Rucete ✏ Campbell Biology In a Nutshell
Unit 1 THE CHEMISTRY OF LIFE — Concept 5.4 Proteins Have Diverse Structures and Functions
Proteins are the workhorses of the cell, performing a vast array of functions that are crucial for life. They are composed of amino acids, and their specific structures determine their biological roles.
1. Amino Acids: The Building Blocks of Proteins
Amino acids have a central carbon bonded to four parts:
An amino group (—NH₂)
A carboxyl group (—COOH)
A hydrogen atom
An R group (side chain) that differs among the 20 standard amino acids
The chemical properties of the R group influence how each amino acid behaves, especially during protein folding.
2. Protein Structure: Four Levels of Organization
Primary structure: The unique sequence of amino acids in a polypeptide chain
Secondary structure: Coils and folds like:
α-helix (spiral)
β-pleated sheet (accordion-like) These are stabilized by hydrogen bonds.
Tertiary structure: The overall 3D shape of a polypeptide, determined by interactions between side chains (R groups)
Quaternary structure: When multiple polypeptide chains form one protein (e.g., hemoglobin)
3. Diverse Protein Functions
Enzymes: Speed up chemical reactions (e.g., amylase, pepsin)
Structural: Provide support (e.g., collagen in skin and cartilage)
Transport: Move substances (e.g., hemoglobin carries oxygen)
Signaling: Hormones like insulin regulate physiological processes
Defensive: Antibodies help fight infections
4. Denaturation: When Proteins Fall Apart
Protein function depends on structure. If the shape unravels due to heat, pH changes, or salt concentration, the protein becomes denatured and non-functional.
Sometimes, this process is reversible—but often, it permanently destroys the protein's activity.
In a Nutshell
- Proteins are made of amino acids, linked by peptide bonds.
- Their structure is organized into four levels, each essential for function.
- Protein function follows form—shape determines what a protein can do.
- Environmental changes can cause denaturation, impairing or destroying the protein's role.