Monosaccharides and Oligosaccharides

9.1 Monosaccharides

Diverse Functions of Carbohydrates

Carbohydrates have diverse functions including:

• Storage and generation of energy (e.g., glucose, glycogen, starch)

• Molecular recognition (e.g., immune system)

• Cellular protection (e.g., bacterial and plant cell walls)

• Cell adhesion (e.g., glycoproteins)

• Biological lubrication (e.g., glycosaminoglycans)

• Building and maintaining biological structure (e.g., cellulose, chitin)

Carbohydrate Terminology

• Monosaccharide: Simple sugars and 0 with 3 to 9 carbon atoms.

• Oligosaccharide: Compound formed by linking several monosaccharides together (e.g., disaccharide, with 2).

• Polysaccharide: Polymer formed from multiple saccharide units; may be homopolysaccharide or heteropolysaccharide.

• Glycan: Generic term for oligosaccharides and polysaccharides.

General Formula

When $(CH<em>2O)</em>n$; where $n=3$, gives compounds with properties of sugars.

There are two classes of monosaccharides:

• Aldehydes are aldoses.

• Ketones are ketoses.

Representative Carbohydrates

• Glucose is a monosaccharide.

• Maltose is a disaccharide containing two glucose units.

• Amylose is a glucose polymer found in starch.

Monosaccharides and Energy Cycle of Life

• Photosynthesis: Plants use the energy of sunlight to combine carbon dioxide and water into carbohydrates, releasing oxygen in the process.

• Respiration: Both plants and animals oxidize the carbohydrates made by plants, releasing energy and re-forming $CO<em>2$ and $H</em>2O$.

Aldoses and Ketoses

• Glyceraldehyde is an aldose, and dihydroxyacetone is a ketose. Both are trioses.

• Monosaccharides are named based on the number of carbon atoms:

  • Tetroses: four carbons

  • Pentoses: five carbons

  • Hexoses: six carbons

  • Heptoses: seven carbons

Enantiomers

• Monosaccharides are chiral. For example, the second carbon of glyceraldehyde carries four different substituents.

• Enantiomers are optical isomers that are nonsuperimposable mirror images.

• Glyceraldehyde has two enantiomers designated as D and L (or alternatively, R and S).

• Fischer projections are the most compact way to represent stereochemistry.

• The wedge-dash representations of the ᴅ- and L-forms glyceraldehyde are also shown.

• The R–S Nomenclature exemplified for Glyceraldehyde is also shown.

Diastereomers

• Compounds with more than one asymmetric carbon may be enantiomers (mirror images) or diastereomers.

• Diastereomers: optical isomers that are not mirror images.

• Enantiomers have three or more carbons, while diastereomers have four or more carbons.

• By convention, D and L refer to configuration about the asymmetric carbon farthest from the carbonyl carbon.

• Examples of aldotetroses are ᴅ-Threose and L-Erythrose, which are diastereomers.

• The ketotetrose erythrulose has only two enantiomers and no diastereomers.

Stereochemical Relationships of Aldoses

• D-Glyceraldehyde is an aldotriose.

• D-Erythrose and D-Threose are aldotetroses.

• D-Ribose, D-Arabinose, D-Xylose, and D-Lyxose are aldopentoses.

• D-Allose, D-Altrose, D-Glucose, D-Mannose, D-Gulose, D-Idose, D-Galactose and D-Talose are aldohexoses.

Natural Occurrence and Biochemical Roles of Monosaccharides

Stereochemical Relationships of Ketoses

• Dihydroxyacetone is a ketotriose.

• D-Erythrulose is a ketotetrose.

• D-Ribulose and D-Xylulose are ketopentoses.

• D-Psicose, D-Fructose, D-Sorbose and D-Tagatose are ketohexoses.

Ring Structures

• Sugars can cyclize to form five-membered (furanose) and six-membered (pyranose) rings.

• Cyclization (ring formation) creates a new asymmetric center (called anomeric center), and the related stereoisomers are designated as $\alpha$ or $\beta$.

• Formed furanose or pyranose rings are shown in Haworth projection.

• Formation of ring structures by pentoses is also shown.

• Conformational isomers of glucose are shown representing the actual stereochemistry resulting from the fact that the ring cannot be planar; two of several conformation isomers are shown.

• The four most common hexoses: Glucose and mannose are epimers their isomers differing only in configuration about one carbon (C2). Glucose and galactose are also epimers (C4 configuration is different).

• Pyranose ring in chair and boat conformations. Conformational isomers are compounds with the same stereochemical configuration but differing in the three-dimensional conformation (bond rotation).

Terminology for Carbohydrate Stereochemistry

• Configurational isomers:

  • Enantiomers: Stereoisomers that are mirror images of one another. The boxed asymmetric carbon (farthest from aldehyde) determines D/L designation.

  • Diastereomers: Stereoisomers that are not mirror images of one another.

• Anomers: Stereoisomers that differ in configuration at the anomeric carbon.

• Epimers: Stereoisomers that differ in configuration at one carbon other than the anomeric carbon.

• Conformational isomers: Molecules with the same stereochemical configuration but differing in three-dimensional conformation

a-D-Sedoheptulopyranose

Plays a role in photosynthesis.

Phosphate Esters

Sugar phosphates are important intermediates in metabolism, functioning as activated compounds in syntheses.

Lactones and Sugar Acids Oxidation of monosaccharides

• Monosaccharides can be oxidized at the C1 to yield aldonic acids, which are in equilibrium with the lactone forms.

• Monosaccharides can also be oxidized at the C6 to yield uronic acid

Alditols

• Reduction of the carbonyl group on a sugar gives rise to the class of polyhydroxy compounds called alditols. Important naturally occurring ones are erythritol, D-mannitol, and D-glucitol, often called sorbitol.

• Reduction of the sugar carbonyl yields an alditol

• When glucose is reduced, ᴅ-glucitol (also called sorbitol) is formed.

Amino Sugars

• Amino sugars are carbohydrates in which at least one hydroxyl group has been replaced with an amine group.

• Amino sugars are found in many polysaccharides and glycoproteins.

• Derivatives of $\beta$-D-Glucosamine are $\beta$-D-N-Acetylglucosamine, Muramic acid, and N-Acetylmuramic acid.

Glycosides

• The elimination of water between the hydroxyl group of the anomeric carbon of a cyclic saccharide and the hydroxyl group of another compound yields an O-glycoside.

• This type of bond is called a glycosidic bond.

• Two naturally occurring glycosides are Ouabain and Amygdalin.

Monosaccharides and Their Abbreviations

9.3 Oligosaccharides

Distinguishing Features of Disaccharides

There are four major features of disaccharides:

1. The sugar monomers involved and their stereochemistry

2. The carbons involved in the linkage

3. The order of sugars (determined by the chemical reactivity of functional groups involved in linkage)

4. The configuration of the anomeric carbon

Writing the Structure of Disaccharides

1. Start by putting the nonreducing end on the left with abbreviated monosaccharide name.

2. Designate anomeric and enantiomeric forms by prefixes.

3. Indicate the ring configuration by a suffix (p for pyranose and f for furanose; often omitted).

4. The carbons involved in glycosidic bond formation are numbered as in open structures of aldoses or ketoses, and the connections are indicated by an arrow.

• **Disaccharides with $\alpha$-connections: **

  • Maltose: $\alpha$-D-glucopyranosyl-(1$\rightarrow$4)-D-glucopyranose

  • $\alpha$,$\alpha$-Trehalose: $\alpha$-D-glucopyranosyl-(1$\rightarrow$1)-$\alpha$-D-glucopyranose

  • Sucrose: $\alpha$-D-glucopyranosyl-(1$\rightarrow$2)-$\beta$-D-fructofuranoside

• Disaccharides with $\beta$ connections:

  • Cellobiose: $\beta$-D-glucopyranosyl-(1$\rightarrow$4)-$\beta$-D-glucopyranose

  • Lactose: $\beta$-D-galactopyranosyl-(1$\rightarrow$4)-$\beta$-D-glucopyranose

  • Gentiobiose: $\beta$-D-glucopyranosyl-(1$\rightarrow$6)-$\beta$-D-glucopyranose

Representative Disaccharides and Their Biochemical Roles

Stability and Formation of Glycosidic Bonds

• Glycosidic bonds are formed between monomeric saccharides by a condensation reaction, which involves the elimination of water.

• However, the reaction (as written) is thermodynamically unfavored.

• The $\Delta G$ for glycosidic bond formation is about +15 kJ/mol; therefore, activation is needed.

• In lactose biosynthesis, the activated sugar is UDP-galactose.

• UDP-galactose is used as a high-energy derivative of galactose that condenses with glucose to form lactose.

• Enzymatic formation of lactose.