Carbohydrates: An Introduction

• Carbohydrates are studied in the context of biochemistry and molecular biology.

Definition of Carbohydrates

• Carbohydrates are biological macromolecules.
• These macromolecules are polymers composed of smaller monomer molecules.
• Monosaccharides are the monomers that make up carbohydrates
• Monosaccharides are the most basic form of carbohydrates.

Classification of Carbohydrates

• Carbohydrates have four primary classifications
• Monosaccharides are the simplest form of carbohydrates, consisting of a single sugar unit
• These are simple sugars and follow the formula CnH2nOn.
• Disaccharides form from the linkage of two monosaccharides through a glycosidic bond
• Oligosaccharides consist of 3-10 monosaccharide units
• Polysaccharides are composed of long chains with more than 10 monosaccharide units

Monosaccharides

• Monosaccharides are further classified based on two criteria

Classification of Monosaccharides by Carbon Number

• Trioses have 3 carbon atoms
• Tetroses have 4 carbon atoms
• Pentoses have 5 carbon atoms
• Hexoses have 6 carbon atoms
• Heptoses have 7 carbon atoms

Classification of Monosaccharides by Functional Group

• Aldoses contain an aldehyde (-CHO) group
• Ketoses contain a ketone (-CO) group.
• Monosaccharides can have detailed names describing their functional groups and carbon count
• Aldotetroses and ketotetroses have four carbons
• Aldopentoses and ketopentoses have five carbons
• Aldohexoses and ketohexoses have six carbons

Characteristics of Monosaccharides

• Monosaccharides exhibit unique characteristics

Asymmetric Carbon Atoms in Monosaccharides

• An asymmetric carbon atom, also known as a chiral carbon, bonds to four dissimilar atoms or groups
• Asymmetric carbons are responsible for isomerism in monosaccharides

Stereoisomerism in Monosaccharides

• Stereoisomers have the same molecular formula but differ in the arrangement of atoms or groups in three-dimensional space
• The number of isomers depends on the number of asymmetric carbon atoms, denoted as "n"
• The number of possible isomers is calculated by 2n

Enantiomerism in Monosaccharides

• Enantiomers are mirror images of each other in two forms
• D and L enantiomers define the stereochemistry of a molecule
• L isomers have the hydroxy group on the left side of the asymmetric carbon furthest from the carbonyl
• D isomers have the hydroxy group on the right side

Epimerism in Monosaccharides

• Epimerism is the stereoisomerism of two monosaccharides that vary in configuration around a single carbon atom.
• Glucose and mannose are epimers, differing at carbon 2 (C2)
• Glucose and galactose are also epimers, differing at carbon 4 (C4)

Anomerism in Monosaccharides

• Anomers are cyclic monosaccharides differing in the arrangement around the anomeric carbon
• D-Glucose has two anomers, α-D-glucose and β-D-glucose
• In α-D-glucose, the OH group is below the molecular plane
• In β-D-glucose, the OH group is above the molecular plane

Disaccharides

• Disaccharides consist of two monosaccharides linked by a glycosidic bond.

Specific Disaccharides

• Lactose, or milk sugar, consists of d-glucose and d-galactose with a beta-1,4 glycosidic bond
• Maltose, commonly known as malt sugar, contains two d-glucose molecules with an alpha-1,4 glycosidic bond
• Sucrose, or table/cane sugar, is made of D-glucose and D-fructose with an (alpha-1,2) glycosidic linkage
• Isomaltose contains two glucose units combined via an alpha-1,6 linkage, where one glucose residue connects to the sixth carbon of another through a glycosidic linkage

Polysaccharides

• Polysaccharides are comprised of numerous monosaccharide units
• Homoglycans consist of a single kind of monosaccharide, such as starch, glycogen, and cellulose
• Heteroglycans contain two or more different monosaccharides, like hyaluronic acid, chondroitin sulfate, and heparin.

Starch

• Starch: reserve carbohydrate of plant kingdom, made of amylose and amylopectin

Amylose

• Amylose comprises about 20% of starch's weight and is water-soluble
• It is also a linear polymer of D-glucose residues linked by alpha-1 → 4 glycosidic bonds

Amylopectin

• Amylopectin constitutes approximately 80% of starch content
• It is a highly branched polymer with D-glucose residues
• The main trunk and branches in amylopectin include D-glucose residues via alpha-1 → 4 glycosidic bonds
• Branching points occur via alpha-1,6 linkages

Glycogen

• Glycogen acts as the primary food reserve in the animal kingdom and is stored in the liver and skeletal muscles
• It's a highly branched polymer of D-glucose residues
• The main trunk of glycogen consists of d-glucose units linked by alpha-1 → 4 glycosidic bonds
• d-glucose residue branches are linked to the primary trunk via alpha-1 → 6 glycosidic bonds

Cellulose

• Cellulose is the main structural component of green plant cell walls
• It is a linear chain polymer composed of D-glucose residues
• Cellulose is linked by beta-(1 → 4) glycosidic bonds