Carbohydrates (Part II)
The Biochemistry of Sugars
Monosaccharides and Disaccharides
Presented by: Dr. Annie Godwin (annie.godwin@port.ac.uk)
References
Lehninger Principles of Biochemistry
Edition: Sixth Edition
Authors: David L. Nelson, Michael M. Cox
Chapter: 7 Carbohydrates and Glycobiology
Checklist: What We Need to Know
Introduction to monosaccharides: understanding the chemistry of sugars, carbonyl groups, and chiral centers.
Understand the open-chain and ring forms of monosaccharides: hemiacetals or hemiketals.
Understand the structures and properties of disaccharides & the glycosidic bond.
Understand the biological function of polysaccharides.
Key Terminology
Monosaccharide: A carbohydrate consisting of a single sugar unit.
Disaccharide: A carbohydrate consisting of two covalently joined monosaccharide units.
Polysaccharide: A linear or branched polymer of monosaccharide units linked by glycosidic bonds.
Carbohydrates: A sugar (monosaccharide) or one of its dimers (disaccharide) or polymers (polysaccharide).
Monosaccharides
Types of Monosaccharides
Glucose: An aldose sugar.
Fructose: A ketose sugar.
Recap: Aldoses & Ketoses
Aldose: A simple sugar in which the carbonyl carbon atom is an aldehyde; the carbonyl carbon is at one end of the carbon chain.
Ketose: A simple monosaccharide in which the carbonyl group is a ketone.
Recap: Chemistry of Sugars
Epimers: Two stereoisomers differing in configuration at one asymmetric center in a compound having two or more asymmetric centers.
Enantiomers: Stereoisomers that are non-superposable mirror images of each other.
Chiral Center: An atom with substituents arranged so that the molecule is non-superposable on its mirror image.
Fischer Projection Formula
Represents the 2D structure of sugars, useful for determining configuration.
Monosaccharides: Straight vs Cyclic Structure
Isomers: Glucose, Galactose & Fructose are isomers (same formula, different structures).
Cyclic Forms: In aqueous solutions, aldotetroses and monosaccharides with 5 or more carbon atoms predominantly occur as cyclic structures.
Haworth Perspective: Visualization of cyclic sugars.
Formation of Cyclic Structures
Electrophilic Carbons: Aldehyde and ketone carbons are electrophilic.
Nucleophilic Alcohols: Alcohol oxygen atom acts as a nucleophile.
Hemiacetal Formation: Occurs when aldehydes react with alcohols; hemiketals form with ketones.
Asymmetric Carbon Formation
The cyclic structure can create an additional asymmetric carbon atom called the anomeric carbon, which exists in two stereoisomeric forms.
Cyclisation of Monosaccharides
Configurations:
α-Glucose: Usable form of glucose.
β-Glucose: Not recognized by the body.
Differentiation: Placement of the hydroxyl group on Carbon 1 distinguishes between α and β.
Hemiacetal Stability
Mutarotation:
Anomers interconvert via changes in the configuration about the carbonyl (anomeric) carbon atom.
Both α and β forms exist in equilibrium with the linear configuration.
Pyranose vs Furanose
6-Membered Ring: Called pyranoses, more stable.
5-Membered Ring: Called furanoses.
Key Points on Cyclisation
Anomeric Carbon: The former carbonyl carbon becomes a new chiral center.
Positioning of Hydroxyl Groups: Determines α (trans) or β (cis) configurations.
From Monosaccharides to Disaccharides
Formation of Disaccharides
Dehydration Reaction: Two monosaccharides bond via a condensation reaction, forming a disaccharide while releasing a small molecule (e.g. H2O).
Glycosidic Bond: The bond formed between the two monosaccharides; can involve anomeric to hydroxyl carbon linkages.
Bond Characteristics
Nature of Bond: An acetal and less reactive compared to hemiacetals.
Reducing vs Nonreducing Sugars:
Example: Maltose (reducing), formed from two glucose units linked by a carbon 1 → 4 bond.
Example: Sucrose (nonreducing), composed of glucose and fructose.
Important Disaccharides
Maltose:
Composed of two glucose units via an α-1,4-glycosidic linkage.
Found in sprouting grains, intermediate in starch hydrolysis.
Lactose:
Composed of galactose and glucose via β-1,4-glycosidic linkage.
Found in milk.
Sucrose:
Composed of glucose and fructose via α-1,β-2-glycosidic linkage.
Sources include sugar cane and beets.
Summary of Key Concepts
Monosaccharides form internal hemiacetals/hemiketals, creating cyclic structures.
The anomeric carbon's configuration can be α or β; interconversion occurs through mutarotation.
A glycosidic bond connects monosaccharides, forming disaccharides through dehydration reactions, reversing through hydrolysis.
Quiz on Cyclic Monosaccharides & Disaccharides
What type of reaction is involved in the cyclization of monosaccharides?
What anomer configurations are formed around the hemiacetal carbon?
If the hydroxyl group is on the ‘cis’ side, what is the configuration?
What is the bond between two monosaccharides called?
What type of reaction forms disaccharides?
True or False: A hemiacetal is more stable than an acetal.
What is a nonreducing disaccharide?
Is trehalose a monosaccharide, disaccharide, or polysaccharide?
Definitions
Anomer: Two stereoisomers differing only in configuration about the carbonyl carbon.
Anomeric Carbon: The new stereocenter formed upon cyclization of a sugar.
Condensation Reaction: A chemical reaction forming a larger molecule while producing a small molecule like H2O.
Disaccharide: A carbohydrate formed by two covalently joined monosaccharides.
Glycosidic Bond: Bonds between sugars and other molecules via an oxygen atom.
Haworth Perspective Formulas: Method for representing cyclic chemical structures in sugars.
Hemiacetal/Hemiketal: Formed from the nucleophilic attack of alcohol on carbonyl carbon.
Non-reducing Sugar: Cannot donate electrons or act as a reducing agent.
Nucleophilic Addition: Initial attack of a nucleophile on the carbonyl group of sugars.
Reducing Sugar: Has a carbonyl carbon that can be oxidized and thus undergoes reduction.