Classification of Proteins

Proteins are long chains of amino acids in a structured form. An amino acid is made up of carbon molecule(s), a hydrogen atom, the carboxyl group (COOH), an amino group (NH₂) and a radical. Protein molecules contain a combination of 20 essential amino acids. The properties and names of protein molecule depend on the number of amino acid molecules and the radical group that attaches to it.
Proteins are an integral part of the food pyramid, and play an important role in building of muscles. They also play other roles for the wellbeing of the human body.
There are four major ways of classifying proteins - according to their chemical composition, their biological function, their nutritional value or the shape of the protein molecule.

Classification of Proteins by Chemical Composition
i. Simple Proteins: Simple proteins are formed by the linking of amino acids via the polypeptide chains. A protein molecule with a single polypeptide chain is said to be monomeric. Proteins that contain two or more polypeptide chains are called as oligomeric proteins. Globulins, albumins and histones are examples of some simple proteins.
ii. Conjugated Proteins: In addition to the amino acids, these type of proteins also contain organic and inorganic radicals. A few examples of conjugated proteins include carbohydrates, lipoproteins and nucleoproteins.
iii. Derived Proteins: They are synthesized from simple proteins by the action of enzymes and chemical agents. They do not occur naturally. Peptones are derived proteins.

Classification of Proteins by Biological Function
i. Enzymic Proteins: They build and break down molecules. Enzymes expedite the growth and digestion within the cell. They are thus biological catalysts.
ii. Carrier (or Transport) Proteins: As the name suggests, the main function of these proteins is to move molecules and nutrients in and out of the cells. One of the best examples of transport proteins is hemoglobin.
iii. Structural Proteins: These proteins strengthen tissues and organs in the body. A prime example is collagen, a protein that strengthens bones, ligaments, cartilage and tendons.
iv. Storage and Nutrient Proteins: Their primary function is to provide nutrition to growing embryos and store ions. They store nutrients and molecules rich in energy.
v. Regulatory (or signaling) Proteins: Regulatory proteins allow communication between cells with the help of signals, receptors and relay proteins. Adrenalin and insulin are two examples of regulatory proteins.
vi. Defense Proteins: Their main job is to protect the body by fighting infection, heal tissue that is damaged and defend against other invaders. Antibodies are defense proteins.
vii. Sensory Proteins: Sensory proteins help us understand the environment around us. Proteins that help us detect light, sound, heat, touch, smell, etc. are sensory proteins. Olfactory receptors help us smell and opsins help us see. These are examples of sensory proteins.
viii. Contractile Proteins: They are present in our muscles, and help us move. The most common examples of contractile proteins are myosin and actin.

Classification of Proteins by Nutritional Value
i. Complete Proteins: A protein that contains all the essential amino acids is called complete proteins. The best source of complete proteins is animal meat. Chicken is a very good source of complete proteins.
ii. Lower / Incomplete Proteins: These proteins have inadequate amount of essential amino acids. They generally come from plants. Of course, it should not be inferred that plant proteins are inferior to animal proteins; it is just that their structure is different.

Classification of Proteins based on Shape of Protein Molecule
Globular (or spheroproteins) Proteins: Globular proteins have an ovoid or spherical shape. They are soluble in blood and other aqueous media present in the cells. They also diffuse readily, and do not aggregate. Salient examples of globular proteins are enzymes, blood proteins and nutrient proteins. Globular proteins are also called as corpuscular proteins.

Fibrous (or scleroproteins) Proteins: They have elongated shape like ribbons or fibers. Fibrous proteins are not soluble in water and other common solvents. Two important properties of fibrous proteins are their ability to stretch and their ability to creep. The best examples of these kinds of proteins are keratin and collagen.