The Structure and Function of Macromolecules

Ch. 3B: The Structure and Function of Macromolecules

Introduction to Macromolecules

Macromolecules are large, complex molecules essential for life. They include carbohydrates, lipids, proteins, and nucleic acids. Their structure and function are determined by the sequence and arrangement of their subcomponents.

• Monomers: Small organic molecules that serve as building blocks for polymers.

• Polymers: Long molecules consisting of many similar or identical monomers linked by covalent bonds.

• Macromolecules: Very large molecules, often composed of two or more polymers bonded together.

Example: Amino acid → peptide → polypeptide → protein (increasing size and complexity)

Dehydration Synthesis and Hydrolysis

Macromolecules are assembled and disassembled through two key chemical reactions: dehydration synthesis and hydrolysis.

• Dehydration Synthesis (Condensation Reaction): A reaction in which two monomers are covalently bonded to each other with the removal of a water molecule. This process builds polymers from monomers.

• Hydrolysis: A reaction that breaks the covalent bond between monomers by adding a water molecule, thus breaking down polymers into monomers.

Equation for Dehydration Synthesis:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Dehydration}} \text{Polymer} + H_2O$

Equation for Hydrolysis:

$\text{Polymer} + H_2O \xrightarrow{\text{Hydrolysis}} \text{Monomer}_1 + \text{Monomer}_2$

Properties Determined by Sequence and Subcomponents

The properties of macromolecules are determined by the sequence and chemical nature of their monomers. Even small changes in sequence can lead to significant changes in function.

• Carbohydrates: Sequence of monosaccharides affects energy storage and structural properties.

• Proteins: Sequence of amino acids determines the protein's shape and function.

• Nucleic Acids: Sequence of nucleotides encodes genetic information.

• Lipids: Types of fatty acids influence membrane fluidity and energy storage.

Major Classes of Biological Macromolecules

Carbohydrates

Carbohydrates serve as fuel and building material. They include simple sugars and polymers such as starch, glycogen, and cellulose.

• Monosaccharides: Simple sugars (e.g., glucose, fructose, ribose).

• Disaccharides: Two monosaccharides joined by a glycosidic linkage (e.g., sucrose, lactose).

• Polysaccharides: Polymers of many monosaccharides (e.g., starch, glycogen, cellulose, chitin).

Example: Starch (energy storage in plants) vs. cellulose (structural component in plant cell walls).

Lipids

Lipids are hydrophobic molecules that include fats, phospholipids, and steroids. They are important for energy storage, membrane structure, and signaling.

• Fats (Triglycerides): Glycerol + 3 fatty acids. Can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Phospholipids: Glycerol, two fatty acids, and a phosphate group. Major component of cell membranes; amphipathic (hydrophilic head, hydrophobic tails).

• Steroids: Four fused carbon rings (e.g., cholesterol, hormones).

Example: Phospholipid bilayer forms the structural basis of all cell membranes.

Proteins

Proteins are polymers of amino acids and perform a vast array of functions, including catalysis, defense, storage, transport, cellular communication, movement, and structural support.

• Monomer: Amino acid (20 different types, each with a unique side chain or R group).

• Peptide Bond: Covalent bond linking amino acids in a protein.

• Directionality: Proteins have an amino (N) terminus and a carboxyl (C) terminus.

Example: Hemoglobin (transports oxygen in blood), enzymes (catalyze biochemical reactions).

Nucleic Acids

Nucleic acids store and transmit hereditary information. The two types are DNA and RNA.

• Monomer: Nucleotide (composed of a phosphate group, a pentose sugar, and a nitrogenous base).

• DNA: Double-stranded helix, bases A, T, C, G; stores genetic information.

• RNA: Single-stranded, bases A, U, C, G; involved in protein synthesis and gene regulation.

• Directionality: Nucleic acids have a 5' end (phosphate group) and a 3' end (hydroxyl group).

Example: mRNA carries genetic information from DNA to ribosomes for protein synthesis.

Protein Structure and Function

Levels of Protein Structure

Proteins have four levels of structure, each critical to their function:

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Local folding into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.

• Tertiary Structure: Overall 3D shape of a polypeptide, stabilized by interactions among R groups (hydrophobic interactions, ionic bonds, disulfide bridges, van der Waals forces).

• Quaternary Structure: Association of two or more polypeptide chains into a functional protein.

Example: Hemoglobin has quaternary structure, consisting of four polypeptide subunits.

Protein Folding and Denaturation

Proteins must fold into their correct conformation to function properly. Environmental factors such as temperature and pH can disrupt folding, leading to denaturation and loss of function.

• Chaperonins: Proteins that assist in the proper folding of other proteins.

• Denaturation: Loss of protein structure due to environmental stress, resulting in loss of function.

Example: High fever can denature enzymes, impairing metabolic processes.

Directionality in Biological Polymers

Directionality refers to the orientation of monomers in a polymer, which influences structure and function.

• Proteins: Synthesized from the amino (N) terminus to the carboxyl (C) terminus.

• Nucleic Acids: Synthesized from the 5' end to the 3' end.

Example: DNA replication and transcription proceed in the 5' to 3' direction.

Summary Table: Monomers, Polymers, and Macromolecules

Type	Monomer	Polymer	Macromolecule
Carbohydrates	Monosaccharide	Polysaccharide	Starch, Glycogen, Cellulose
Lipids	Glycerol, Fatty Acids	Triglyceride, Phospholipid	Fats, Membranes
Proteins	Amino Acid	Polypeptide	Enzymes, Structural Proteins
Nucleic Acids	Nucleotide	Polynucleotide	DNA, RNA