Classification of Proteins
Because of structural complexities, the classification of proteins cannot be a perfect one. However, based on their solubility and chemical properties, these have been classified into three major groups —
I. Simple proteins
II. Conjugated proteins
III. Derived proteins
Simple proteins contain only ordinary α-amino acids. These have been further divided into seven major groups based on their solubility.
Albumins are water-soluble proteins but are also soluble in dilute salts, acids, and alkalis. These coagulate by heat. Common albumins are legumelin from legume seeds, β-amylose from barley, leucosin from cereals and albumins from soybeans.
These are sparingly soluble in water but dissolve in salt solution of moderate concentration. Globulins are less soluble in concentrated salt solutions than albumins. On heating, these coagulate. Among examples legumin of peas and tuberin of potato are common.
Glutelins are insoluble in water and salt solutions but are soluble in weak acids or alkalies. These do not heat sensitive and are confined to plants. Common examples are seed proteins particularly of cereals such as glutenin in wheat and oryzalin in rice.
These are insoluble in water but frequently dissolve in 70-80 per cent ethanol (ethyl alcohol). These have relatively few polar groups and are usually high in proline and amide groups. Examples are gliadin (C635 H1068 N196 O211 S5 from wheat, hordein from barley and zein (C736 H1161 N174 O208 S3) from maize.
Scleroproteins are insoluble in most of the common solvents. Examples are structural and fibrous proteins. (Absent in plants).
These are soluble in water but insoluble in dilute ammonia. Being high in lysine and arginine these are quite basic and are found biologically associated with certain acidic structures. Globin, haemoglobin, and protein components of nucleoproteins are common examples of histones.
These are small basic proteins soluble in water, dilute acids and ammonia and are not coagulated by heat. Like histones, protamines are rich in basic amino acids while tryptophan and tyrosine are not found. This group is known only in combination with nucleoprotein of sperms of fishes.
Proteins that contain non-amino acid components in addition to the amino acids are termed conjugated proteins. These additional non-amino acid components are referred to as prosthetic groups. The conjugated proteins are classified into seven major types based on the nature of prosthetic groups.
These are found in the nucleus conjugated with the nucleic acid. On hydrolysis, nucleoproteins give rise to simple proteins and nucleic acid. These are weakly acidic and soluble in water.
Glycoproteins or mucoids —
These are conjugated with carbohydrates usually macromolecular polysaccharides containing acetylglucosamine, sugars, sugar acids and sulphate or phosphate esters. They tend to have unusual properties based on the presence of many polar groups Gonadotropic hormone is an example, though not much is known about it. The cell membranes are also thought to possess some amount of glycoproteins.
They are conjugates of lipids and proteins. The prosthetic groups of them are lipids such as lecithin and cephalin and are commonly found in cell membranes forming lipoprotein complex Nuclear, mitochondrial and chloroplast membranes also possess them.
Chromoproteins possess pigment groups as their prosthetic groups and are coloured. The colour may be due to metals such as Cu and Va or due to metals with organic groups such as in Fe and Mg-porphyrins. The chromoproteins include a diverse group of compounds such as flavoproteins, carotenoid proteins, chlorophyll proteins and haemoglobins.
These proteins represent the group of enzymes that require metals as activators. Many enzymes involved in respiration represent this group of proteins.
Lecithoproteins contain fats such as lecithin as the prosthetic group, eg, white of an egg.
These are soluble in alkalis and insoluble in water and contain phosphoric acid as a prosthetic group in them. The common examples are casein in milk and vitelline in eggs.
Derived proteins are the degradation products obtained by hydrolysis of natural proteins with acids, alkalis, and enzymes. Two categories of such proteins are recognised.
1) Primary derived protein
2) Secondary derived proteins
1) Primary derived proteins –
These have been further grouped into two :
Metaproteins — Meta Proteins are insoluble in water and dilute salt solution but soluble in acids and alkalis. These are produced by hydrolysis of natural proteins, by alkalis or prolonged treatment with acids
Coagulated protein — These are insoluble in water and are produced out of the action of heat or alcohol. These are coagulated with heat, eg coagulated egg white
2) Secondary derived proteins –
Three categories of these proteins are recognised
Proteoses — Proteoses are soluble in water but are not coagulated by heat. Commonly these are produced by prolonged hydrolysis of metaproteins e.g. albuminose from albumin.
Peptones — Like proteoses, these are soluble in water and are not coagulated by heat. Further hydrolysis of proteoses by the action of HCL, H2SO4, or certain enzymes yields peptones. These give biuret tests.
Peptides — These are soluble in water and are not coagulated by heat. Peptides are produced by extensive hydrolysis with HCl or H2SO4 of natural proteins. They do not give biuret tests.