Proteins are classified in several ways. Explore the classification of proteins according to their function, chemical nature and solubility properties and nutritional properties.
What are proteins in biochemistry?
Proteins are nitrogen containing organic macromolecules widely present in animals and plants. These are the polymers of L-α-amino acids.
They occur in every part of the cell. 50% of the cellular dry weight is protein. Proteins form the fundamental structure and function of life.
For more details about proteins read What are proteins?
What are the classifications of proteins?
Proteins are classified into three major types those are based on their function, chemical nature and nutritional importance.
A) Functional classification of proteins
Based on the function’s proteins are classified into the following groups.
| Protein Type | Examples |
|---|---|
| Structural Proteins | Keratin (hair and nails), Pepsin, Collagen (bone). |
| Enzymes/Catalytic Proteins | Hexokinase, Pepsin. |
| Transport Proteins | Hemoglobin, Serum Albumin. |
| Hormonal Proteins | Insulin, Growth Hormone. |
| Contractile Proteins | Actin, Myosin. |
| Storage Proteins | Ovalbumin, Glutelin. |
| Genetic Proteins | Nucleoproteins. |
| Defense Proteins | Snake Venoms, Immunoglobulins. |
| Receptor Proteins | Hormone Receptor Proteins, Virus Receptor Proteins |
B) Based on chemical nature and solubility
It is based on the amino acid composition, structure, shape and solubility properties. Proteins are broadly classified into 3 major groups. This is a popular and compressive classification of proteins.
- Simple proteins.
- Conjugated proteins.
- Derived proteins.
Simple proteins
They are composed of only amino acid residues. These are also known as homoproteins. Simple proteins are further classified in 2 types.
- Globular proteins.
- Fibrous proteins.
| Simple protein types | Definition | Examples |
| A) Globular proteins | ||
| 1) Albumins | Soluble in water and dilute salt solutions and coagulated by heat. | Serum albumin, ovalbumin (egg), lactalbumin (milk). |
| 2) Globulins | Soluble in neutral and dilute salt solutions. | Serum globulins, vitelline (egg yolk). |
| 3) Glutelins | Soluble in dilute acids and Alkalies, mostly found in plants. | Glutelin (wheat), oryzenin (rice). |
| 4) Prolamines | Soluble in 70% alcohol. | Gliadin (wheat), zein (maize) |
| 5) Histones | Strongly basic, soluble in water and dilute acids, insoluble in dilute ammonium hydroxide. | Thymus histones |
| 6) Globins | Considered with histones, not basic, not precipitated by NH4OH. | Associated with hemoglobin. |
| 7) Protamines | Strongly basic, smaller than histones, soluble in NH4OH, found with nucleic acids. | Sperm proteins. |
| 8) Lectins | Carbohydrate-binding proteins, involved in cell-protein interaction. | Concanavalin A, agglutinin. |
| B) Fibrous Proteins | | |
| 1) Collagens | Connective tissue proteins lacking tryptophan, yield gelatin on boiling. | Component of connective tissues. |
| 2) Elastins | Found in elastic tissues like tendons and arteries. | Component of elastic tissues. |
| 3) Keratins | Present in exoskeletal structures (e.g., hair, nails, horns). | Human hair keratin. |
Conjugated proteins
These are a type of protein that contains a non-amino acid component, known as a prosthetic group, in addition to amino acid chains.
| Conjugated protein types | Definition | Examples |
| 1) Nucleoproteins | Nucleic acid (DNA or RNA) is the prosthetic group. | Nucleohistones, nucleoprotamines. |
| 2) Glycoproteins | Carbohydrate is the prosthetic group (<4% protein for glycoproteins, >4% for mucoproteins). | Mucin (saliva), ovomucoid (egg white). |
| 3) Lipoproteins | Protein found in combination with lipids. | Serum lipoproteins. |
| 4)Phosphoproteins | Phosphoric acid is the prosthetic group. | Casein (milk), vitelline (egg yolk). |
| 5) Chromoproteins | Prosthetic group is colored in nature. | Hemoglobins, cytochromes. |
| 6) Metalloproteins | Contain metal ions (e.g., Fe, Co, Zn, Cu, Mg). | Ceruloplasmin (Cu), carbonic anhydrase (Zn). |
Derived proteins
These are substances formed through the alteration or breakdown of proteins. They can be categorized into two main types: primary derived proteins and secondary derived proteins.
| 1) Primary derived | | |
| Coagulated proteins | Denatured proteins produced by agents like heat, acids, alkalis. | Cooked proteins, coagulated albumin. |
| Proteans | Earliest products of protein hydrolysis, insoluble in water. | Fibrin formed from fibrinogen. |
| Metaproteins | Second-stage products of protein hydrolysis, obtained with slightly stronger acids and alkalis. | Acid and alkali metaproteins. |
| 2) Secondary derived | These are the progressive hydrolytic products of protein hydrolysis. | Polypeptides, peptones. |
C) Nutritional classification of proteins
The nutritive value of proteins is determined by the composition of essential amino acids. From the nutritional point of view, proteins are classified into 3 categories.
| Sub-types | Definition | Examples |
|---|---|---|
| 1) Complete Proteins | These are containing all ten essential amino acids. | Egg albumin, milk casein. |
| 2) Partially incomplete proteins | Proteins that partially lack one or more essential amino acids. | Wheat proteins (limiting Lys, Thr), Rice proteins (limiting Lys, Thr). |
| 3) Incomplete proteins | Proteins lacking one or more essential amino acids, not promoting significant growth. | Gelatin (lacks Trp), Zein (lacks Trp, Lys). |
Also read What are proteins?
FAQ
What are Globular proteins with examples?
These are water-soluble proteins with a spherical shape, typically involved in biological processes. For examples, serum albumin, ovalbumin, oryzenin, lactalbumin etc.
What are fibrous proteins with examples?
These are fiber like in shape, insoluble in water and resistant to digestion. For examples, keratins, collagens, elastins etc.
What are derived proteins with examples?
These are the proteins which produce from the hydrolysis of simple protein or due to breakdown of
peptide bonds. For example, cooked proteins, coagulated albumin
What are conjugated proteins?
These proteins have non-protein components (prosthetic groups) in their structure. For examples, nucleohistones, nucleoprotamines, glycoproteins, lipoproteins etc.
What is difference between globular protein and fibrous protein?
Globular proteins have a rounded structure and water-soluble. In another side, fibrous proteins have thread-like structures and insoluble in water.
What is protean?
This is the earliest products of protein hydrolysis by enzymes, dilute acids, Alkalies etc. which are insoluble in water. For example, fibrin formed from fibrinogen.
What is coagulated protein?
These are the denatured proteins produced by agents such as heat, acids, Alkalies etc. For example, cooked proteins, coagulated albumin (egg white).
What is main difference between primary and secondary derived protein?
Primary derived protein produces due to hydrolysis of simple proteins into peptides and amino acids. The secondary derived protein is produced due to degrading (breakdown of peptide bonds) of proteins.
What do mean by metaproteins?
Proteins which contain metal ions such as Fe, Co, Zn, Cu, Mg etc., e.g. ceruloplasmin (Cu), carbonic anhydrase (Zn).
What is difference between derived protein and conjugated protein?
Derived proteins produce from the hydrolysis of simple proteins and breakdown into peptides and amino acids, does not have any non-proteins components. But in Conjugated proteins, these have non-protein components (prosthetic groups) in their composition.