Macromolecules in Microbiology: Structure, Function, and Relevance

Macromolecules: An Overview

Definition and Major Classes

Macromolecules are large, complex molecules essential for life. Living organisms are primarily composed of four major types of macromolecules: carbohydrates, proteins, nucleic acids, and lipids. Each class plays distinct and critical roles in cellular structure and function.

• Carbohydrates: Energy storage and structural components

• Proteins: Catalysis, structure, transport, regulation, and more

• Nucleic acids: Storage and transmission of genetic information

• Lipids: Membrane structure, energy storage, and signaling

Monomers and Polymers

Building Blocks and Assembly

Most macromolecules are polymers, constructed from repeating subunits called monomers. The process of linking monomers to form polymers is called dehydration synthesis (or condensation), while breaking polymers into monomers is called hydrolysis (or decomposition).

• Carbohydrates: Monomer = monosaccharide (e.g., glucose); Polymer = polysaccharide (e.g., starch, cellulose, glycogen)

• Proteins: Monomer = amino acid; Polymer = polypeptide/protein

• Nucleic acids: Monomer = nucleotide; Polymer = DNA or RNA

• Lipids: Not true polymers, but built from fatty acids and glycerol (e.g., triglycerides, phospholipids)

Carbohydrates

Structure and Function

Carbohydrates are a major energy source for most living things and are important structural components in cells (e.g., cell wall, capsule). The primary carbohydrate used by human cells for energy during cellular respiration is glucose.

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

• Disaccharides: Two monosaccharides joined together (e.g., sucrose)

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

Concept Check Example

• Which of these is a monomer? Glucose (starch, cellulose, and glycogen are polymers)

Polymers and Monomers: Synthesis and Breakdown

Dehydration Synthesis and Hydrolysis

Polymers are formed by dehydration synthesis, where a water molecule is removed to join two monomers. Polymers are broken down by hydrolysis, where water is added to split the bond.

• Dehydration synthesis: Builds macromolecules

• Hydrolysis: Breaks macromolecules into monomers

Equation for dehydration synthesis:

$\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

Equation for hydrolysis:

$\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Proteins

Structure and Function

Proteins are polymers of amino acids and are the most functionally diverse macromolecules. They serve as enzymes, structural components, transporters, regulators, and more. The tetanus toxin is an example of a bacterial protein that disrupts human physiology by blocking inhibitory neurons, leading to rigid paralysis.

• Structural proteins: Provide support (e.g., collagen)

• Motility proteins: Enable movement (e.g., actin, myosin)

• Transport proteins: Move substances across membranes (e.g., hemoglobin, membrane channels)

• Enzymes: Catalyze biochemical reactions (e.g., DNA polymerase)

• Regulatory proteins: Control cellular processes (e.g., transcription factors)

Protein Structure

Proteins are made of amino acids linked by peptide bonds. The general structure of an amino acid includes an amino group, a carboxyl group, and a variable side chain (R group). Proteins fold into specific shapes, which determine their function.

• Primary structure: Sequence of amino acids

• Secondary structure: Local folding (α-helix, β-sheet)

• Tertiary structure: Overall 3D shape

• Quaternary structure: Multiple polypeptide chains

Protein Denaturation

Denaturation is the loss of protein structure (and function) due to heat, chemicals, or pH changes. Only proteins in their correct tertiary or quaternary structure are fully functional.

• Functional proteins: Tertiary and quaternary structures

• Non-functional proteins: Primary or secondary structure only

Nucleic Acids

DNA and RNA: Structure and Function

Nucleic acids store and transmit genetic information. DNA contains the instructions for building and controlling cells, while RNA is involved in protein synthesis and, in some viruses, also stores genetic information.

• DNA: Double helix, antiparallel strands, base pairing (A-T, G-C), hydrogen bonds

• RNA: Single-stranded, contains ribose and uracil (instead of deoxyribose and thymine)

• mRNA: Messenger RNA, copy of a gene

• rRNA: Ribosomal RNA, part of ribosome structure

• tRNA: Transfer RNA, delivers amino acids during translation

Concept Check Example

• What is unique to RNA compared to DNA? RNA contains uracil (U) and ribose sugar.

Lipids

Types and Functions

Lipids are hydrophobic molecules with diverse structures and functions. They are not true polymers but are essential for cell membrane structure, energy storage, and signaling.

• Phospholipids: Main component of cell membranes; structure includes phosphate, glycerol, and fatty acids

• Steroids: Four carbon-based rings; strengthen cell membranes (e.g., cholesterol in eukaryotes, hopanoids in prokaryotes)

• Triglycerides: Glycerol and three fatty acids; energy storage

Applications and Microbial Relevance

Examples of Macromolecule Use in Microbes

• Carbohydrates: Bacterial cell walls (peptidoglycan), energy storage (glycogen)

• Proteins: Enzymes for metabolism, toxins (e.g., tetanus toxin)

• Nucleic acids: Plasmids for antibiotic resistance, viral genomes

• Lipids: Membrane structure (phospholipids), energy reserves (polyhydroxyalkanoates)

Additional info: Understanding macromolecules is foundational for topics such as microbial metabolism, genetics, pathogenesis, and biotechnology.