Loading [MathJax]/jax/output/HTML-CSS/jax.js
Skip to main content
Library homepage
 

Text Color

Text Size

 

Margin Size

 

Font Type

Enable Dyslexic Font
Biology LibreTexts

7.1: Monosaccharides and Disaccharides

( \newcommand{\kernel}{\mathrm{null}\,}\)

Search Fundamentals of Biochemistry

 

Learning Goals (ChatGPT o1, 1/30/25)
  • Define Key Terminology:

    • Differentiate between “sugar,” “carbohydrate,” and “glycan” and explain how these terms are used to describe molecules ranging from simple monosaccharides to complex glycan polymers.
  • Understand Monosaccharide Structures:

    • Describe the chemical structure of monosaccharides as polyhydroxy-aldehydes or ketones, and explain the significance of stereochemistry in sugars.
    • Convert and interpret various structural representations (Fischer projections, Haworth projections, chair, and wedge/dash forms) for common monosaccharides like D-glucose, D-ribose, and D-fructose.
  • Cyclic Conformations and Anomer Formation:

    • Explain the process by which monosaccharides cyclize to form furanose or pyranose rings, including the formation of hemiacetals.
    • Distinguish between α- and β-anomers in cyclic sugars and understand the factors that influence their stability in solution versus in polysaccharides.
  • Isomerism in Sugars:

    • Identify and differentiate among configurational isomers (enantiomers, diastereomers, epimers, and anomers) and conformational isomers (chair and boat forms) of sugars.
  • Formation of Glycosidic Bonds:

    • Describe the chemical mechanism of hemiacetal and acetal formation in monosaccharides leading to the formation of disaccharides and polysaccharides, including the significance of glycosidic linkages (e.g., 1→4, 1→6).
  • Reducing vs. Nonreducing Sugars:

    • Explain the concept of reducing sugars in terms of the reversible opening of the cyclic form to expose an aldehyde group, and contrast these with nonreducing sugars such as sucrose.
  • Monosaccharide Derivatives:

    • Recognize common chemical modifications of monosaccharides (e.g., oxidation, phosphorylation, amination, acetylation, lactonization) and discuss their biological significance.
  • Biological and Clinical Implications:

    • Discuss how the structural diversity of glycans affects their function in cellular recognition, stability, and signaling.
    • Explain the molecular basis of alpha-gal syndrome, including how the disaccharide Gal(α1,3)Gal, found in tick saliva and red meat, triggers an immune response leading to allergic reactions.

Achieving these goals will equip students with the knowledge to navigate the complexity of glycan structures and their multifaceted roles in biological systems.

Introduction

Carbohydrate or glycan biochemistry is very complex and challenging owing to the stereochemical complexity of simple sugars, the large number of positions on the sugars used to form linkages between other sugars to create polymers, the large number of chemical modifications to base sugars, and the lack of a genetic template to instruct glycan polymer formation. No wonder our understanding of complex glycans has developed after that of the chemically simpler polymers like nucleic acids and proteins.

In addition, the terminology used to describe them varies as well. We use these general descriptions of them:

Sugar: usually refers to low molecular weight carbohydrates like glucose, lactose, and sucrose, but it can also refer broadly to any carbohydrate.

Carbohydrate: a general term that applies to simple sugars to complex sugar polymers like glycogen, starch, and cellulose. The name derives from the formula for simple sugars like glucose (C6H12O6), which can be written as C6(H2O)6 - a carbo (C) - hydrate (H2O).

Glycan: a general term for molecules containing simple sugars and sugar derivatives linked in a polymer, either standalone molecules or attached to other molecules like proteins.

Monosaccharides Structures

The above definition of sugar needs some further nuance. From a chemical perspective, sugars can be defined as polyhydroxy-aldehydes or ketones. The simplest sugars contain at least three carbon atoms, and the most common are the aldo- and keto-trioses, tetroses, pentoses, and hexoses. The 3C sugars are glyceraldehyde and dihydroxyacetone, as shown in Figure 7.1.1.

FisherProj_Perspect_3CSugarsV3.svg

 

Figure 7.1.1: Three-carbon sugars

Glucose, an aldohexose, is a central sugar in metabolism. It and other 5C and 6C sugars can cyclize through intramolecular nucleophilic attack of one of the free hydroxyl groups on the carbonyl carbon of the aldehyde or ketone. Such intramolecular reactions occur if stable 5- or 6-member rings can form. The resulting rings are labeled furanose (5-member) or pyranose (6-member) based on their similarity to furan and pyran. On nucleophilic attack to form the ring, the carbonyl O becomes an OH that points either below (α anomer) or above (β anomer) the ring.

Figure 7.1.2 shows different representations of the linear and cyclic forms of the sugars D-glucose, D-ribose, and D-fructose

glcfrccyclizV2.svg
Figure 7.1.2: Linear and cyclic forms of D-glucose, D-ribose and D-fructose

Monosaccharides in solution exist as equilibrium mixtures of the straight and cyclic forms. In solution, glucose (Glc) is mainly in the pyranose form, fructose is 67% pyranose and 33% furanose, and ribose is 75% furanose and 25% pyranose. However, in polysaccharides, Glc is exclusively pyranose, and fructose and ribose are furanoses.

Sugars can be drawn in the straight chain form as Fischer projections or perspective structural formulas.

In the Fisher projection, the vertical bonds point down into the plane of the paper. That's easy to visualize for 3C sugars but more complicated for larger ones. For those, draw a wedge and dash line drawing of the molecule. When determining the orientation of the OHs on each C, orient the wedge and dash drawing in your mind so that the C atoms adjacent to the one of interest are pointing down. Sighting towards the carbonyl C, if the OH is pointing to the right in the Fisher project, it should be pointing to the right in the wedge and dash drawing, as shown below for D-threose and D-glucose. Figure 7.1.3 shows how to convert Fisher projections to wedge dash representations.

SaveFischertoWedge.svg
Figure 7.1.3: Converting Fisher projections to wedge dash representations.

Figure 7.1.4 shows an interactive iCn3D model of D-glucose in a linear form.

Figure 7.1.4: D-glucose (Copyright; author via source).

Orient the molecule as shown in Figure 7.1.5 below, with the carbonyl oxygen pointed to the far right, and compare it to the orientation shown in Figure 7.1.5 to reinforce your understanding of Fisher and wedge/dash projections.

clipboard_e4399a17849be2424f3813876ca6d1574.png

Figure 7.1.5:

Cyclic forms can be drawn either as the Haworth projections, which show the molecule as cyclic and planar with substituents above or below the ring) or the more plausible bent forms (showing glucose in the chair or boat conformations). β-D-glucopyranose is the only aldohexose that can be drawn with all its bulky substituents (OH and CH2OH) in equatorial positions, which probably accounts for its widespread prevalence in nature. Figure 7.1.6 shows four different representations of glucose.

glcchair.svg
Figure 7.1.6: Fisher and cyclic Haworth, chair and wedge/dash representations of glucose

Haworth projections are more realistic than the Fisher projections, but you should be able to draw both structures. Generally, if a substituent points to the right in the Fisher structure, it points down in the Haworth. If it points left, it points up. Generally, the OH on the α-anomer points down (αnts down) while on the β-anomer, it points up (βutterflies up) as illustrated in Figure 7.1.7

choalphabeta2.svg
Figure 7.1.7: Alpha and beta Haworth representations of sugars

In the Haworth projections, the bulky R group of the next carbon after the carbon whose OH group was the nucleophile for ring formation is pointed up if the OH engaged in the attach was on the right-hand side in the straight chain Fisher diagram (as in α-D-glucopyranose above when the CH2OH group is up). It is pointed down if the OH engaged in the attack was on the left-hand side in the straight chain Fisher diagram (as in α-D-galactofuranose above when the (CHOH)CH2OH group is down). The rest of the OH groups still follow the simple rule that if they point to the right in the Fisher straight chain form, they point down in the Haworth form.

The Fisher structures of most common monosaccharides (other than glyceraldehyde and dihydroxyacetone), which you will encounter most frequently, are shown in Figure 7.1.8.

sugarsknow.svg

Figure 7.1.8: Most common monosaccharides discussed in this book

The mirror image of D-Glc is L-Glc. The D- and L- designations refer to the center of asymmetry most remote from the aldehyde or ketone. By convention, all chiral centers are related to D-glyceraldehyde, so sugar isomers related to D-glyceraldehyde at their last asymmetric center are D sugars.

Figure 7.1.9 shows multiple renderings of common hexoses.

CHOMonomers.svg
Figure 7.1.9: Multiple renderings of common hexoses

Isomers

Sugars can be configurational (interconverted only by breaking covalent bonds) or conformational isomers. Figure 7.1.10 reviews different configurational isomers.

IsomerFlowChart.svg
Figure 7.1.10: Types of isomers

The configurational isomers include enantiomers (stereoisomers that are mirror images of each other), diastereomers (stereoisomers that are not mirror images), epimers (diastereomers that differ at one stereocenter), and anomers (a particular form of stereoisomer, diastereomer, and epimer) shows enantiomers, diastereomers, epimers and anomers of 6 carbon sugars.

sugarisomers.svg
Figure 7.1.11: Enantiomers, diastereomers, epimers, and anomers of 6 carbon sugars.

Sugars can also exist as conformational isomers, which interchange without breaking covalent bonds. These include chair and boat conformations of the cyclic sugars as shown in Figure 7.1.12.

BetaDGlcChairConformers.svg
Figure 7.1.12: Conformational isomers of b-D-glucopyranose

Monosaccharide Derivatives

Many derivatives of monosaccharides are found in nature. These include

  • oxidized forms in which the aldehyde and/or alcohol functional groups are oxidized to carboxylic acids
  • phosphorylated forms in which phosphates are transferred from ATP to form phosphoester derivatives
  • amine derivatives such as glucosamine or galactosamine
  • acetylated amine derivatives such as N-Acetyl-GlcNAc (GlcNAc) or GalNAc
  • lactone forms (intramolecular esters) in which an OH group attacks a carbonyl C that was previously oxidized to a carboxylic acid
  • condensation products of sugar derivatives with lactate (CH3CHOHCO2-) and pyruvate (CH3COCO2- ), both from the glycolytic pathway, to form muramic acid and neuraminic acids (also called sialic acids), respectively.

Figure 7.1.13 some simple monosaccharide derivatives.

sugarderiv.svg
Figure 7.1.13: Monosaccharide derivatives

Figure 7.1.14 shows some additional oxidative derivatives of glucose shown in Fischer projections.

redoxderivativesofGlc.svg
Figure 7.1.14: Redox derivatives of glucose

Other important derivatives of monosaccharides are sialic acids. N-acetylmuramic acid, found in bacterial cell walls, consists of GlcNAc in ether link at C3 with lactate, while N-acetylneuraminic acid results from an intramolecular cyclization of a condensation product of ManNAc and pyruvate. These sialic acids are shown in Figure 7.1.15.

SialicAcid.svg
Figure 7.1.15: Sialic Acids

Sugars are very complicated as the linkages and substituents are so diverse. Figure 7.1.16 shows differences in sialic acids between humans and chimps.

sialicacid_Humans_Chimps.svg
Figure 7.1.16: Sialic acids between humans and chimps.

What happens when non-vegan humans eat animal products (meat, milk) with N-glycoyl neuraminic acids (Neu5Gc)? Some get incorporated into human membrane glycans. Sialic acids on surface proteins can serve as "receptors" that allow the binding of self-cells as well as foreign cells or proteins that have evolved to bind them. A toxin, SubAB, secreted by E. Coli 0157, can bind Neu5Gc. Hence eating meat products can make us more susceptible to bacteria that recognize Neu5Gc.

Formation of Hemiacetals, Acetals, and Disaccharides

Monosaccharides that contain aldehydes can cyclize through an intramolecular nucleophilic attack of an OH at the carbonyl carbon in an addition reaction to form a hemiacetal.  In the past, the group was called a hemiketal if the attack was on a ketone, but now they are also called hemiacetals. On the addition of acid (which protonates the anomeric OH, forming water as a potential leaving group), another alcohol can add, forming an acetal with water leaving. These reactions are shown in Figure 7.1.17.

hemiacetalchemistry.svg
Figure 7.1.17: Hemiacetal and acetal formation

If the other alcohol is a second monosaccharide, a disaccharide results. The acetal link bonding to the two monosaccharides is called a glycosidic link. If the link between the two sugars involves an anomeric carbon, the newly formed OH group at the link can be designated either as α (if the O on the carbon involved in the glycosidic link is pointing down) or β (if the O is pointing up). For a 2-2 link between hexoses (i.e., between two non-anomeric carbons, the α/β designation is not used.  Since sugars contain so many OH groups, which can act as the "second" alcohol in acetal formation, links between sugars can be quite diverse. These include α and β forms of 1-2, 1-3, 1-4, 1-5, 1-6, etc. links. Here are examples of disaccharides:

  • maltose: Glc(α 1,4)Glc, which can be considered a disaccharide hydrolysis product of the polysaccharide glycogen or starch (discussed in the section)
  • cellobiose: Glc(Glc(α 1,4)Glc 1,4)Glc, which can be considered a disaccharide hydrolysis product of cellulose.
  • lactose: Gal(β 1,4)Glc, also known as milk sugar.
  • sucrose: Glc(α 1,2)Fru. Since fructose is attached through the anomeric OH of this ketose, it is not in equilibrium with its straight-chain keto form; hence, sucrose is a nonreducing sugar.  Note also that since the anomeric C-OH or each sugar is used, the α/β designation in the disaccharide is used.  Hence, sucrose would be abbreviated as Glc(α1,2β)Fru

Figure 7.1.18 illustrates the differences between lactose and sucrose. Note that the β-D-fructofuranose ring is flipped (left to right as in turning one of your hands over) compared to Figure 7.1.16.

 

disacchformationV2.svg

Figure 7.1.18: Structures of lactose and fructose

Acetal links between sugars in glycans can be hydrolyzed by water (catalyzed by H+), as with the other key biological polymers, proteins, and nucleic acids.

The disaccharides described above that are linked through a 1,4 linkage are called reducing sugars since they can act as reducing agents in reactions in which they get oxidized. For example, in lactose, since galactose is attached to glucose through the OH on C4, the anomeric glucose carbon, C1, could revert to the noncyclic aldehyde form. This aldehyde is susceptible to oxidation by reagents (Benedict's Solution - with citrate, Fehling's reagent - with tartrate) as these reagents are subsequently reduced. In both reagents, reducing sugars reduce a basic blue solution of CuSO4 (Cu2+) to a brick-red precipitate of Cu2O (Cu+). Sugars (monosaccharides, disaccharides and polysaccharides) that have a potentially open aldehyde at C1 or have an α-hydroxymethyl ketone group which can isomerize to an aldehyde under basic conditions (such as fructose) are called reducing sugars. These oxidizing agents are mild and react with aldehydes and not ketones.

If a monosaccharide, disaccharide, or even polysaccharide has at least one hemiacetal link (for instance, the second sugar in lactose), it is a reducing sugar, as the monomer with the cyclic hemiacetal can reversibly open to form an aldehyde. However, if the only links in sugars are full acetals (such as in sucrose when the link is between the two anomeric carbons), the sugar is not reducing.

Alpha-gal syndrome

Alpha-gal syndrome (AGS) is a relatively newly discovered disease caused by the bite of a tick. Tick saliva contains the disaccharide galactose-α-1,3-galactose (alpha-gal).  After a tick bite, people develop an immune response to the disaccharide through IgE antibodies.  Further bites could cause a mild rash up to an anaphylactic response. 

What makes AGS worse is that red meat contains the disaccharide but is not found in fish, birds, or people. Hence, people who mount a strong IgE response to the disaccharide can also elicit the same response when they eat red meat or even drink cow's milk.  Estimates that up to 450,000 people in the US may develop serious and even life-threatening symptoms after eating red meat.

The structure of Gal(α1,3)Gal is shown in Figure 7.1.19 below.

Galalpha13Gal.svg

Figure 7.1.19:  The structure of the disaccharide Gal(α1,3)Gal

Summary

This chapter provides an in-depth exploration of the complexity of carbohydrate chemistry, emphasizing the structural, stereochemical, and functional diversity of sugars and glycans. It highlights why glycan biochemistry is particularly challenging due to the vast array of possible structures that arise from simple monosaccharide building blocks.

Key topics include:

  1. Terminology and Definitions:
    The chapter begins by clarifying the terms "sugar," "carbohydrate," and "glycan." While "sugar" typically refers to small, low molecular weight carbohydrates like glucose, the term "carbohydrate" encompasses everything from simple sugars to complex polymers such as glycogen and cellulose. "Glycan" is used more generally to describe any polymer of sugars, whether free or attached to proteins and lipids.

  2. Monosaccharide Structures and Representations:
    An overview of monosaccharide structures is provided, including the chemical basis of sugars as polyhydroxy-aldehydes or ketones. The chapter explains how simple sugars (e.g., trioses, tetroses, pentoses, hexoses) can cyclize to form stable 5-membered (furanose) or 6-membered (pyranose) rings. Various methods of structural representation are compared, such as Fischer projections, Haworth projections, and chair conformations, highlighting the stereochemical intricacies and the significance of α- and β-anomers.

  3. Isomerism and Structural Diversity:
    The text reviews different types of isomers found in sugars, including configurational isomers (enantiomers, diastereomers, epimers, and anomers) and conformational isomers (chair and boat forms). This section underscores how subtle differences in stereochemistry can have profound effects on glycan function and recognition.

  4. Monosaccharide Derivatives and Modifications:
    Beyond the basic sugar units, the chapter discusses various chemical modifications of monosaccharides, such as oxidation, phosphorylation, amination, acetylation, lactone formation, and complex condensation reactions that lead to the formation of important derivatives like sialic acids. These modifications are critical in determining the biological roles and properties of glycans.

  5. Glycosidic Bond Formation and Disaccharide Assembly:
    The formation of glycosidic bonds via hemiacetal and acetal reactions is explained, along with the implications for disaccharide and polysaccharide synthesis. The chapter details how different linkages (e.g., α-1,4, β-1,4, α-1,2) lead to diverse structures, which in turn affect the reducing or nonreducing nature of the resulting carbohydrates.

  6. Biological Implications and Clinical Relevance:
    Finally, the chapter explores the physiological significance of glycan diversity. An example is provided through alpha-gal syndrome, an allergic response triggered by the disaccharide Gal(α1,3)Gal found in tick saliva and red meat. This section highlights how glycan structures can influence cell–cell interactions, pathogen recognition, and immune responses.

Overall, this chapter lays a foundation for understanding the intricate world of carbohydrate biochemistry by examining the molecular structures, stereochemical variations, and modifications that contribute to the functional complexity of glycans in biological systems.


This page titled 7.1: Monosaccharides and Disaccharides is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Henry Jakubowski and Patricia Flatt.

Support Center

How can we help?