Protein and their Classification
Proteins
The term protein was derived from the greek word proteios which means primary or principal, was first of all used by Mulder in 1839 at the suggestion of Berzelius. However, proteins are now defined as complex nitrogeneous substances which are found in the protoplasm of all animal and plant cells. Actually, the proteins are biopolymers containing large number of amino acids joined to each other by peptide bonds.
All proteins contain C, H, O and N, the presence of N distinguishing them from carbohydrates and fats. On an average proteins contain 16% nitrogen. Some proteins also have S in addition, and in a few proteins P and other elements may be present. The molecular weight of proteins varies from about 12,000 daltons (bovine insulin) to several million. Cells contain a very large number of proteins. The number may vary from 1000-2000 in the simplest bacteria to as many as 100,000 different proteins in human cells.
| Proteins and Their Proportions | ||
|---|---|---|
| Proteins | Proportion | |
| Carbon | 50 – 55% | |
| Oxygen | 19 – 24% | |
| Hydrogen | 6 – 7.3% | |
| Nitrogen | 13 – 19% | |
| Sulfur | 0 – 4% | |
Classification of Proteins
Classification of Proteins Based on Solubility
According to this, proteins may be divided into two groups: Fibrous proteins and Globular proteins.
Fibrous proteins:
Fibrous proteins are mostly the linear condensation products of neutral amino acids. Since, these are formed by the condensation of monoaminocarboxylic acids there is a little or no branching or cross linking. The long linear protein chains are held together by intermolecular hydrogen bonds. Fibrous proteins are generally insoluble in water but soluble in strong acids abd bases. These are highly resistant to digestion by proteolytic enzymes. Examples of fibrous proteins are proteins of silk, wool, skin, hair, horn, nails, hoofs, quills, connective tissue and bone.
Globular proteins:
Unlike fibrous proteins, the gobular proteins are more highly branched and cross linked condensation products of basic or acids amino acids. Since, the globular proteins have diaminocarboxylic acids there are good chances for branching and cross linking by the usual peptide linkage involving the second amino group of one amino acid and the second carboxyl group of the second amino acid. The globular protein molecules may also be folded to three dimentional shape, the peptide chain is stabilized by intramolecular hydrogen bonds. Globular proteins are generally soluble in water, acids, bases and salts. Example of globular proteins are casein of milk an dglobin of haemoglobin.
| Differences Between Fibrous Proteins and Globular Proteins | ||
|---|---|---|
| Feature | Fibrous Proteins | Globular Proteins |
| Shape | Long and Narrow | Compact and Spherical |
| Solubility | Insoluble in water | Soluble in water |
| Function | Primarily structural roles | Diverse functions (enzymes, transport, etc.) |
| Amino Acid Sequence | Highly repetitive sequence | Irregular sequence |
| Resilience | Less sensitive to temperature and pH | More sensitive to temperature and pH |
| Example | Collagen, keratin, elastin | Hemoglobin, enzymes, antibodies |
Classification of protens based on Complexity of Structure
Proteins on the basis of increasing complexity in their structure are of three types
- Simple Proteins
- Conjugated Proteins
- Derived Proteins
Simple Proteins
Simple proteins are those which yield only α-amino acids on hydrolysis. They are further classified into following classes on the basis of the decreasing solubility.
Albumins: These are soluble in water, acids and alkalies. These are coagulated by heat and precipitated by saturated salt solutions like ammonium sulphate. These are usually low or deficient in glycine.
Examples of albumins are lactalbumin, serum albumin, egg albumin and lactalbumin.
Globulins: These are soluble in dilute salt solutions of the strong acids and bases.
These are insoluble in pure water or in moderately concentrated salt solutions. These are coagulated by heat. These are precipitated by half saturated their solutions with ammonium sulphate. Globulins generally contain glycine.
Examples of globulins are serum globulin, tissue globulin and vegetable globulin.
Glutelins: These are soluble in dilute acids and alkalis. These are insoluble in neutral salts. These are coagulated by heat.
Glutelins are comparatively rich in arginine, proline and glutamic acid.
Examples of glutelins are glutenin from wheat and oyrzenin from rice.
Prolamins: These are soluble in 70-90 per cent ethanol. These are insoluble in absolute alcohol, water and other neutral solvents. Prolamins contain large amounts of proline but are defficient in lysine.
Examples of prolamins are zein from maize, gliadin from wheat and hordein from barley.
Albuminoids (Scleroproteins): These are insoluble in all neutral solvents and in dilute acids and alkalis. These are the proteins of supportive tissues.
Examples of albuminoids are keratin from hair, hoof, etc. and fibroin from silk. These are attacked by enzymes.
Albuminoids are further subdivided into two types:
Collagens: More than half the total protein in the mammalian body is collagen. When collagens are boiled with water, gelatin, a water soluble protein, is obtained. The collagens appear to be unique in their high content of hydroxyproline and in containing hydroxylysine. They are poor in sulphur, since cysteine and cystine are absent and contain no tryptophan. Collagens are found in skin, tendons and bones. These are attacked by pepsin or trypsin.
Elastins: These are present in tendon, arteries and other elastic tissues. Although similar to collagens in many respects, they cannot be converted into gelatins and are attacked slowly by trypsin.
Basic proteins: These are strongly basic and are further divided into two subclasses.
Histones: These are basic proteins which are soluble in water or dilute acids but are insoluble in dilute ammonia. These are not coagulated by heat. Histones are rich in basic amino acids like histidine and arginine but deficient in tryptophan and contain little cystine or methionine. Histones are readily hydrolysed by pepsin and trypsin. Histones can be extracted in large amounts from certain glandular tissues, such as thymus and pancreas. Most histones are combined with nuclei acids, hemoglobins, etc.
Protamines: These are more basic than histones. These have a simpler structUl'e of relatively low molecular weight. These are soluble in water or in NH 0H. These are not coagulated by heat. They contain no sulphur but have a high nitrogen content (2530 per cent) due to the presence of large quantities of arginine. Tyrosine and tryptophan are absent. Protamines are h:ydrolysed by enzymes like trypsin, papain but not by pepsin. Typical protamines are salmine from salmon sperm, clupeine from herring and sturine from sturgeon. Protamines are principally found in egg cells.
Conjugated Proteins:
These are proteins which contain some non-protein substance called prosthetic group. This group may be separated from the protein part by carrying out hydrolysis very carefully. Sub-members of conjugated proteins are:
Nucleoproteins: In these proteins, the prosthetic group is a nucleic acid. Examples of nucleoproteins are nuclein, nucleo histone from nuclei-rich material (e.g., gladular tissue).
Chromoproteins: In these proteins, the prosthetic group is a chromophoric group called coloured prosthetic group. Examples of chromoproteins are hemoglobin, hemocyanin, cytochrome, flavoproteins.
Glycoproteins: These are the proteins having carbohydrate phosthetic groups and these on hydrolysis yield amino sugars (hexoseamines). These are also known as mucoproteins.
Examples of glucoproteins are egg albumin, some serum albumins and certain serum globulins.
Phosphoproteins: In these proteins, the prosthetic group possesses phosphoric acid in some form other than in the nucleic acids or in the lipoproteins.
Example of phosphoproteins is casein.
Lipoproteins: These are water soluble proteins in which the prosthetic groups are such as phospholipid and cholesterol.
Examples are several lipoproteins of serum.
Metalloproteins: These are conjugated proteins that contain a metal which is an integral part of the structure. Metals found in metalloproteins are generally iron, magnesium, copper and m'anganese. Examples of metalloproteins are hemoglobin and chlorophyll.
Derived Proteins:
When proteins are hydrolysed by acids, alkalis or enzymes, the degradation products obtained from them are called derived proteins. The derived proteins are further classified on the basis of progressive cleavage as proteins, primary proteoses, secondary proteoses, peptones, polypeptides, simple peptides and amino acids.
Proteins → Denatured proteins
↓
Primary proteoses → Secondary proteoses.
(metaproteins)
Peptones → Polypeptides → Simple peptides → Amino acids.
Denatured proteins are insoluble proteins which are obtained by the action of heat on proteins. Primary proteoses (also called metaproteins) are insoluble in water or dilute salt solution. However, they are soluble in acids or alkalis. These get precipitated by half-saturation with
ammonium sulphate. Secondary proteoses are soluble in water. They are precipitated by saturation with ammonium sulphate and are not coagulated by heat.
Peptones, polypeptides and simple peptides are soluble in water. They are all coagulated by heat. However, they are not precipitated by saturation with ammonium sulphate.
Source: Biochemistry by C.B. POWAR and G.R. CHATWAL
