Components of Molecules
The proteins are of 3 types according to their constitutents: simple, conjugated and derived.
1) Simple Proteins. These are formed of amino acids only. They include albumens of egg, serum and milk; globulins of egg, serum and milk; collagen of connective tissues ; keratin of hair, feathers, claws, nails, hoofs, horns and scales ; gluten of wheat and rice; histones, protamines and enzymes.
2) Conjugated Proteins. These are composed of a protein molecule joined to a nonprotein group called the prosthetic group. These include -
a) Nucleoproteins. The prosthetic group is nucleic acid. Deoxyribonucleoproteins (DNPs)of chromosomes and ribonucleoproteins (RNPs) of ribosomes are examples.
b) Chromoproteins. The prosthetic group is some pigment. Common examples are haemoglobin, haemocyanin, cytochromes, visual purple, and flavoprotein.
c) Phosphoproteins. The prosthetic group is phosphoric acid. Examples : casein of milk, vitellin of egg yolk.
d) Glycoproteins. The prosthetic group is a carbohydrate. The glycoproteins occur in blood plasma, egg albumen, saliva, mucus and blood group substances.
e) Lipoproteins. The prosdietic group is mainly phospholipid. The lipoproteins occur in membranes, milk (casein), blood serum and egg yolk (vitellin).
f) Metalloproteins. The prosthetic group is some metal ion. The enzyme carbonic anhydrase is an example. It contains zinc ions besides amino acids in its molecule. Ferritin is another example. It is iron and protein complex.
3) Derived Proteins. These are formed by partial breakdown of natural proteins in digestion or otherwise. They include proteoses, peptones and short peptides formed in protein digestion; and fibrin formed from fibrinogen in blood clotting. Peptones are formed in partial hydrolysis of proteins, are soluble in water and diffusible. Proteoses are intermediate products of protein hydrolysis between protein and peptones.
Nature of Molecules
The proteins are of 2 types according to the nature of their molecules: acidic and basic.
(i) Acidic Proteins. These have acidic amino acids (aspartic acid, glutamic acid, exist as anions and behave as acids. Examples : most blood proteins.
(if) Basic Proteins. These are rich in basic amino acids (lysine, arginine), exist as cations and behave as bases. Example : histones.
Protein Specificity. Every cell has hundreds of different proteins. Each kind of cell has some proteins unique to itself, and these proteins make it distinct from all other types of cells. Every specie likewise has a set of proteins found in no other species. Similar species share many of their proteins, whereas dissimilar species share fewer. Study of proteins, thus, shows the evolutionary relationship among species.
The principle of protein specificity has practical application in blood transfusion, tissue grafting and organ transplants which succeed only in animals that have similar proteins.
Functions
The proteins have a number of functions.
1) Structural Components. Many proteins primarily serve as building material of cells and tissues. They are called the structural proteins. They are essential for growth, repair and reproduction. They are components of -
(a) Cell organelles such as membranes, ribosomes, mitochondria, plastids, microfilaments, micro tubules and chromosomes. Walls of pollen grains contain proteins.
(b) Tissues, e.g., matrix of connective tissues contains collagen and elastin protein fibres, matrix of cartilage contains the protein chondrin, and the matrix of bone has the protein ossein.
(c) Protective coverings, such as horny layer of skin, and exoskeletal structures, scales, feathers, hairs, wool, nails, claws, hoofs and horns, are formed of the protein keratin sulphate.
2) Metabolic regulation. Certain proteins form enzymes, some coenzymes, and many hormones (insulin, parahormone) and regulate metabolism. They are called functional proteins.
3) Fuel. The proteins may act as fuel to supply energy when glucose or fats are in short supply.
4) Transport. The proteins also serve as physical carriers of other functionally important smaller molecules. A protein “globin”, for example, carries the pigment “heme”, the two forming haemoglobin. The heme transports oxygen in the body. Myoglobin in muscle cells stores oxygen. The a-globulin of blood transports thyroxine and bilirubin. The -globulin carries vitamins A, D and K and cholesterol and ions in the blood. Serum albumen transports fatty acids and lipids in the blood. In plants, P-protein present in the sieve tubes in vibratile. It is believed to help in the transport of organic compounds through phloem. Protein carrier molecules (permeases) in cell membranes actively transport molecules across the membranes.
5) Visual Pigments. Rhodopsin of rod cells and iodopsin of cone cells of retina are proteins. They enable animals to see.
6) Movements. Myosin and actin make muscle fibres contractile to bring about movements. These are called contractile proteins.
7) Defence. Some proteins act as antibodies which defend the organism against foreign materials entering the body.
8) Raw Materials. Proteins may break down into amino acids to provide materials to synthesize new proteins or other compounds (carbohydrates and fats).
9) Biological Buffers. Proteins also assist in maintaining a balance of acidity and alkalinity in the cell by combining with excess acids and bases. They are, thus, biological "buffers" against acids and bases that might harm the cell.
10) Receptor Molecules. The receptor molecules of the cells are proteins. The hormone molecules bind to these receptors to produce their effect in the cells.
11) Toxins. Many toxins of animals are proteins, e.g., snake venom. The ricin of castor oil plant, Ricinus communis, is a highly toxic protein. If eaten, acts as a violent irritant and may be fatal. Bacterial toxins are also proteins.
12) Support. Collagen and keratin provide rigidity and support to the tissues, organs and body.
13) Osmotic Pressure. Protein contents of a cell influence the osmotic pressure in it.
14) Blood Clotting. The proteins thrombin and fibrinogen help in blood clotting to check bleeding from injuries.
15) Regulation of Gene Action. The repressors that regulate gene action are proteins in nature.
17) Pollination. Pollen grains have specific proteins in the wall to help them in pollinating the stigmas of the flowers of specific plants only.
18) Storage Proteins. These include oval-bumen of egg white, glutens of grains, casein of milk. Ferritin stores iron. It also regulates iron transport from the intestinal lumen to the plasma.
19) Perception of Pain and Pleasure. Small peptides, called enkephalins, having only 5 amino acids each, bind to specific receptor sites in the brain, and influence the perception of pain and pleasure.
20) Blood Transfusion. Compatibility of antigens on the RBCs with antibodies in the plasma determines the type of blood group fit for transfusion. Both antigens and antibodies are proteins.
21) Sweetest Substance. Monellin, a pr derived from an African berry is 2000 times sweeter than sucrose. Being nontoxic and noncaloric, it may prove suitable for the diabetics.
22) Mucus. Mucus secreted by mucous glands protects the delicate cellular lining of alimentary canal from digestive enzymes and friction with food.