The Structure and Function of Large Biological Molecules: Proteins

Chapter 5: The Structure and Function of Large Biological Molecules

Introduction

Large biological molecules, also known as macromolecules, are essential for life. Among these, proteins are the most diverse in structure and function, accounting for more than 50% of the dry mass of most cells. This section explores the diversity of protein structures and their wide range of biological functions.

Proteins: Diversity of Structure and Function

Overview of Protein Functions

• Enzymatic proteins speed up chemical reactions (catalysis).

• Other protein functions include defense, storage, transport, cellular communication, movement, and structural support.

Major Types of Proteins and Their Functions

Type	Function	Example(s)
Enzymatic proteins	Selective acceleration of chemical reactions	Digestive enzymes catalyze the hydrolysis of bonds in food molecules.
Defensive proteins	Protection against disease	Antibodies inactivate and help destroy viruses and bacteria.
Storage proteins	Storage of amino acids	Casein (milk protein), ovalbumin (egg white protein), storage proteins in plant seeds.
Transport proteins	Transport of substances	Hemoglobin transports oxygen in blood; membrane transport proteins move substances across cell membranes.
Hormonal proteins	Coordination of an organism's activities	Insulin regulates blood sugar concentration.
Receptor proteins	Response of cell to chemical stimuli	Receptors in nerve cell membranes detect signaling molecules.
Contractile and motor proteins	Movement	Actin and myosin in muscle contraction; motor proteins in cilia and flagella.
Structural proteins	Support	Keratin in hair, feathers, and skin; silk fibers in insects and spiders; collagen and elastin in connective tissues.

Protein Structure: Four Levels of Organization

The function of a protein is directly related to its structure, which is organized into four hierarchical levels:

1. Primary Structure: The unique sequence of amino acids in a polypeptide chain. This sequence is determined by genetic information.

2. Secondary Structure: Local folding of the polypeptide chain into structures such as alpha helices and beta pleated sheets, stabilized by hydrogen bonds.

3. Tertiary Structure: The overall three-dimensional shape of a polypeptide, resulting from interactions between side chains (R groups) of amino acids. These interactions include hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and sometimes covalent disulfide bridges.

4. Quaternary Structure: The association of two or more polypeptide chains (subunits) into a functional protein. Examples include collagen (three polypeptides coiled together) and hemoglobin (four subunits: two alpha and two beta).

Protein Denaturation and Folding

• Denaturation is the loss of a protein's native structure due to changes in pH, salt concentration, temperature, or other environmental factors. Denatured proteins are usually biologically inactive.

• Protein folding is a complex process, and misfolding can lead to diseases such as Alzheimer's, Parkinson's, and mad cow disease.

• Techniques such as X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy are used to determine protein structures.

Key Terms

• Amino acid: The monomer unit of proteins, containing an amino group, a carboxyl group, a hydrogen atom, and a variable R group attached to a central carbon.

• Peptide bond: The covalent bond formed between amino acids during protein synthesis.

• Polypeptide: A polymer of amino acids linked by peptide bonds.

Example: Hemoglobin Structure

• Hemoglobin is a globular protein with quaternary structure, consisting of four polypeptide subunits (two alpha and two beta chains).

• It functions in oxygen transport in vertebrate blood.

Summary Table: Types of Protein Structures

Structure Level	Description	Stabilizing Forces
Primary	Sequence of amino acids	Peptide bonds
Secondary	Alpha helix, beta pleated sheet	Hydrogen bonds
Tertiary	Three-dimensional folding	Hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges
Quaternary	Association of multiple polypeptides	Same as tertiary (between subunits)

Additional info: The notes are based on Campbell Biology, Chapter 5, and are suitable for college-level General Biology students. The content covers the essential aspects of protein structure and function, including examples and key terminology.