Carbohydrates

General Formula and Main Types

Carbohydrates are essential biomolecules that serve as energy sources, structural components, and mediators of cell recognition. Their general formula is , where n is typically 3 or more. For example, glucose has the formula .

• Functional Groups: Carbohydrates contain hydroxyl (-OH) and carbonyl (C=O) groups.

• Oxidation: Carbohydrates can be oxidized to during cellular respiration, releasing energy.

• Main Types:

  • Monosaccharides: Single sugar units (e.g., glucose, fructose).

  • Disaccharides: Two monosaccharide units joined by a glycosidic bond (e.g., sucrose, lactose).

  • Polysaccharides: Long chains of monosaccharide units (e.g., starch, glycogen, cellulose).

Types of Monosaccharides According to the Functional Group

Monosaccharides are classified based on the type of carbonyl group present:

• Aldoses: Contain an aldehyde group at C1 (e.g., glucose is an aldohexose).

• Ketoses: Contain a ketone group at C2 (e.g., fructose is a ketohexose).

Fischer Projection to Represent 3D Structures

• The Fischer projection is a two-dimensional representation of three-dimensional sugar structures.

• Horizontal bonds project out of the plane; vertical bonds go into the plane.

• Still widely used for depicting monosaccharide stereochemistry.

Classification by Number of Carbon Atoms

• Triose: 3 carbons (e.g., glyceraldehyde)

• Tetrose: 4 carbons (e.g., erythrose)

• Pentose: 5 carbons (e.g., ribose)

• Hexose: 6 carbons (e.g., glucose)

Cyclic Forms of Monosaccharides

Monosaccharides with five or more carbons can cyclize in solution. The carbonyl group reacts with a hydroxyl group to form a ring structure:

• 5-membered rings: Furanose forms

• 6-membered rings: Pyranose forms

• This cyclization creates a new chiral center called the anomeric carbon.

Anomers: Stereoisomers of Cyclic Monosaccharides

Anomers are stereoisomers that differ in configuration around the anomeric carbon (C1 in aldoses, C2 in ketoses):

• Alpha (α) anomer: The OH group on the anomeric carbon is trans (opposite side) to the CH2OH group.

• Beta (β) anomer: The OH group is cis (same side) to the CH2OH group.

Examples of Monosaccharides

• Glucose: The most important monosaccharide in human metabolism; building block for many polysaccharides.

• Galactose: Isomer of glucose (differs at C4); component of lactose; found in milk and peas.

• Fructose: The most important keto-monosaccharide; found in honey and fruit juices.

Disaccharides ()

Disaccharides are formed by a condensation reaction (dehydration synthesis) between two monosaccharides, creating a glycosidic bond (covalent bond between the anomeric carbon of one sugar and a hydroxyl group of another).

• Nomenclature: Glycosidic bonds are named by the carbons involved and the anomeric configuration (e.g., α-1,4 bond).

Examples of Disaccharides

Glycosidic Bond Formation and Hydrolysis

• Condensation: Formation of glycosidic bond with the release of water ().

• Hydrolysis: Breaking of glycosidic bond by addition of water.

Polysaccharides

Polysaccharides are long polymers of monosaccharides formed via condensation reactions. They serve as energy storage or structural molecules.

• Starch: Storage carbohydrate in plants; mixture of amylose (straight chain, α-1,4 bonds) and amylopectin (branched, α-1,4 and α-1,6 bonds).

• Glycogen: Storage carbohydrate in animals, fungi, and bacteria; highly branched (α-1,4 and α-1,6 bonds); stored in liver and muscle.

• Cellulose: Structural carbohydrate in plants; straight chains of β-glucose (β-1,4 bonds); dietary fiber for animals and humans.

Nucleosides and Nucleotides

Nucleosides and Nucleotides: Structure

• Nucleoside: Composed of a pentose sugar and a nitrogenous base (purine or pyrimidine) attached at the 1' position (e.g., adenosine).

• Nucleotide: Nucleoside with one or more phosphate groups attached at the 5' position (e.g., adenosine monophosphate, AMP).

Pentoses: Ribose and 2-Deoxyribose

• Ribose: Found in RNA; has an OH group at the 2' position.

• 2-Deoxyribose: Found in DNA; has an H at the 2' position (lacks the 2' OH group).

Nitrogenous Bases

• Pyrimidines: Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only)

• Purines: Adenine (A), Guanine (G)

• The C=O, NH2, and R2N groups confer unique hydrogen bonding properties.

DNA and RNA Nucleosides

Nucleotide Polymers

• Nucleotides are joined by phosphodiester bonds (between the 5' phosphate of one nucleotide and the 3' OH of the next).

• This forms the sugar-phosphate backbone of DNA and RNA.

• Enzymes such as DNA polymerase and DNA ligase catalyze this process.

DNA Strand Polarity: 5' and 3' Ends

• 5' end: Free phosphate group attached to C5'.

• 3' end: Free hydroxyl group attached to C3'.

The History of the Structure of DNA

• Chargaff's Rule: In DNA, the amount of purines equals the amount of pyrimidines (A = T, G = C).

• Watson and Crick (1953) proposed the double helix model, using X-ray diffraction data from Rosalind Franklin and Maurice Wilkins.

A-T and G-C Base Pairing

• Adenine (A) pairs with Thymine (T) in DNA via 2 hydrogen bonds.

• Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.

• In RNA, Uracil (U) replaces Thymine (T).

DNA Double Helix Structure

• DNA consists of two antiparallel strands (one 5'→3', the other 3'→5').

• Hydrophilic phosphate groups face outward; bases are stacked inside.

• Water is expelled from the interior of the helix.

Major and Minor Grooves

• Major groove: Wider and deeper; base pairs are more accessible for protein binding.

• Minor groove: Narrower and shallower; less accessible but still important for some protein interactions.

Functions of DNA

• Stores genetic information required for building and maintaining an organism.

• Contains genes that code for proteins and functional RNAs.

• Passes genetic information from parent to offspring.

Replication and Expression

• Transcription: DNA is copied into messenger RNA (mRNA).

• Translation: mRNA directs the assembly of amino acids into proteins.

Ribonucleic Acid (RNA)

Major Classes of RNA

• Messenger RNA (mRNA): Template for protein synthesis; carries genetic code from DNA to ribosome.

• Transfer RNA (tRNA): Adaptor molecule; brings specific amino acids to the ribosome during translation.

• Ribosomal RNA (rRNA): Structural and catalytic component of ribosomes; forms the core of ribosome's structure and catalyzes peptide bond formation.

Differences Between DNA and RNA

Transcription of DNA to mRNA

• DNA is partially unwound.

• The anti-sense strand serves as the template for mRNA synthesis.

• mRNA is released and DNA rewinds.

Codons

• There are 64 possible codons (triplets of nucleotides) that code for 20 amino acids.

• Several codons can code for the same amino acid (degeneracy).

• AUG codes for methionine (start codon); UAA, UAG, UGA are stop codons.

Structure of tRNA

• Short chains of 73–93 nucleotides.

• Each tRNA carries a specific amino acid and recognizes the corresponding codon on mRNA via its anticodon loop.

• Unusual bases in tRNA create loops that do not form standard hydrogen bonds.

Translation of tRNA to Protein

• Two tRNA molecules bind to the ribosome at the mRNA binding sites.

• A peptide bond forms between the amino acids carried by the tRNAs.

• The ribosome moves along the mRNA, and new tRNAs bring in additional amino acids, elongating the protein chain.

Summary Table: Key Carbohydrates and Nucleic Acid Components

Additional info: This guide expands on the provided slides by including definitions, examples, and tables for clarity and exam preparation.