What Are Proteins?

Introduction to Proteins and Polypeptides

Hemoglobin is a protein consisting of four polypeptide subunits.
Hemoglobin is a protein consisting of four polypeptide subunits. LAGUNA DESIGN/SCIENCE PHOTO LIBRARY, Getty Images

Proteins are a large class of biological molecules consisting of chains of amino acids called polypeptides. A single polypeptide can make a protein, although many proteins consists of multiple polypeptide subunits.

Protein Functions

Proteins perform important functions in organisms. In fact, this class of molecules is found in every cell and is essential for life. Here are some examples of functions performed by proteins:

  • form the scaffolding that maintains cell shape
  • catalyze metabolic reactions
  • essential in animal diets as a source of certain amino acids
  • transport molecules within cells and throughout organism
  • essential for replicating DNA
  • act in the immune response

Protein Structure

A protein may consist of a single polypeptide or multiple polypeptide subunits. Some proteins include non-peptide groups, called cofactors. A cofactor may be an organic group (e.g., coenzyme, prosthetic group) or an inorganic group (e.g., metal ion or iron-sulfur cluster).

Each polypeptide is a linear molecule made from a string of amino acids that are bonded into a chain by peptide bonds. An organism's DNA or RNA codes for the sequence of amino acids that form proteins. Building each amino acid chain from the genetic code is termed translation. Once a polypeptide has been translated, it usually undergoes additional chemical changes, called posttranslation modification.

There are four levels of protein structure:

  1. Primary structure refers to a protein's amino acid sequence.
  2. Secondary structure refers to repeating regions that are stabilized by hydrogen bonding. Examples of secondary structure include the alpha helix, beta sheet, and turns within a molecule.
  3. Tertiary structure is the three-dimensional shape of a single protein molecule. This structure determines the functionality of a protein.
  1. Quaternary structure is the structure formed by multiple protein subunits to form a protein complex. All proteins have a primary, secondary, and tertiary structure. Quaternary structure only comes into play when the molecule contains subunits. For example, hemoglobin has a quaternary structure.

Protein Examples

Proteins are common in everyday life. Most of the structure of organs and tissues consists of proteins. Here are some examples:

  • Keratin
  • Actin
  • Myosin
  • Hemoglobin
  • RuBisCO
  • Collagen
  • Elastin
  • Albumin
  • Fibrin
  • Insulin
  • Immunoglobulins (antibodies)
  • Catalase (and all enzymes)