CARBOHYDRATE STRUCTURES Student Edition 5/24/13 Version Pharm. 304 Biochemistry Fall 2014 Dr. Brad Chazotte 213 Maddox Hall Web Site: Original material only © 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

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 Epimers Lehninger Biochemistry 2000 Fig 9.4

Carbohydrate Major Classes Carbohydrates (“hydrate of carbon”) have empirical formulas of (CH 2 O) 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: C 3 trioseC 4 tetroseC 5 Pentose C 6 hexoseC 7 heptoseC 8 Octose C 9 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 6-Carbon D -Aldoses Horton et al 2012 Fig 8.3b H O C H -- C -- OH CH 2 OH 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. 9.5 Voet, 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 , reaction with the other side gives  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.

Anomers Definition: 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 ab

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 CH 2 OH at the carbon atom that determine whether the sugar is designated ( D or L ). Haworth Fischer Haworth projection indicates sterochemistry and is easily related to Fischer projection 1 1 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  -D-glucopyranose (b) Stereo view of chair (left), boat (right) Horton et al 2012 Fig 8.11 Voet, Voet & Pratt 2013 Figure 8.3

Conformations of  -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 8.5Berg, Tymoczko, & Stryer 2012 p.334

 Conformations of  - 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) D. Sugar Alcohols (polyhydroxy alcohols) Sugar alcohols: carbonyl oxygen is reduced Horton et al 2012 Fig 8.16

E.Sugar Acids 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 Disaccharides (cont.) Structures of (c) lactose, (d) sucrose Horton et al 2012 Fig 8.20c,d

Rules for Disaccharide Structures 1.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., (1  4) means a bond from carbon 1 of the residue on the left to carbon four of the residue on the right.) Example :  - D -Glcp(1  2)-  - D -Fruf Matthews et al, 2000 Chap 9

Reducing vs Nonreducing Sugars Reducing: 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.,Cu 2+ 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 8.22 Voet, 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 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-CH 3 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 8 Voet, 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)  -(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 Chains 1.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