Chapter 11

Sugars and Polysaccharides

Instructor: Rashid Syed

Textbook: Biochemistry (4th Edition ) by Donald Voet Judith G. Voet

Carbohydrates

General formula (CH2O)n or Cn(H2O)n (carbon-hydrates) Also known as saccharides or sugars They serve as energy stores or fuel They are components of DNA and RNA They are the primary component of cell walls of bacteria

and plants Glycoconjugates are carbohydrates linked to proteins and

lipids to form key intermediates and regulators of physiological processes

Glycobiology is the study of carbohydrate-containing biomolecules

Monosaccharides

They are polyhydroxy aldehydes (aldose) or ketones (ketose).

Carbon chain from 3 to 9 with general name of triose, hexose etc or aldohexose, ketohexose etc.

Fischer projection formulas are written as linear C chains with the aldehyde numbered as C1 in aldoses and ketone as C2 in ketoses.

Each monosaccharide (except dihydroxyacetone) has at least one chiral carbon. The highest numbered C is generally achiral

The orientation of the OH around the chiral carbons determines the identity of the sugar.

PresenterPresentation Notes The enantiomers are mirror images of each other. Ball-and-stick models show the actual configuration of molecules. Recall (see Figure 1-17) that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away.

Stereoisomers

PresenterPresentation NotesThe enantiomers are mirror images of each other. Ball-and-stick models show the actual configuration of molecules. Recall (see Figure 1-17) that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away.

D-Glyceraldehyde L-Glyceraldehyde

PresenterPresentation NotesThree ways to represent the two enantiomers of glyceraldehyde. The enantiomers are mirror images of each other. Ball-and-stick models show the actual configuration of molecules. Recall (see Figure 1-17) that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away.

Aldoses and Ketoses

PresenterPresentation Notes(a)Two trioses, an aldose and a ketose. The carbonyl group in each is shaded. (b) Two common hexoses.

Stereoisomers The (n-1) C determines the enantiomeric form of the sugar.

If the hydroxyl on Cn-1 is to the left, it is the L-isoform; if the n-1 hydroxyl is to the right, it is the D isoform.

Enantiomers are mirror-images of each other. Most sugars in biomolecules exist in the D-form

All the aldose and ketose stereoisomers with the same number of C atoms but not mirror images of each other are called diastereomers.

Usually there are n-2 chiral carbons in aldoses (n-3 in ketoses). There are 2(n-2) possible aldose stereoisomers (enantiomers + diastereoisomers)

Two sugars that differ in the orientation of only one hydroxyl are called as epimers of each other. Eg: glucose, mannose and galactose

D-Aldoses

PresenterPresentation NotesThe series of (a) D-aldoses and (b) D-ketoses having from three to six carbon atoms, shown as projection formulas. The carbon atoms in red are chiral centers. In all these D isomers, the chiral carbon most distant from the carbonyl carbon has the same configuration as the chiral carbon in D-glyceraldehyde. The sugars named in boxes are the most common in nature; you will encounter these again in this and later chapters.

D-Ketoses

PresenterPresentation NotesThe series of (a) D-aldoses and (b) D-ketoses having from three to six carbon atoms, shown as projection formulas. The carbon atoms in red are chiral centers. In all these D isomers, the chiral carbon most distant from the carbonyl carbon has the same configuration as the chiral carbon in D-glyceraldehyde. The sugars named in boxes are the most common in nature; you will encounter these again in this and later chapters.

Epimers

PresenterPresentation NotesEpimers. D-Glucose and two of its epimers are shown as projection formulas. Each epimer differs from D-glucose in the configuration at one chiral center (shaded pink).

Formation of Hemiacetals and Hemiketals

PresenterPresentation NotesFormation of hemiacetals and hemiketals. An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon. Substitution of a second alcohol molecule produces an acetal or ketal. When the second alcohol is part of another sugar molecule, the bond produced is a glycosidic bond (p. 243).

Cyclization of Glucose

Mutarotation: interconversion of and anomers:

PresenterPresentation NotesFormation 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, producing either of two stereoisomers, the and anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of and anomers is called mutarotation.

Haworth Projections

Alcohols react with aldehydes to form hemiacetals and with ketones to form hemiketals

Most long-chain aldoses and ketoses form intramolecular hemiacetals / hemiketals to form 5-(furanose) or 6- (pyranose) membered ring structures.

The ring-form depictions of monosaccharide structures are called Haworth projections. When converting Fischer to Haworth formulas follow the rule: right down (except for C5)

Ring formation results in C1 becoming a chiral carbon. The -anomer is when OH is below and -anomer is when OH is above the plane of the ring

Complete name of sugar includes: anomer, enantiomer, sugar identity and ring type. Eg: -D-Glucopyranose

Pyranose and Furanose

PresenterPresentation NotesPyranoses and furanoses. The pyranose forms of D-glucose and the furanose forms of D-fructose are shown here as Haworth perspective formulas. The edges of the ring nearest the reader are represented by bold lines. Hydroxyl groups below the plane of the ring in these Haworth perspectives would appear at the right side of a Fischer projection (compare with Figure 7-6). Pyran and furan are shown for comparison.

Chair Conformations

-D-Glucopyranose puts all the hydroxyls equatorial nice and stable!

PresenterPresentation NotesConformational formulas of pyranoses. (a) Two chair forms of the pyranose ring. Bonds to substituents and hydrogen atoms on the ring carbons may be either axial (ax), projecting parallel to the vertical axis through the ring, or equatorial (eq), projecting roughly perpendicular to this axis. Two conformers such are these are not readily interconvertible without breaking the ring. However, when the molecule is "stretched" (by atomic force microscopy; see Box 11-1), an input of about 46 kJ of energy per mole of sugar can force the interconversion of chair forms. Generally, substituents in the equatorial positions are less sterically hindered by neighboring substituents, and conformers with bulky substituents in equatorial positions are favored. Another conformation, the "boat" (not shown), is seen only in derivatives with very bulky substituents. (b) The preferred chair conformation of -D-glucopyranose.

Important Modified Monosaccharides

Glucosamine/Galactosamine: -OH on C2 is replaced by NH2 The amines are usually esterified with acetic acid to form N-

acetylglucosamine and N-acetylgalactosamine Glucuronic / galactouronic acid: C6 is oxidized to COOH. These modified monosaccharides are abundantly found in

mucopolysaccharides and glycosaminoglycans Neuraminose is a 9 C ketose. In sialic acid (N-

acetylneuraminic acid) C1 is oxidized to COOH and C5 has a N-acetyl group. Sialic acid is the terminal sugar of oligosaccharides found on glycoproteins

Sulfated polysaccharides: The OH on C4 or C6 may be sulfated (-OH to -OSO3-) on several glycosaminoglycans. The degree of sulfation is variable

Reactions of monosaccharides

Monosaccharides have a free carbonyl group which is prone to oxidation. Thus, monosaccharides reduce other compounds. Any sugar with a free carbonyl C is called a reducing sugar

A hemiacetal can react with an alcohol or an amine to form an adduct with the removal of H2O. The resulting bond with an alcohol results in an O-glycosidic linkage. Reaction with an amine results in a N-glycosidic bond

O-glycosidic linkages can join one monosaccharide to another to form polymers. These are called as disaccharides, oligosaccharides or polysaccharides.

Polysaccharides may be linear or branched. The repeating unit may be a monosaccharide or a disaccharide.

Sugars are reducing agents

PresenterPresentation NotesSugars as reducing agents. Oxidation of the anomeric carbon (and probably the neighboring carbon) of glucose and other sugars under alkaline conditions is the basis for Fehling's reaction. The cuprous ion (Cu+) produced forms a red cuprous oxide precipitate. In the hemiacetal (ring) form, C-1 of glucose cannot be oxidized by complexed Cu2+. However, the open-chain form is in equilibrium with the ring form, and eventually the oxidation reaction goes to completion. The reaction with Cu2+ is complex, yielding a mixture of products and reducing 3 mol of Cu2+ per mole of glucose. (b) The glucose oxidase reaction, used in the measurement of blood glucose. A second enzyme, a peroxidase, catalyzes the reaction of the H2O2 with a colorless compound to produce a colored product, which is measured spectrophotometrically.

O-Glycosidic Linkages

Two monosaccharides are linked covalently by a O-glycosidic linkage

Formed when OH of one sugar reacts with the anomeric carbon of a second sugar to form an acetal

The acetal C is no longer a reducing sugar. A polysaccharide has reducing and non-reducing

ends O-glycosidic linkages are hydrolyzed by acid but

resistant to base.

Formation of Maltose

PresenterPresentation NotesFormation of maltose. A disaccharide is formed from two monosaccharides (here, two molecules of D-glucose) when an OH (alcohol) of one glucose molecule (right) condenses with the intramolecular hemiacetal of the other glucose molecule (left), with elimination of H2O and formation of a glycosidic bond. The reversal of this reaction is hydrolysisattack by H2O on the glycosidic bond. The maltose molecule, shown here as an illustration, retains a reducing hemiacetal at the C-1 not involved in the glycosidic bond. Because mutarotation interconverts the and forms of the hemiacetal, the bonds at this position are sometimes depicted with wavy lines, as shown here, to indicate that the structure may be either or .

Rules for naming oligosaccharides Oligosaccharide names are written in the following

sequence: Anomeric configuration of sugar 1 Non-reducing sugar (sugar 1) including it ring

conformation derivatized to end in -yl In parentheses carbon atoms linked by bond are

identified with an arrow going from carbon of first to second sugar

Second sugar is named. Eg. Maltose is: -D-glucopyranosyl (14) D-glucopyranose

Names can be abbreviated: 3-letter code for sugar, anomeric orientation included in parentheses with linkage. Eg: Maltose: Glc(14)Glc

Complex Carbohydrates

Disaccharides and polysaccharides formed by O-glycosidic linkages between monosaccharides are called complex carbohydrates

In sucrose, the carbonyl carbons of both glucose and fructose participate in the O-glycosidic linkage, so it is a non-reducing sugar.

Lactose formed from glucose and galactose is the disaccharide of milk.

Polysaccharides contain more than 2 monomeric sugars Homopolysaccharides are repeats of the same monomer,

heteropolysaccharides contain a disaccharide repeating unit

Carbohydrate Nomenclature

http://www.chem.qmul.ac.uk/iupac/2carb/

A useful website if you need a detailed explanation or clarification for naming any carbohydrate molecule.

Common Disaccharides:

PresenterPresentation Notes Some common disaccharides. Like maltose in Figure 7-11, these are shown as Haworth perspectives. The common name, full systematic name, and abbreviation are given for each disaccharide. Formal nomenclature for sucrose names glucose as the parent glycoside, although it is typically depicted as shown, with glucose on the left.

Sucrose is the most chemically stable of the four: the only one without a hemiacetal.

Polysacchardies

PresenterPresentation NotesHomo- and heteropolysaccharides. Polysaccharides may be composed of one, two, or several different monosaccharides, in straight or branched chains of varying length.Unlike proteins polysaccharides generally do not have defined molecular weights (see page 245 textbook for details)

Polysaccharides

Storage polysaccharides: Starch and glycogen are polymers of glucose in (14) linkages with (16) branches. Starch is a mixture of amylose (linear) and amylopectin (branched). Glycogen is highly branched (every 8-12 units)

Structural polysaccharides: Cellulose makes up the cell wall of plant cells. It is a polymer of glucose in (14) linkages. Other eg: chitin, pectin.

Mucopolysaccharides/glycosaminoglycans: Very viscous heteropolysaccharides found in extracellular cavities and connective tissue. Hyaluronic acid and chondroitin sulfate serve as lubricants for joints. Heparin is an anticoagulant.

Glycosaminoglycans attach to proteins to form proteoglycans. Carbohydrates, rather than protein are the main component

Plants store glucose as starch, animals as glycogen Both polymers are greatly hydrated; total mol. wt. up to

1 million. 3-D structure is coiled-helical. Glycogen is highly branched; each molecule has one

reducing end and multiple non-reducing ends. Enzymatic hydrolysis proceeds from non-reducing ends.

Glycogen is abundant in the liver and exists in dense granules also containing enzymes for glycogen metabolism

Storage of glucose as glycogen prevents increase in osmolarity.

Starch and Glycogen

Starch: Amylose (linear) and Amylopectin (branched)

PresenterPresentation Notes7-14b Glycogen and starch. (a) A short segment of amylose, a linear polymer of D-glucose residues in (14) linkage. A single chain can contain several thousand glucose residues. Amylopectin has stretches of similarly linked residues between branch points. Glycogen has the same basic structure, but has more branching than amylopectin. (b) An (16) branch point of glycogen or amylopectin. (c) A cluster of amylose and amylopectin like that believed to occur in starch granules. Strands of amylopectin (red) form double-helical structures with each other or with amylose strands (blue). Glucose residues at the nonreducing ends of the outer branches are removed enzymatically during the mobilization of starch for energy production. Glycogen has a similar structure but is more highly branched and more compact.

3-D structure of amylose

PresenterPresentation NotesFIGURE 7-20a Starch (amylose). (a) In the most stable conformation, with adjacent rigid chairs, the polysaccharide chain is curved, rather than linear as in cellulose (see Figure 7-15).

FIGURE 7-20b Starch (amylose). (b) A model of a segment of amylose; for clarity, the hydroxyl groups have been omitted from all but one of the glucose residues. Compare the two residues shaded in pink with the chemical structures in (b). The conformation of (14) linkages in amylose, amylopectin, and glycogen causes these polymers to assume tightly coiled helical structures. These compact structures produce the dense granules of stored starch or glycogen seen in many cells (see Figure 20-2).

Cellulose and Chitin

These are structural polysaccharides Cellulose is glc (14). It forms the cell wall of plant cells

(cotton, wood, dietary fiber from veg.) Chitin is glcNac (14). It is the component of the hard

exoskeleton of insects. Linear, fibrous and water-insoluble; straight extended

geometry stabilized by inter-chain and intra-chain H-bonds. Most animals lack the enzyme cellulase that hydrolyzes glc

(14) linkages. Bacteria present in rumen of cattle secrete cellulase.

PresenterPresentation NotesTherefore, chitin may be described as cellulose with one hydroxyl group on each monomer substituted with an acetyl amine group. This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength.Cellulose is a fibrous, tough water insoluble substance is found in the cell walls of plants particularly in stalks, stems, trunks, and all woody portions of the plant body. Glycogen ingested in the diet are hydrolyzed by alpha-amylase and glycosidases, enzymes in saliva and the intestine that break alpha 1-4 linkages. Most animals can not digest cellulose as they lack cellulase enzyme which is capable of hydrolyzing beta 1-4 linkages. Termites readily digest cellulose (and therefore wood) but only because their intestines harbors a symbiotic microorganism, Trichonympha, that secretes cellulase.

Cellulose and Chitin

PresenterPresentation NotesCellulose. (a) Two units of a cellulose chain; the D-glucose residues are in (14) linkage. The rigid chair structures can rotate relative to one another.Chitin. (b) A short segment of chitin, a homopolymer of N-acetyl-D-glucosamine units in (14) linkage. Therefore, chitin may be described as cellulose with one hydroxyl group on each monomer substituted with an acetyl amine group. This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength.

Heteropolysaccharides found in extracellular matrix along with fibrous proteins. They facilitate cell-cell adhesion. Found as lubricant in joints.

Linear, disaccharide is repeating unit. One of the repeating dimer is glcNac or galNac; it may be sulfated at C-4 or C-6. in the other sugar, C-6 is COOH.

High charge on molecule. Assumes extended geometry to avoid repulsion. Binds to basic proteins.

Eg: Hyaluronic acid: found in vitrous humor of eye. Being developed as a pharmaceutical agent to facilitate wound-healing. Chondroitin sulfate: used to alleviate joint aches. Heparin: anti-coagulant.

Glycosaminoglycans

Glycosamino-glycans

Structures to Memorize

*Glyceraldehyde *Ribose *mannose *Glucose *Galactose *Dihydroxyacetone *Fructose

*Lactose: galactose (14)glucose *Sucrose: glucose ,(12)fructose (non-reducing sugar) *Starch (Amylose): glucose (14) glucose *Starch (Amylopectin) / Glycogen: glucose (14) glucose;

branches by (16) linkages *Cellulose: glucose (1 4) glucose *Chitin: (1 4) N-acetylglucosamine *Hyaluronate: {Glucuronic acid (13) N-acetylglucosamine (14)}n

Proteoglycans

Macromolecules with glycosaminoglycans (GAG) that are covalently attached to a core protein; GAG mass % greater than that of protein

Found on outer cell surface (integral protein) or extracellular matrix & connective tissue (secreted protein)

Functions: cell-cell recognition, adhesion, cell migration, immune response, wound healing, blood clotting etc.

GAG polymer attaches to ser on sequence S-G-X-G of core protein through a trisaccharide bridge

Proteoglycan aggregates form when many core proteins associate with a single molecule of hyaluronic acid. Link proteins at the junction of HA and core protein stabilize the aggregate.

Proteoglycan Structure

Proteoglycan Aggregate

PresenterPresentation NotesThe proteoglycan aggregate is the major structural component of the extracellular matrix of the cartilage, composed of aggrecan, hyaluronan (HA) and link protein (LP). The aggregates provide cartilage with unique gel-like property and resistance to deformation through water absorption

Glycoproteins

Glycoproteins are proteins with covalently attached oligosaccharide groups

Most cell surface proteins as well as secreted proteins are glycoproteins with the oligosaccharide component essential for protein activity and function.

Carbohydrates are linked to proteins by O-glycosidic linkages with ser or thr. They also form N-glycosidic linkages with Asn. Asn has to be in the seq NXS or NXT. No specific motif requirement for O-linked Ser / Thr.

All N-linked oligosaccharides have a pentasaccharide core that grows to various oligosaccharide patterns. Two most common ones are the high-mannose type and complex type. The first sugar in O-linked oligosaccharides is usually galNac.

O-linked glycosylation

Takes place in the Golgi apparatus by the serial addition of monosaccharide units to a completed polypeptide chain

End sugar monomer on each branch is sialic acid Oligosaccharides are synthesized by the action of

enzymes called glycosyl transferases. Many different glycosyl transferases, each specific for

formation of a specific glycosidic linkages The monomeric sugar has to be activated in the form

of a sugar nucleotide (UDP- or GDP-sugar) prior to attachment to the growing oligosaccharide

Specificity of glycosyl transferases is demonstrated by human ABO blood groups.

The basic O-type oligosaccharide antigen on RBC membranes is extended with galNac by the action of N-acetylgalactosamine transferease to form A antigen.

In individuals with the B antigen, galactosyl transferase adds on galactose to the oligosaccharide.

The two enzymes differ by only 4 aa and are absent in O-type individuals due to premature termination of polypeptide synthesis.

Importance of Glycosyl transferases in determining Blood Groups

Red Blood Cell

Red Blood Cell

Red Blood Cell

N-linked glycosylation

Starts co-translationally in the ER lumen and processing continues in the Golgi complex.

A 14-unit oligosaccharide complex is first assembled on dolichol phosphate, a lipid present in the ER membrane

The 14-unit oligosaccharide is transferred from dolichol phosphate to an acceptor Asn on the nascent protein in the ER

The oligosaccharide is trimmed before the protein leaves the ER and further modified as it passes through the Golgi

Covalent conjugates of lipids and oligosaccharides

Cerebrosides and gangliosides are found in the brain and nervous tissue

In biomembranes, glycolipids are oriented asymmetrically with the sugar units always on the extracellular side of the membrane

Lipopolysaccharides are found on outer membrane of bacterial cells and are targets of antibodies

Glycolipids

Functions of Glycoconjugates Lectins are proteins that bind oligosaccharides with high

affinity and specificity. Important for cell-cell recognition and cell adhesion

Removal of sialic acid cap makes glycoprotein a ligand for galactosyl receptor resulting in its degradation. Similar mechanism for degradation of RBCs.

P-Selectins on PM of capillary endothelial cells bind to oligosaccharides on circulating T-lymphocytes and initiate immune response.

Monocyte adhesion to lectins on endothelial cells of vasculature initiate the atherosclerotic process.

Viruses and bacteria bind to cell surface glycoproteins Cholera and pertussis toxins bind to glycolipids.

PresenterPresentation NotesPM = plasma membrane

Oligosaccharide interactions with lectins

PresenterPresentation NotesFIGURE 7-35 Roles of oligosaccharides in recognition and adhesion at the cell surface. (a) Oligosaccharides with unique structures (represented as strings of hexagons), components of a variety of glycoproteins or glycolipids on the outer surface of plasma membranes, interact with high specificity and affinity with lectins in the extracellular milieu. (b) Viruses that infect animal cells, such as the influenza virus, bind to cell surface glycoproteins as the first step in infection. (c) Bacterial toxins, such as the cholera and pertussis toxins, bind to a surface glycolipid before entering a cell. (d) Some bacteria, such as H. pylori, adhere to and then colonize or infect animal cells. (e) Selectins (lectins) in the plasma membrane of certain cells mediate cell-cell interactions, such as those of neutrophils with the endothelial cells of the capillary wall at an infection site. (f) The mannose 6-phosphate receptor/lectin of the trans Golgi complex binds to the oligosaccharide of lysosomal enzymes, targeting them for transfer into the lysosome.

Slide Number 1CarbohydratesMonosaccharidesSlide Number 4StereoisomersSlide Number 6Aldoses and KetosesSlide Number 8StereoisomersD-AldosesD-KetosesEpimersFormation of Hemiacetals and HemiketalsCyclization of GlucoseHaworth ProjectionsPyranose and FuranoseChair ConformationsImportant Modified MonosaccharidesSlide Number 19Reactions of monosaccharides Sugars are reducing agentsO-Glycosidic LinkagesFormation of MaltoseRules for naming oligosaccharidesComplex CarbohydratesCarbohydrate NomenclatureSlide Number 27Slide Number 28PolysacchardiesPolysaccharidesStarch and GlycogenStarch: Amylose (linear) and Amylopectin (branched)3-D structure of amylose Cellulose and ChitinCellulose and ChitinGlycosaminoglycansGlycosamino-glycansSlide Number 38Structures to MemorizeProteoglycansProteoglycan StructureProteoglycan AggregateGlycoproteinsSlide Number 44O-linked glycosylationSlide Number 46Slide Number 47N-linked glycosylationGlycolipidsFunctions of GlycoconjugatesOligosaccharide interactions with lectins