyclic monsaccharide structures and anomeric forms

•Cyclic forms possess anomeric carbons.•For D-sugars, has OH down, has OH up.

For L-sugars, the reverse is true.•Mutarotation: The optical rotation of glucose

solution could change with time. It involves int

erconversion of - and -D-glucose.

•[]D20 = 112.2 for -D-glucose

[]D20 = 18.7 for -D-glucose

Monosaccharide Derivatives• Reducing sugars: sugars with free anomeric carbons - they will reduce oxidizing agents, such as peroxide, ferricyanide and certain metals (Cu2+ and Ag+).

• Fehling’s reagent: CuSO4 (blue) + RC(=O)H Cu2O (red) + RCO2-

• Tollen’s reagent: Ag+ Ag0• These redox reactions convert the sugar to a sugar acid.

• Glucose is a reducing sugar --- so these reactions are the basis for diagnostic tests for blood sugar.

More Monosaccharide Derivatives

• Sugar alcohols (alditols): sweet-tasting, from mild reduction of sugars

• Deoxy sugars: constituents of DNA, etc. • Sugar esters: phosphate esters like ATP

are important.• Amino sugars contain an amino group in

place of a hydroxyl group. • Acetals, ketals and glycosides: basis for

oligo- and poly-saccharides.

7.3 – What is the Structure and Chemistry of Oligosaccharides?

It’s not important to memorize structures, but you should know the important features.

• Be able to identify anomeric carbons and reducing and nonreducing ends.

• Sucrose is NOT a reducing sugar.

• Browse the structures in Figure 7.19 and Figure 7.20.

• Note carefully the nomenclature of links! Be able to recognize (1,4), (1,4), etc.

7.4 – What is the Structure and Chemistry of Polysaccharides?

Functions: storage, structure, recognition

• Nomenclature: homopolysaccharide vs. heteropolysaccharide.

• Lower the osmotic pressure.

• Starch and glycogen are energy storage molecules.

• Chitin and cellulose are structural molecules.

• Cell surface polysaccharides are recognition molecules.

Starch

A plant storage polysaccharide

• Two forms: amylose and amylopectin • Most starch is 10-30% amylose and 70-

90% amylopectin. • Amylose has (1,4) links and one

reducing end.• Amylopectin has (1,6) branches in

every 12-30 residues.

Starch• Amylose and amylopectin are poorly

soluble in water, but form micellar suspensions.

• In these suspensions, amylose is helical and iodine fits into the helices to produce a blue color. Amylopectin produces a red-violet color with I2.

• Salivary -amylase, an endoamylase, is (14)-glucan 4-glucanhydrolase.

-amylase is an exoamylase, cleaving maltose units.

(16)-glucosidase is required for complete hydrolysis of amylopepctin.

Why branching in Starch?

Consider the phosphorylase reaction...

• Phosphorylase releases glucose-1-P, products from the amylose or amylopectin chains.

• The more branches, the more sites for phosphorylase attack.

• Branches provide a mechanism for quickly releasing (or storing) glucose units for (or from) metabolism.

Glycogen--- the glucose storage device in animals

• Glycogen constitutes up to 10% of liver mass and 1-2% of muscle mass.

• Glycogen is stored energy for the organism.

• Only difference from amylopectin: number of branches.

(1,6) branches every 8-12 residues .• Like amylopectin, glycogen gives a red-

violet color with iodine.• Hydrolyzed by -, -amylase, and glycog

en phosphorylase.

DextransA small but significant difference from starch and

glycogen.

• If you change the main linkages between glucose from (1,4) to (1,6), you get a new family of polysaccharides – dextrans.

• Branches can be (1,2), (1,3), or (1,4).• Dextrans formed by bacteria are components of

dental plaque. • Cross-linked dextrans are used as "Sephadex"

gels in column chromatography. • These gels are up to 98% water!

Structural Polysaccharides

Composition similar to storage polysaccharides, but small structural differences greatly influence properties.

• Cellulose is the most abundant natural polymer on earth.

• Cellulose is the principal strength and support of trees and plants .

• Cellulose can also be soft and fuzzy - in cotton.

Other Structural Polysaccharides

• Chitin - exoskeletons of crustaceans, insects and spiders, and cell walls of fungi.

– similar to cellulose, but C-2s are N-acetyl

– cellulose strands are parallel, chitins can be parallel or antiparallel.

• Alginates – Ca2+-binding polymers in algae.

• Agarose and agaropectin - galactose polymers

• Glycosaminoglycans - repeating disaccharides with amino sugars and negative charges.

Bacterial Cell WallsComposed of 1 or 2 bilayers and peptidoglycan shell

• To resist high internal osmotic pressure, to maintain cell shape and size of bacteria.

• Gram-positive: One bilayer and thick peptidoglycan outer shell.

• Gram-negative: Two bilayers with thin peptidoglycan shell in between .

• Gram-positive: pentaglycine bridge connects tetrapeptides.

• Gram-negative: direct amide bond between tetrapeptides.

More Notes on Cell Walls

• Note the -carboxy linkage of isoglutamate in the tetrapeptide

• Peptidoglycan is called murein - from Latin "murus", for wall

• Gram-negative cells are hairy! Note the lipopolysaccharide "hair" in Figures 7.35 and 7.36.

Cell Surface PolysaccharidesA host of important functions!

• Animal cell surfaces contain an incredible diversity of glycoproteins (on the dell surface) and proteoglycans (in the extracellular matrix).

• In glass dishes, heart myocytes “beat” and liver cells avoid contact with kidney cells. Cancer cells grow without contact inhibition.

• These polysaccharide structures regulate cell-cell recognition and interaction. They contain several points for linkage (-OH) and are more informative than linear proteins and nucleic acids.

• The uniqueness of the "information" in these structures is determined by the enzymes that synthesize these polysaccharides.

7.5 – What Are Glycoproteins, and How Do They Function in Cells?

Many structures and functions!

• May be N-linked or O-linked.

• N-linked saccharides are attached via the amide nitrogens of asparagine residues.

• O-linked saccharides are attached to hydroxyl groups of serine, threonine or hydroxylysine.

• See structures in Figure 7.39

O-linked Saccharides of Glycoproteins

• Function in many cases is to adopt an extended and relatively rigid conformation.

• These extended conformations resemble "bristle brushes“.

• Bristle brush structure extends functional domains up out of the glycocalyx.

• See Figure 7.40

N-linked OligosaccharidesMany functions known or suspected

• N-glycosylation of proteins can alter the chemical and physical properties of proteins, altering solubility, mass, and electrical charges.

• N-linked oligosaccharide moieties can (1) stabilize protein conformations, (2) protect against proteolysis and (3) promote correct folding of certain globular proteins (p. 239).

• Cleavage of monosaccharide units from N-linked glycoproteins in blood targets them for degradation in the liver. - see pages 238, 239

7.6 - Proteoglycans

--- Glycoproteins whose carbohydrates are mostly glycosaminoglycans.

• Components of the cell membrane and glycocalyx.

• Consist of proteins with one or two types of glycosaminoglycan.

• See structures, Figure 7.44

7.6 – How Do Proteoglycans Modulate Processes in Cells and Organisms?

• Proteoglycans are glycoproteins whose carbohydrate moieties are predominantly glycosaminoglycans.

• Example: syndecan - transmembrane protein - inside domain interacts with cytoskeleton, outside domain interacts with fibronectin.

• Highly sulfated glycosaminoglycans bind specific proteins (e.g. fibronectin) at sites containing basic amino acid residues. (charge interactions)

• A particular pentasaccharide sequence in heparin finds to antithrombin III. (sequence-specific)

Proteoglycan Functions

• Modulation of cell growth processes – Binding of growth factor proteins by

proteoglycans in the glycocalyx provides a reservoir of growth factors at the cell surface.

• Cushioning in joints – Cartilage matrix proteoglycans absorb large

amounts of water. During joint movement, cartilage is compressed, expelling water!