CARBOHYDRATE STRUCTURES

Student Edition 5/24/13 Version

Pharm. 304 Biochemistry

Fall 2014

Dr. Brad Chazotte 213 Maddox Hall

chazotte@campbell.edu

Web Site:

http://www.campbell.edu/faculty/chazotte

Original material only ©2003-14 B. Chazotte

Goals• Review stereochemistry with emphasis on carbohydrates

• Learn the various projections & carbon numbering for carbohydrate structures

• Learn the structures of a few, selected biologically important aldoses & ketoses

• Understand that sugars are conformationally variable and this effects their properties.

• Understand some of the basic reactions of carbohydrate functional groups

• Become familiar with the difference types of monosaccharide derivatives

• Learn the differences and similarities among disaccharides, polysaccharides, glycoconjugates, peptidoglycans, proteoglycans, & glycoproteins

Stereochemistry and Carbohydrates: A Quick Review

• Optically active molecules rotate plane polarized light. D-dextrorotary (right, clockwise). L-levorotary (left, counterclockwise).

• Optically active molecules have an asymmetry such that they are not superimposable on their mirror image.

• This situation is characteristic of substances that contain tetrahedral carbons having four different substituents.

• Stereoisomers compounds that have the same molecular formula but differ in the configuration of their atoms in space, about one of more of their chiral centers.

• Enantiomers are stereoisomers, molecules, that are not superimpossible on their mirror images. Such molecule are physically and chemically indistinguishable by most techniques, except probing by plane polarized light

Chiral (Asymmetric) Carbons, Optical Activity, & Stereoisomers

Chiral (Asymmetric) Carbons, Optical Activity, & Stereoisomers

Berg, Tymoczko, & Stryer 2012 Fig 11.1

• Stereoisomers compounds that have the same molecular formula but differ in the configuration of their atoms in space, about one or more of their chiral centers.

• Enantiomers are stereoisomers, molecules, that are not superimposable on their mirror images. Such molecule are physically and chemically indistinguishable by most techniques, except probing by plane polarized light

• Optically active molecules rotate plane polarized light. D-dextrorotary (right, clockwise). L-levorotary (left, counterclockwise). [Small capital letters]

• Optically active molecules have an asymmetry such that they are not superimposable on their mirror image.

• This situation is characteristic of substances that contain tetrahedral carbons having four different substituents.

Configuration Sequence Rules about an Asymmetric Carbon Cahn-Ingold-Prelog System

Rule 1. If the four atoms attached to the asymmetric carbon are all different, priority depends on atomic number, with the atom of higher atomic number getting priority. If two atoms are isotopes of the same element, the atom of the higher mass number has the priority

Rule 2. If the relative priority of two groups cannot be decided by Rule 1, it shall be determined by a similar comparison of the atoms next in the groups (and so on, if necessary, working out from the asymmetric carbon).

Morrison & Boyd, 1966 Chapter 3; Voet & Voet 2003 Chapter 4;Matthew et al Fig 9.6

Designation:

R – Rectus (right) the order of the groups about the asymmetric centers is clockwise

S – Sinister (left) the order of the groups about the asymmetric center is counter clockwise

Fisher Convention for Viewing Carbohydrates

Barker 1971 Chaper 5

RULES

1. The carbon chain is vertical with the lowest numbered carbon at the top.

2. The numbering usually follows the convention that the most oxidized end of the molecule has the lowest number.

This “system” relates the configuration of the groups about an asymmetric center to that of glyceraldehyde. Glyceraldehyde has one asymmetric center.

In a Fisher projection on paper:

Horizontal bonds extend above the plane of the paper.

Vertical bonds extend below the plane of the paper

Lehninger Biochemistry 2000 Fig 9.2 a

Enantiomers and Epimer s

• D-Sugars predominate in nature

• Enantiomers –pairs of D-sugars and L-sugars (one type of stereoisomer)

• Epimers - sugars that differ at only one of several chiral centers

• Example: D-galactose is an epimer of D-glucose at C-4

Enantiomers (Mirror Images)

Lehninger Biochemistry 2000 Fig 9.2

Glucose and Two EpimersLehninger Biochemistry 2000 Fig 9.4

Carbohydrate Major Classes

• Carbohydrates (“hydrate of carbon”) have empirical formulas of (CH2O)n , where n ≥ 3

• Monosaccharides one monomeric unit Oligosaccharides ~2-20 monosaccharides

• Polysaccharides > 20 monosaccharides

• Glycoconjugates (carbohydrate derivative) linked to proteins or lipids

Horton et al 2002 Chapter 8

Number of Carbons in a Sugar is Indicated by a Prefix

All common monosaccharides and disaccharide names end in –”ose”

(e.g. glucose, sucrose, fructose, ribose, mannose)

The number of carbons in a sugar is indicated by a prefix:

C3 triose C4 tetrose C5 Pentose

C6 hexose C7 heptose C8 Octose

C9 Nonose

Most Monosaccharides are Chiral Compounds

O

• Aldoses - polyhydroxy aldehydes -C- CH

OH O OH

• Ketoses - polyhydroxy ketones –CH – C - CH2

• Most oxidized carbon: aldoses C-1, ketoses usually C-2

• Trioses (3 carbon sugars) are the smallest monosaccharides

Horton et al 2002 Chapter 8.1

Aldoses and Ketoses

• Aldehyde C-1 is drawn at the top of a Fischer projection

• Glyceraldehyde (aldotriose) is chiral (C-2 carbon has 4 different groups attached to it)

• Dihydroxyacetone (ketotriose) does not have an asymmetric or chiral carbon and is not a chiral compound

Horton et al 2002 Chapter 8

Fischer projections of: (a) L- and D-glyceraldehyde, (b) dihydroxyacetone

Horton et al 2012 Fig 8.1

Fisher projections of 3 to 6 carbon D-aldoses

• D-sugars have the same configuration, by convention, as D-glyceraldehyde when their chiral carbon most distant from the carbonyl carbon (highest number) is the same as C-2 of glyceraldehyde. That is, the –OH group is to the right in the Fisher projection.

• Aldoses those shown in blue (next slide) are the most important in biochemistry

Horton et al 2002 Fig 8.3

3, 4, & 5-Carbon D-Aldoses

Horton et al 2012 Fig 8.3

6-Carbon D-Aldoses

Horton et al 2012 Fig 8.3b

H O

C

H -- C -- OH

CH2OH

D-glyceraldehyde

Fisher projections of L- and D-glucose

Horton et al 2002 Fig 8.4

Fisher projections of the 3,4, & 5-Carbon D-ketoses (blue structures most common)

Horton et al 2012 Fig 8.5

6-Carbon D-Ketoses

Horton et al 2012 Fig 8.5b

Relevant Reactions of Aldehydes & Ketones

Lehninger 2000 Fig. 9.5Voet, Voet & Pratt 2013 Page 219

Cyclization of Aldoses and Ketoses

Reaction of an alcohol with:

(a) An aldehyde to form a hemiacetal

(b) A ketone to form a hemiketal

Horton et al 2002 Fig 8.6

alcohol

aldehyde

Voet, Voet & Pratt 2013 Page 219

Cyclization of D-glucose to form Glycopyranose Haworth Projection

• Fischer projection (top left)

• Three-dimensional figure (top right)

• C-5 hydroxyl close to aldehylde group (lower right)

Horton et al 2012 Fig 8.8

Cyclization of D-Glucose (continued) • Reaction of C-5 hydroxyl with

one side of C-1 gives a, reaction with the other side gives b

Horton et al 2002 Fig 8.8b

Anomers: Isomeric forms of monosaccharides that differ only about the hemiacetal or hemiketal carbon.

The hemiacetal or carbonyl carbon (the most oxidized carbon, i.e. attached to two oxygen atoms) is called the anomeric carbon.

The and anomers of D-glucose interconvert in solution by a process called mutarotation.

AnomersDefinition: Isomeric forms of monosaccharides that differ only about the hemiacetal or hemiketal carbon are called anomers

The hemiacetal or carbonyl carbon (the most oxidized carbon, i.e. attached to two oxygen atoms) is called the anomeric carbon.

The and anomers of D-glucose interconvert in solution by a process called mutarotation.

Voet, Voet & Pratt 2013 Figure 8.4

Pyran (a) and furan (b) ring systems

• (a) Six-membered sugar ring is a “pyranose”

• (b) Five-membered sugar ring is a “furanose”

Voet, Voet & Pratt 2013 p. 220

a b

Haworth Projections

Rules:

1. Cyclic monosaccharide is drawn with the anomeric carbon on the right.

2. Other carbons are numbered in a clockwise direction

3. Hydroxyl groups (-OH) on right of carbon skeleton in Fischer projection point down in Haworth projection (and vice versa).

4. (For D sugars) Anomeric carbon is if its hydroxyl group is trans to the CH2OH at the carbon atom that determine whether the sugar is designated (D or L).

Haworth

FischerHaworth projection indicates sterochemistry and is easily related to Fischer projection

11

Berg, Tymoczko, & Stryer 2012 Fig 11.3

Conformations of Monosaccharides

Horton et al 2002 Chapter 8.3

Conformation:

Three-dimensional shape having the same configuration.

Sugars are conformationally variable!

Conformations of b-D-glucopyranose

(b) Stereo view of chair (left), boat (right)

Horton et al 2012 Fig 8.11Voet, Voet & Pratt 2013 Figure 8.3

Conformations of b-D-glucopyranose

• Conformer on the left is more stable because it has the bulky hydroxyl substituents in equatorial positions (less steric strain)

Voet, Voet & Pratt 2013 Figure 8.5 Berg, Tymoczko, & Stryer 2012 p.334

Conformations of b-D-ribofuranose

Horton et al 2012 Fig 8.10

Derivatives of Monosaccharides

• Many sugar derivatives are found in biological systems

• Some are part of monosaccharides, oligosaccharides or polysaccharides

• These include sugar phosphates, deoxy and amino sugars, sugar alcohols and acids

Horton et al 2012 Table 8.1

Abbreviations for some Monosacchardies and their Derivatives

Monosaccharide Derivatives

A. Sugar Phosphates

Some important sugar phosphates

Horton et al 2012 Fig 8.13

B. Deoxy Sugars

• In deoxy sugars an H replaces an OH

Deoxy sugars

Horton et al 2012 Fig 8.14

C. Amino Sugars

• An amino group replaces a monosaccharide OH

• Amino group is sometimes acetylated

• Amino sugars of glucose and galactose occur commonly in glycoconjugates

Horton et al 2002 Chapter 8

Several amino sugars

• Amino and acetylamino groups are shown in red

Horton et al 2012 Fig 8.15

D. Sugar Alcohols (polyhydroxy alcohols)

• Sugar alcohols: carbonyl oxygen is reduced

Horton et al 2012 Fig 8.16

E. Sugar Acids

• Sugar acids are carboxylic acids • Produced from aldoses by:

(1) Oxidation of C-1 to yield an aldonic acid

(2) Oxidation of the highest-numbered carbon to an alduronic acid

Lehninger Biochemistry 2000 Fig 9.9

Some Biologically Important Hexose Derivatives

Disaccharides and Other Glycosides

• Glycosidic bond - primary structural linkage in all polymers of monosaccharides

• An acetal linkage - the anomeric sugar carbon is condensed with an alcohol, amine or thiol

• Glucosides - glucose provides the anomeric carbon

Glucopyranose + methanol yields a glycoside

Voet, Voet & Pratt 2013 Figure 8.7

Structures of Disaccharides

Structures of (a) maltose, (b) cellobiose

Horton et al 2012 Fig 8.20a,b

Systematic name

Systematic Description: The linking atoms, the configuration of the glycosidic bond, and the name of each monosaccharide, including its designation as a pyranose or furanose, MUST be specified.

Structures of Disaccharides (cont.)

Structures of (c) lactose, (d) sucrose

Horton et al 2012 Fig 8.20c,d

Rules for Disaccharide Structures1. The structure is written starting with the non-reducing end at the

left and standard accepted abbreviation are used. (see Table 8.1 in Horton et al.,)

2. Anomeric and enantiomeric forms are designated by prefixs, e.g. - and D-.)

3. The ring structure is indicated by a suffix (p for pyranose and f for furanose)

4. The atoms between which glycosidic bonds are formed are indicated by numbers in parentheses between residue designations (e.g., (14) means a bond from carbon 1 of the residue on the left to carbon four of the residue on the right.)

Example : -D-Glcp(12)--D-Fruf Matthews et al, 2000 Chap 9

Reducing vs Nonreducing SugarsReducing:

• Monosaccharides and most disaccharides are hemiacetals with a reactive carbonyl group.

• Carbonyl group can be readily oxidized to diverse products; will reduce metal ions, e.g.,Cu2+ or Ag+ to precipitates.

Non reducing:

• Carbohydrates that are acetals cannot reduce metals, sucrose with both anomeric carbons in a glycosidic bond is one example.

Oligo- and poly-saccharides: with a linear polymer they show only one reducing end. All the glycosidic bonds are acetals which are not in equilibrium with the open chain structure and therefore cannot reduce metal ions.

Polysaccharides

• Homoglycans - homopolysaccharides containing only one type of monosaccharide

• Heteroglycans - heteropolysaccharides containing residues of more than one type of monosaccharide

• Lengths and compositions of a polysaccharide may vary within a population of these molecules

Horton et al 2002 Chapter 8

Horton et al 2012 Table 8.2

The nature of a polysaccharide’s biological role is commonly used to classify them, such

as: structural or (energy) storage.

A. Starch and Glycogen• D-Glucose is stored intracellularly in polymeric forms

• Plants and fungi - starch

• Animals - glycogen

• Starch is a mixture of amylose (unbranched) and amylopectin (branched)

(a) Amylose is a linear polymer

(b) Assumes a left-handed helical conformation in water

Horton et al 2002 Fig 8.22Voet, Voet & Pratt 2013 p.227

Glycogen• Storage polysaccharide for animals: greatest in skeletal muscle and liver cells, but present

in all cells.

• Primary structure resembles amylopectin but more highly branched, i.e. every 8-14 residues.

• In the cell degraded for use by glycogen phosphorylase cleaving (1-4) bonds working from the nonreducing end onward.

• Highly branched structure provides rapid access.

• Debranching enzyme cleaves the (1-6) bonds at the branch points.

Stryer et al., 2002 Fig 21.1

B. Cellulose and Chitin (structural homopolysaccharidies)

Structure of cellulose

(a) Chair conformation

(b) Haworth projection

Horton et al 2002 Fig 8.25

O

- NH-C-CH3

substituted in chitin

Voet, Voet & Pratt 2013 Figure 8.9

A linear, unbranched polymer of 10 –50 thousand glucose units. One key difference is that cellulose is in the beta configuration and has (1-4) glycosidic bonds compared to amylose & glycogen

Lehninger Biochemistry 2000 Fig 9.17a

Structure of Cellulose: 2 Chain Units

intrachain

interchain

Glycoconjugates

• Heteroglycans appear in three types of glycoconjugates:

Proteoglycans

Peptidoglycans

Glycoproteins

Horton et al 2012 Chapter 8

Proteoglycans• Proteoglycans - glycosaminoglycan-protein complexes

• Glycosaminoglycans - unbranched heteroglycans of repeating disaccharides (many sulfated hydroxyl and amino groups)

• Disaccharide components include:

(1) amino sugar (D-galactosamine or D-glucosamine), (2) an alduronic acid

Horton et al 2012 Chapter 8Voet, Voet & Pratt 2013 Figure 8.15

Repeating disaccharide of hyaluronic acid

• GlcUA =D-glucuronate

• GlcNAc= N-acetylglucosamine

Horton et al 2012 Fig 8.28

Peptidoglycans• Peptidoglycans - heteroglycan chains linked to peptides

• Major component of bacterial cell walls

• Heteroglycan composed of alternating GlcNAc and N-acetylmuramic acid (MurNAc)

• b-(1-4) linkages connect the units

Glycan moiety of peptidoglycan

Horton et al 2012 Fig 8.30

Bacterial Cell Walls

Matthews et al, 2000 Fig 9.25

Voet, Voet & Pratt 2013 Figure 8.16

Peptodiglycan Layer of Gram

Positive Bacteria

Matthews et al, 2000 Fig 9.26

Glycoproteins• Proteins that contain covalently-bound

oligosaccharides

• “Class” includes: enzymes, hormones, transport & structural proteins

• O-Glycosidic and N-glycosidic linkages

• Oligosaccharide chains exhibit great variability in sugar sequence and composition

• Glycoforms - proteins with identical amino acid sequences but different oligosaccharide chain composition

Horton et al 2012 Chapter 8

Diversity in Glycoprotein Oligosaccharide Chains1. Chain can contain several different sugars (predominant in eukaryotes) such as: (6

carbon) L-fucose, D-galactose, D-glucose, D-mannose; N-acetyl-galactosamine, N-acetyl-glucosamine; (9-carbon) sialic acids; (5 carbon) D-xylose.

2. Sugars can be joined by either or glycosidic linkages

3. Linkages can join various carbon atoms. In 6 carbon all involve C-1 of one sugar, but C-2,3,4, or 6 of another hexose or C-3,4 or 6 of hexosamines; C-2 not C-1 of sialic acid links to other sugars.

4. Chains can contain up to four branches

Horton et al 2012 Chapter 8

IMPORTANT: The addition of one or more oligosaccharide chains affects a protein’s PHYSICAL PROPERTIES (size, shape, charge, stability, etc.) which, in turn, affects the BIOLOGICAL PROPERTIES such as: secretion rate, circulation ½-life, immunogenicity, targeting within the cell, cell signaling.

Stryer et al., 2002 Fig 11.X

Blood Groups & Glycoproteins

End of Lectures