Carbohydrates, Nucleosides, and Nucleotides: Structure and Function

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 $(CH_2O)_n$, where n is typically 3 or more. For example, glucose has the formula $C_6H_{12}O_6$.

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

• Oxidation: Carbohydrates can be oxidized to $CO_2$ 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 ($C_{12}H_{22}O_{11}$)

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

Name	Constituent Sugars	Bond
Sucrose	Glucose + Fructose	α-1,2
Lactose	Galactose + Glucose	β-1,4
Maltose	Glucose + Glucose	α-1,4

Glycosidic Bond Formation and Hydrolysis

• Condensation: Formation of glycosidic bond with the release of water ($H_2O$).

• 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

DNA Nucleoside	Abbreviation
Deoxyadenosine	dA
Deoxyguanosine	dG
Deoxythymidine	dT
Deoxycytidine	dC

RNA Nucleoside	Abbreviation
Adenosine	A
Guanosine	G
Uridine	U
Cytidine	C

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

Feature	DNA	RNA
Pentose Sugar	Deoxyribose (H at 2')	Ribose (OH at 2')
Pyrimidine Base	Thymine	Uracil
Strandedness	Double-stranded	Single-stranded
Base Modification	Bases not modified	Bases can be modified
Hydrolysis	Resistant to alkali	Susceptible to alkali
Location	Nucleus, mitochondria	Nucleus, cytoplasm

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

Type	Example	Key Features
Monosaccharide	Glucose	Energy source, building block
Disaccharide	Sucrose	Glucose + Fructose, α-1,2 bond
Polysaccharide	Starch	Plant energy storage, α-1,4 and α-1,6 bonds
Nucleoside	Adenosine	Pentose + Adenine
Nucleotide	AMP	Adenosine + Phosphate

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