Biomolecules and Carbohydrates

Carbohydrates

• Carbohydrates are the most abundant biomolecule on Earth, with approximately 100 billion metric tons produced annually through photosynthesis.
• They are a vital dietary staple in many parts of the world, and their functions include energy source, energy storage, structural component, and informational molecules.
• Carbohydrates can be covalently linked with proteins to form glycoproteins, glycolipids, and proteoglycans.

Classification of Carbohydrates

• Carbohydrates are classified based on the number of carbon atoms they have, with the formula (CH2O)n, where 'n' is the number of carbon atoms ≥ 3.
• They can range in size from small (e.g., glyceraldehyde, Mw = 90 Da) to huge (e.g., amylopectin, Mw = 200,000,000 Da).

Functional Groups of Biomolecules

• A functional group is a group of atoms within a molecule with distinctive chemical properties.
• When two or more substituents are shown in a molecule, they are designated as R1, R2, and so forth.

Monosaccharides

• Monosaccharides are classified into two classes: aldoses and ketoses, based on the type of carbonyl group they possess.
• Aldoses have an aldehyde group, while ketoses have a ketone group.
• Monosaccharides can also be classified based on the number of carbon atoms they have, such as trioses, tetroses, pentoses, and hexoses.

Fischer Projection Formulas

• Fischer projection formulas are a 2D method for representing 3D molecules to show the configuration of groups around chiral centers.
• The carbonyl group is situated at the top, and the last achiral CH2OH is at the bottom.

Chiral Centers and Enantiomers

• Chiral centers are atoms with substituents arranged so that the molecule is non-superposable on its mirror image.
• Enantiomers are stereoisomers that are non-superposable mirror images of each other.
• Chiral compounds can interact with polarized light, causing the plane of vibration to rotate.

Importance of Enantiomers

• Obtaining enantiomerically pure compounds is crucial in medicine, as the two enantiomers of a chiral molecule can have very different biological properties.
• Enantiomers can differ in bioavailability, rate of metabolism, metabolites, excretion, potency, selectivity, and toxicity.

Epimers

• Epimers are two sugars that differ only in the configuration around one carbon atom.
• D-glucose and D-mannose are examples of epimers.

Definitions

• Aldose: A simple sugar in which the carbonyl carbon atom is an aldehyde.
• Carbohydrate: A sugar (monosaccharide) or one of its dimers (disaccharide) or polymers (polysaccharide).
• Chiral center: An atom with substituents arranged so that the molecule is non-superposable on its mirror image.
• Chiral compound: A compound that contains an asymmetric center (chiral atom or chiral center) and thus can occur in two non-superposable mirror-image forms.
• Disaccharide: A carbohydrate consisting of two covalently joined monosaccharide units.
• Enantiomers: Stereoisomers that are non-superposable mirror images of each other (L - laevorotatory and D - dextrorotatory).
• Fischer projection formulas: A 2D method for representing 3D molecules to show the configuration of groups around chiral centers.
• Glycosidic bond: Bonds between a sugar and another molecule (typically an alcohol, purine, pyrimidine, or sugar) through an intervening oxygen.
• Isomers: Any two molecules with the same molecular formula but a different arrangement of molecular groups.
• Ketose: A simple monosaccharide in which the carbonyl group is a ketone.
• Monosaccharide: A carbohydrate consisting of a single sugar unit.
• Oligosaccharide: Several monosaccharide groups joined by glycosidic bonds.
• Polysaccharide: A linear or branched polymer of monosaccharide units linked by glycosidic bonds.

Monosaccharides

• Monosaccharides can exist in a straight or cyclic structure.
• In aqueous solutions, aldotetroses and monosaccharides with 5 or more carbon atoms occur predominantly as cyclic structures.
• Glucose, Galactose, and Fructose are all isomers of one another (same chemical formula, different structural formula).

Cyclic Structure: Hemiacetals and Hemiketals

• Aldehyde and ketone carbons are electrophilic, while alcohol oxygen atoms are nucleophilic.
• When aldehydes are attacked by alcohols, hemiacetals form, and when ketones are attacked by alcohols, hemiketals form.
• Formation of an additional asymmetric carbon atom (anomeric carbon) can exist in two stereoisomeric forms.

Cyclisation of Monosaccharides

• There can be multiple configurations of a ring structure.
• Two possible ring structures for Glucose exist: α-Glucose and β-Glucose.
• The difference between α-Glucose and β-Glucose is the placement of the hydroxyl group on Carbon 1.
• Aldopyranose is more stable than aldo furanose.
• Mutarotation occurs when the anomeric carbon is attacked by a nucleophile.

Anomers

• Anomers are two stereoisomers of a given sugar that differ only in the configuration about the carbonyl (anomeric) carbon atom.
• The anomeric carbon is the carbon atom in a sugar at the new stereocenter formed when a sugar cyclizes to form a hemiacetal.

Disaccharides

• Two monosaccharides bond together (dehydration reaction) to form a carbohydrate called a disaccharide.
• A dehydration reaction is a condensation reaction, where two molecules combine to form one single molecule, losing a small molecule in the process.
• The bond between two monosaccharides is known as a glycosidic bond.

The Glycosidic Bond

• Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon and a hydroxyl carbon.
• The glycosidic bond (an acetal) between monomers is less reactive than the hemiacetal at the second monomer.
• A second monomer with a hemiacetal is reducing, while the anomeric carbon involved in the glycosidic linkage is non-reducing.

Non-Reducing Disaccharides

• Two sugar molecules can also be joined via a glycosidic bond between two anomeric carbons.
• No remaining hemiacetals are present, and the product has two acetal groups and no hemiacetals.
• There are no reducing ends, and this is a non-reducing sugar.

Key Concepts

• Haworth perspective formulas are a method for representing cyclic chemical structures to define the configuration of each substituent group.
• Hemiacetal/Hemiketal is formed when an alcohol oxygen atom adds to the carbonyl carbon of an aldehyde or a ketone following nucleophilic attack.
• Nucleophilic addition is the initial attack of a nucleophile (hydroxyl group) on the slightly positive carbon center of the carbonyl group.
• Reducing sugar is a sugar in which the carbonyl (anomeric) carbon is not involved in a glycosidic bond and can therefore undergo oxidation.
• Non-reducing sugar is a carbohydrate that cannot donate electrons to other molecules and therefore cannot act as a reducing agent or be oxidized in aqueous solution.

Polysaccharide Structure

• Polysaccharides can be composed of 1, 2, or several different monosaccharides in straight or branched chains of varying length.

Homopolysaccharides

• A homopolysaccharide is a polysaccharide made up of one type of monosaccharide unit.
• Examples: starch, glycogen, chitin, cellulose.

Heteropolysaccharides

• A heteropolysaccharide is a polysaccharide containing more than one type of sugar.
• Examples: agarose, peptidoglycans, glycosaminoglycans.

Starch

• Starch is a mixture of two homopolysaccharides of D-glucose: amylose and amylopectin.
• Amylose is an unbranched polymer of (α1 → 4) linked residues with a molecular weight that varies from a few thousand to over 1 million.
• Amylopectin is branched like glycogen, but the branch-points with (α1 → 6) linkers occur every 24–30 residues, with a molecular weight of up to 200 million.
• Starch is the main storage polysaccharide in plants and is heavily hydrated due to the number of exposed -OH groups.

Glycogen

• Glycogen is a branched homopolysaccharide of glucose.
• Glucose monomers form (α1 → 4) linked chains.
• Branch-points with (α1 → 6) linkers occur every 8–12 residues, making it more compact than starch.
• Molecular weight reaches several millions, and it functions as the main storage polysaccharide in animals, abundant in the liver and skeletal muscle.

Other Polysaccharides

• Dextrans: bacterial and yeast polysaccharides, with poly-D-glucose forming (α1 → 6) linked chains.
• Cellulose: an unbranched homopolysaccharide of glucose, with D-glucose monomers forming (β1 → 4) linked chains.
• Chitin: a linear homopolysaccharide of N-acetylglucosamine (NAG), found in cell walls of mushrooms and exoskeletons of insects, spiders, and other arthropods.
• Agar and Agarose: complex mixtures of heteropolysaccharides containing modified galactose units, used in laboratory settings.
• Glycosaminoglycans: linear polymers with repeating disaccharide units, found in animals and bacteria, containing sulfate groups.

Polysaccharide Functions

• Serve as stored fuel and structural components of cell walls and extracellular matrix.
• Some homopolysaccharides fold in 3D, forming helical structures with interchain hydrogen bonding.
• Heteropolysaccharides, such as peptidoglycan and agar, strengthen bacterial and algal cell walls.
• Glycosaminoglycans provide support and aid diffusion in the extracellular matrix.