Proteins are made up of carbon, hydrogen, nitrogen, oxygen and usually sulphur. Proteins are built up of a larger number of amino acids molecules linked by peptides bonds.
Beside classifying of protein on the basis of soluble and insoluble proteins have been further classified based on the following –
- Classification Based On Chemical Nature.
- Classification Based On Chemical Properties.
- Classification Based On Amino Acid Composition.
In this article i will discuss only The classification based on chemical nature ( Based on solubility and physical properties )
Proteins Classification Based On Chemical Nature
- Simple Proteins
- Conjugated Proteins, And
- Derived Proteins
Simple proteins are those which contain only amino acids or their derivatives and no prosthetic group. They yield only amino acids or their derivatives on hydrolysis. Let us see which are these and where they are found.
- Albumins: Proteins such as egg albumin and serum albumin are soluble in water and coagulable by heat.
- Globulins: These proteins are insoluble in pure water, but soluble in neutral salt solutions. For example, serum globulin, tuberin (potato), arachin and conarachin (peanuts).
- Glutelins: These are insoluble in all neutral solvents but soluble in very dilute acids and alkalis. e.g. glutenin of wheat.
- Prolamins: Proteins soluble in 70-80% alcohol. e.g. gliadin and zein
- Fibrous proteins: These proteins are characteristic of the skeletal structures of animals and also of the external protective tissues, such as the skin, hair, .etc. e.g., collagen, elastin and keratin.
- Histones: Soluble in water and insoluble in very dilute ammonia. On hydrolysis, they yield several amino acids, among which the basic ones predominate. The important proteins of this group are the thymus, histones and the globin of haemoglobin.
Conjugated proteins contain some non-protein substances. Most proteins occur in cells in combination with prosthetic groups and hence are important for the nutritionist. These include:
- Glycoproteins: Most of the naturally occurring conjugated proteins are glycoproteins. Sugar molecules are covalently bound to them, especially those secreted from the cell. They range in size from a molecular weight of 15,000 to more than one million. The carbohydrate component varies from 1 to 85%. Glycoproteins with more than 80% of their molecules as carbohydrates are called ‘proteoglycans.’
- Lipoproteins: These are the multicomponent complexes of lipids and proteins that fomz distinct molecular- aggregates. They contain polar and neutral lipids, cholesterol or cholesterol esters in addition to protein. The proteins and lipids are held together by non covalent bonds. Lipids are primarily hydrophobic and cannot be easily transported through an aqueous environment as blood. The lipoprotein combination renders the lipid molecule hydrophilic and is transported in the blood to tissues which can use or store the lipids.
- Nucleoproteins: Nucleoproteins are combinations of nucleic acids and simple proteins, which usually consists of a large rzunzber of basic amino acids. Nucleoproteins have very complex structures and numerous functional activities. All living cells contain nucleoproteins. Some cells, such as viruses, are composed of nucleoprotein.
- Other conjugated proteins: The phospho proteins and the metallo proteins are loose (as with phosphate carrying protein) or tight (as with the phosphate in casein or the iron in ferritin) associations of proteins with phosphate groups or such ions as zinc, copper and iron.
Derived proteins are the derivatives of the protein molecule, apparently formed through hydrolytic changes in the molecule. These are either primary or secondary protein derivatives. Let us get to know them.
Primary Protein Derivatives: These are the derivatives of the protein molecule formed by hydrolysis involving slight alterations. Examples include:
- Proteins: These are the insoluble products which result from the incipient action of very dilute acids or enzymes. e.g. casein (curdled milk), fibrin (coagulated fibrinogen).
- Metaproteins: Proteins resulting from the action of acids and alkalis whereby the molecule is sufficiently altered to form proteins soluble in weak acids and alkalis, but insoluble in neutral solvents.
- Coagulated Proteins: Insoluble proteins which result from the action of heat on protein solutions or the action of alcohol on the protein. e.g. cooked egg albumin or egg albumin precipitated by alcohol.
Secondary Protein Derivatives: These are the products of further hydrolytic i cleavage of the protein molecule, Examples include:
- Proteose: Soluble in water, not coagulable by heat, precipitated by saturating their solutions with ammonium.
- Peptones: Soluble in water, not coagulable by heat and not precipitated by saturating their solutions with ammonium sulphate. These represent a further stage of cleavage than the proteose.
- Peptides: These are the compounds containing two or more amino acids. An anhydride of two amino acids is called a ‘dipeptide’, one having three amino acids, a ‘tripeptide’ and containing several amino acids, a ‘polypeptide’. Peptides result from further hydrolytic cleavage of the peptones.