Study Guide: Proteins, Nucleic Acids, Carbohydrates, and Lipids (Lectures 2–5)

Proteins

Structure and Function of Proteins

Proteins are complex macromolecules essential for various biological functions, including catalysis, structure, transport, and regulation. Their structure is organized into four hierarchical levels.

• Primary Structure: The linear sequence of amino acids in a polypeptide chain, held together by peptide bonds.

• Secondary Structure: Local folding patterns such as α-helix and β-sheet, stabilized by hydrogen bonds between backbone atoms.

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide, stabilized by interactions including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

• Quaternary Structure: The arrangement of multiple polypeptide subunits in a protein complex.

Peptide Bond: A covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water (condensation reaction).

• R-group Interactions: Side chains (R-groups) of amino acids interact to stabilize tertiary and quaternary structures.

Example: Hemoglobin is a quaternary protein composed of four polypeptide subunits.

Nucleic Acids

Structure and Types of Nucleic Acids

Nucleic acids, including DNA and RNA, store and transmit genetic information. They are polymers of nucleotides.

• Nucleotide Components: Each nucleotide consists of a phosphate group, a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

• DNA vs. RNA: DNA contains deoxyribose and the bases A, T, C, G; RNA contains ribose and the bases A, U, C, G.

• Phosphodiester Bond: Nucleotides are linked by phosphodiester bonds between the 3' hydroxyl and 5' phosphate groups (condensation reaction).

• Double Helix Structure: DNA is typically double-stranded, forming a right-handed double helix. RNA is usually single-stranded but can form secondary structures.

• Antiparallel Strands: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

• Complementary Base Pairing: In DNA, A pairs with T, and C pairs with G. In RNA, A pairs with U, and C pairs with G.

Base Pairing Rules Table:

	DNA	RNA
Adenine (A)	Thymine (T)	Uracil (U)
Cytosine (C)	Guanine (G)	Guanine (G)
Guanine (G)	Cytosine (C)	Cytosine (C)
Thymine (T)	Adenine (A)	—
Uracil (U)	—	Adenine (A)

Example: The sequence 5'-ATGC-3' in DNA pairs with 3'-TACG-5'.

Carbohydrates

Structure and Classification of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, serving as energy sources and structural components.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) with the general formula (CH2O)n. They contain a carbonyl group (aldehyde or ketone) and multiple hydroxyl groups.

• Structural Features: Monosaccharides are classified by the number of carbon atoms (triose, pentose, hexose) and the position of the carbonyl group (aldose or ketose).

• Ring Formation: In aqueous solutions, monosaccharides often form ring structures (α and β anomers).

• Disaccharides and Polysaccharides: Formed by glycosidic linkages between monosaccharides via condensation reactions.

• Major Linkages: α-1,4-glycosidic and β-1,4-glycosidic bonds are common in starch, glycogen, and cellulose.

• Polysaccharides: Starch and glycogen (energy storage), cellulose (structural support in plants).

• Hydrolysis: Enzymes like amylase break glycosidic bonds, but only if the monomers are in the correct form (e.g., α-form for starch).

Example: Sucrose is a disaccharide formed from glucose and fructose via an α-1,2-glycosidic bond.

Table: Comparison of Major Polysaccharides

Polysaccharide	Monomer	Linkage	Function
Starch	Glucose (α)	α-1,4 and α-1,6	Energy storage in plants
Glycogen	Glucose (α)	α-1,4 and α-1,6	Energy storage in animals
Cellulose	Glucose (β)	β-1,4	Structural support in plants

Lipids

Structure and Types of Lipids

Lipids are hydrophobic molecules that include fats, phospholipids, and steroids. They play roles in energy storage, membrane structure, and signaling.

• Fats (Triglycerides): Composed of three fatty acids linked to a glycerol backbone via ester bonds. Used for long-term energy storage.

• Phospholipids: Contain two fatty acids and a phosphate group attached to glycerol. They are amphipathic, with hydrophilic heads and hydrophobic tails, forming the basis of biological membranes.

• Steroids: Characterized by a four-ring structure. Cholesterol is a common steroid, important for membrane fluidity and as a precursor for steroid hormones.

• Amphipathic Nature: Phospholipids have both hydrophilic and hydrophobic regions, enabling the formation of lipid bilayers.

Example: The phospholipid bilayer forms the fundamental structure of cell membranes.

Summary Table: Types of Lipids

Lipid Type	Main Components	Function
Fats (Triglycerides)	Glycerol + 3 fatty acids	Energy storage
Phospholipids	Glycerol + 2 fatty acids + phosphate group	Membrane structure
Steroids	Four fused rings	Membrane fluidity, hormones

Additional info:

• Condensation reactions (dehydration synthesis) are responsible for forming peptide, glycosidic, and phosphodiester bonds, releasing water as a byproduct.

• Hydrolysis is the reverse process, breaking bonds by adding water.