As the proteins possess, a very complex structure, their classification also poses a problem as many categorizations are possible based on structure, function etc.
The following is a generalized classification. The major groups are recognized in proteins based on their solubility and chemical properties.
I. Simple proteins
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II. Conjugated proteins and
III. Derived proteins.
I. Simple proteins:
Chemically these have only amino acids. No additional compounds are present in the molecule. Simple proteins are of the following kinds –
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1. Albumins:
These are soluble in water, dilute salts, acids and alkalies. Heat denatures the protein structure by coagulation. Some of the commonly occurring albumins are Legumelin (from legume seeds), Amylase (from barely), Leucosin (from cereals) etc.
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2. Globulins:
Less soluble in water than albumins but soluble in less concentrated salt solutions heat coagulation is seen in globulins also Eg. Legumin (pears), Tuberin (potato) etc.
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3. Glutelins:
Insoluble in water and salt solutions, but soluble in weak acids and alkalies glutelins are generally found in plants and are heat immune. Seed proteins found in cereals like Wheat (glutenin) and rice, oryzenin) are best examples of glutelins.
4. Prolamines:
These proteins are water insoluble, but dissolve in 70-80% alcohol. Prolamines have a high concentration of the amino acid proline and other amides. E.g. Gliadin (Wheat), Hordein (barley) and Zein (maize)
5. Scleroproteins:
These are fibrous proteins found as an integral part of the structural framework of plants. These proteins are insoluble in most of the solvents.
6. Histones:
Most of the nucleoproteins, hemoglobin and globins are histones. They are insoluble in water but insoluble in dilute ammonia. Histones have a high concentration of lysine and arginine.
7. Protamines:
These are soluble in water, dilute acids and ammonia and do not undergo heat coagulation. Basic amino acids are rich in protamines, while tryptophan and tyrosine are completely absent. E.g. Nucleoproteins of sperms in fish
II. Conjugated Proteins:
In these proteins, non amino acid components are associated with the protein molecule. The non amino components are called prosthetic groups. There are seven major categories of conjugated proteins.
1. Nucleoproteins:
Proteins are found in combination with nucleic acids. Hydrolysis can separate nucleic acids and simple proteins. Nucleoproteins are water soluble.
2. Glycoproteins:
Also called mucoids, these are conjugated with carbohydrates like macromolecular polysaccharides, sugars, sugar phosphate etc. Glycoproteins are found in cell membranes.
3. Chromoproteins:
These constitute the pigments and are colored; the colored appearance is due to the presence of metals such as Cu, Fe, Mg etc. Chlorophylls, flavor proteins, carotenoids, cytochromes etc are typical examples of chromoproteins.
4. Lipoproteins:
Proteins in conjugation with lipids are called lipoproteins. Lecithin, Cephalin etc are the lipids found in combination with proteins. Lipoproteins are an integral part of all cell membranes.
5. Metalloproteins:
Certain types of enzymes which require metals as activators belong to this group. Many of the respiratory enzymes belong to this group.
6. Phosphoproteins:
These are proteins with phosphoric acid as the prosthetic group. Phosphoproteins are insoluble in water, but soluble in alkalies E.g. milk and egg protein (casein and viteline).
7. Lecithoprotein:
Proteins with lecithin as the prosthetic group (E.g. white of egg) belong to this group.
III. Derived Proteins:
These are degradation products of proteins. Primary proteins on hydrolysis with acids or alkalies yield derived proteins.
(A) Primary derived proteins and
(B) Secondary derived proteins
A. Primary derived Proteins:
There are two categories of primary derived proteins. These are metaproteins and Coagulated proteins.
(i) Metaproteins:
These are insoluble in water but dissolve in dilute salt solutions, acids and alkalis. Hydrolysis of natural proteins on long treatment with alkalies or acids yield metaproteins
(ii) Coagulated proteins:
Treatment with heat or alcohol of natural proteins yields these proteins.
B. Secondary derived proteins:
Three types of secondary derived proteins have been identified. These are protoeses, peptones and peptides.
(i) Proteoses:
Prolonged hydrolysis of metaproteins yields proteoses (eg. Albuminoses from albumin). These are water soluble but immune to heat treatment.
(ii) Peptones:
Peptones are obtained by continued hydrolysis of proteoses. Like proteoses, these are also water soluble and heat insensitive.
(iii) Peptides:
Prolonged hydrolysis of natural proteins yields peptides. These are also water soluble and not coagulated by heat.