Introduction to Carbohydrates

• Carbohydrates are organic molecules, which are abundant in nature
• They have a wide range of functions
• Some functions include providing energy and acting as an energy storage within the body
• Cell membranes have components that the carbohydrates serve as
• Nucleic acids are components which have carbohydrates such as D-ribose (RNA) and 2-deoxyribose(DNA)

Structural Components

• Carbohydrates serve as structural components of many organisms
• The cell walls of bacteria include carbohydrates
• The exoskeleton of many insects has carbohydrates
• The fibrous cellulose of plants are carbohydrates

Chemical Formula

• The empiric formula for the carbohydrates is (CH2O)n
• This is where the name "hydrate of carbon" is derived

Classification of Carbohydrates

• Carbohydrates can be classified as Monosaccharides, Disaccharides, Oligosaccharides, and Polysaccharides

Monosaccharides

• Monosaccharides are simple sugars that cannot be hydrolyzed to simpler carbohydrates

Disaccharides

• Disaccharides are 2 monosaccharides covalently linked

Oligosaccharides

• Oligosaccharides are a few monosaccharides covalently linked (3- 10 monosaccharides)

Polysaccharides

• Polysaccharides are polymers consisting of chains of monosaccharide or disaccharide

Building blocks

• Monosaccharides are building blocks of all carbohydrates
• Classified according to the number of carbon atoms the monosaccharides contain

Atoms by Count

• 3C: trioses
• 4C: tetroses
• 5C: pentoses
• 6C: hexoses

Types

• Monosaccharides can be either aldoses, containing an aldehyde group
• Or ketoses, containing a ketone group

Cyclization

• Monosaccharides are predominantly found in a cyclic, ring formation
• Aldehyde or ketone reacts with an alcohol group on the same sugar

Linear Chain

• Less than 1% of each of the monosaccharides with five or more carbons exists in the open-chain (acyclic) form

Six-Membered Ring

• Pyranose refers to a six-membered ring consisting of five carbons and one oxygen (e.g., glucopyranose)

Five-Membered Ring

• Furanose denotes a five-membered ring with four carbons and one oxygen
• The pyranose form of glucose dominates (>99%) at equilibrium in aqueous solution

Anomeric Carbon

• Cyclization creates an anomeric carbon
• The former carbonyl carbon generates the a and β configurations of the sugar
• Glucose's a-D-glucopyranose and β-D-glucopryanose

Isomers

• Two sugars are both glucose but are anomers

Alpha configuration

• The OH on the anomeric C projects to the same side as the ring in a modified Fischer projection formula
• Of Haworth projection formula of the α configuration, this OH is trans to the CH2OH group

Fischer Projection

• A modified Fischer projection formulas shows the interconversion of α and β anomeric forms of glucose

Haworth projection formulas

• A Haworth projection formulas shows the interconversion

Fischer Projection

• Bonds are written in a two dimensional form depicting vertical and horizontal relationships with tetrahedral stereocenters
• For sugars with more than one chiral centre, D or L refers to the asymmetric C farthest from the aldehyde or keto group
• The most naturally sugars are D

D- Monosaccharide

• The -OH is on the right when written as Fischer projection

L- Monosaccharide

• The -OH is on the left when written as Fischer projection

Isomers and Epimers

• Isomers are compounds with the same chemical formula but different structures
• Fructose, glucose, mannose, and galactose are isomers having the same chemical formula C6H12O6
• Carbohydrate isomers with different configurations around one specific carbon atom (not carbonyl) are called epimers
• Glucose and galactose, for example, are C-4 epimers, only differing by their -OH group's position on carbon 4
• Glucose and mannose are C-2 epimers
• Galactose and mannose are not epimers due to differences in -OH at carbons 2 and 4, simply isomers

Examples

• Mannose is a C-2 epimer of glucose
• Galactose is a C-4 epimer of glucose
• Fructose is an isomer of glucose
• Mannose is also a component of glycoproteins

Enantiomers

• Have a special type of isomerism in the pairs of structures, which are mirror images
• These mirror images are enantiomers, named as a D- and an L- sugar
• The vast majority of human sugars are D-sugars
• In the D isomeric form, the OH group is on the right of the asymmetric carbon(groups) farthest from the carbonyl carbon

Joining Monosaccharides

• Monosaccharides are joined to form disaccharides, oligosaccharides, and polysaccharides
• Glycoside: The -OH of an anomeric carbon in sugar molecules are replaced by -OR (another group)
• Glycosidic bond: The bond that connects sugars
• Bond from the anomeric carbon to the -OR group
• The anomeric carbon of one sugar is linked to -OH groups of another
• Happens when forming an α- or β-glycosidic linkages

Glycosidic Bonds

• Glycosidic bonds between sugars are named according to the location of connected carbons in this structure
• Can also be named according to position of the anomeric hydroxyl group of the sugar (Alpha and Beta bonds)
• In Lactose: It is synthesized by glycosidic bonding between carbon 1 of β-galactose and carbon 4 of glucose, making up a β(1→4) glycosidic-bond

Disaccharides Compositions

• Lactose: D-galactose and D-glucose, by way of β-1,4 glycosidic bond
• Sucrose (table sugar): D-glucose and D-fructose joined by α-1,2 glycosidic bond
• Maltose: two units of D-glucose, joined by α-1,4 glycosidic bond

Polysaccharides

• Most carbohydrates exist as polysaccharides
• The scientific term for this is "glycans"
• These differ from each other because of monosaccharide units, length of chains, types of linking bonds and degree of branching
• Glycogen (animal sources)- Polymer of α-D-glucose units, joined by α-1,4 and α-1,6 glycosidic bond
• Starch (plant sources)- Polymer of α-D-glucose units, joined and α-1,6 glycosidic bond
• Cellulose (plant sources)- Polymer of D-glucose per a singular molecule, joined by β-1,4 glycosidic bond

Glycogen Details

• Glycogen has mobilized storage of glucose
• It is very sizable, with highly branched polymer of glucose residues to break down for high amount of energy
• Glucose residues in glycogen happen by α-1,4-glycosidic bonds
• Branches at every 10 residue by α-1,6-glycosidic bonds
• Glycogen is typically stored in the liver and skeletal muscle
• Blood-glucose will be maintained through glycogen; which is what the brain uses

More on Storage

• Glycogen is ingested with diet, and is hydrolyzed by α-amylases to break (α 1-4 ) glycosidic bonds between glucose molecules
• Followed by using debranching enzymes to break for (α 1-6 ) glycosidic bonds

Starch Details

• A Polymeric carbohydrate of the numerous glucose units joined by way of 𝛼 1-4 and 𝛼 1-6
• Main format of glucose stored in most plant life
• Glycogen serves as this analogue
• Consists of a linear amylose and branched amylopectin
• Once It enters the diet, it breaks downed again, by α-amylases to also break (α 1-4) branches of glucose molecules, and with debranching enzymes, breaks (α 1-6) for more glycosidic bonds