A protein is a naturally occurring, extremely complex substance that is made up of amino acid residues joined by peptide bonds.

A protein may be classified based on its form and main functions.

Types of proteins based on their function

Fibrous proteins

Fibrous proteins form muscle fiber, tendons, connective tissue and bone. Examples of fibrous proteins are;

• Actin

• Collagen

• Coronin

• Elastin

• Fibronectin

• Keratin

• Myosin

Globular proteins

These are more water soluble than the other classes of proteins and they have many functions including transporting, catalyzing, and regulating.  Examples of globular proteins are;

• Albumins

• Alpha globulin

• Beta globulin

• Carboxypeptidase

• C-reactive protein

• Fibrin

• Hemoglobin

• Myoglobin

• Thrombin

Membrane proteins

They play many roles including relaying signals within cells, allowing cells to interact, and transporting molecules. Examples of membrane proteins include:

• Glucose transporter

• Histones

• Hydrolases

• P53

• Rhodopsin

• Transferases

Types of proteins based on their structure

Primary Structure

Linear/straight chain of amino acids that describes the unique order in which amino acids are linked together to form a protein. Proteins are constructed from a set of 20 amino acids. The three-dimensional shape of a protein is determined by its primary structure. The order of amino acids establishes a protein's structure and specific function.

Secondary Structure

Secondary Structure refers to the coiling or folding of a polypeptide chain that gives the protein its 3-D shape. It is a regularly repeating local structures foldings. There are two types of secondary structures observed in proteins. One type is the alpha (α) helix structure. This structure resembles a coiled spring and is secured by hydrogen bonding in the polypeptide chain. The second type of secondary structure in proteins is the beta (β) pleated sheet. This structure appears to be folded or pleated and is held together by hydrogen bonding between polypeptide units of the folded chain that lie adjacent to one another. Examples of a secondary structure are α-helix, β-sheet, and Turns and Loops

Tertiary Structure

Tertiary Structure refers to the comprehensive 3-D structure of the polypeptide chain of a protein. There are many types of bonds and forces that hold a protein in its tertiary structure.

• Hydrophobic interactions

• Hydrogen bonding

• ionic bonding

• covalent bonding

• van der Waals forces

• disulfide bonds

• salt bridges

Quaternary Structure

Quaternary Structure refers to the structure of a protein macromolecule formed by interactions between multiple polypeptide chains. Each polypeptide chain is known as a subunit. Proteins with quaternary structure may consist of more than one of the same type of protein subunit. They may also be composed of different subunits. Hemoglobin is an example of a protein with quaternary structure.

Protein classification based on chemical composition

Simple proteins

These are also known as Homoproteins, made up of only amino acids. Examples are plasma albumin, collagen, and keratin.

Conjugated proteins

They are also called Heteroproteins, contain a non-protein portion in their structure. Examples are glycoproteins, chromoproteins, and phosphoproteins.

Protein classification based on biological functions

From the functional point of view, proteins may be divided into many groups.

Enzymes
In living organisms, almost all reactions are catalyzed by specific proteins called enzymes. They have a high catalytic power, increasing the rate of the reaction.

Transport proteins
Many small organic and inorganic molecules are transported in the bloodstream and extracellular fluids, across the cell membranes, and inside the cells from one compartment to another, by specific proteins.

Storage proteins

Ferritin, that stores iron intracellularly in a non-toxic form.
Milk caseins, that reserves amino acids for the milk. Egg yolk phosvitin, that contains high amounts of phosphorus. Prolamins and glutelins are the storage proteins of cereals.

Mechanical support
Proteins have a pivotal role in the stabilization of many structures. Examples are α-keratins, collagen and elastin.

Hormones
They are regulatory molecules involved in the control of many cellular functions, from metabolism to reproduction. Examples are insulin, glucagon, and thyroid-stimulating hormone (TSH).

Protection against harmful agents
Examples are antibodies or immunoglobulins.