Proteins: Classification and Functions

This is not a function of protein.

It is an example of conjugated proteins.

Chromo-proteins  are pigmented proteins, e.g., haemoglobin (red), haemocyanin (blue) and chlorocruorin (green).

These are naturally occurring proteins which yield only amino acids or their derivatives on hydrolysis. Examples are albumins, globulins and scleroproteins.

Phospho-proteins  are the proteins linked to phosphate groups; hence are acidic, e.g., casein of milk; vitellin of egg-yolk.

Glycoproteins are proteins that contain oligo-saccharide chains (glycans) co-valently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a co-translational or post-translational modification. This process is known as glycosylation.

The conjugated proteins are simple globular proteins linked to non-protein substances called prothetic group, e.g., Glyco-proteins or Muco-proteins are proteins bound to carbohydrate groups. Glyco-proteins contain less carbohydrate (4%) when compared to mucoproteins,e.g., egg albumin and mucin of saliva.

As the name suggests, they are derivatives of proteins resulting from the action of enzymes, heat or chemical agents. Proteoses, peptones and peptides are examples for derived proteins. They are produced during the stepwise digestion of a protein.

Nucleo-proteins  are complexes of nucleic acid and proteins. Chromosomes of animal and plant cells are complexes of DNA and protein. Ribosomes, found in all cells, are complexes of RNA and proteins.

As the name suggests, they are derivatives of proteins resulting from the action of enzymes, heat or chemical agents. Proteoses, peptones and peptides are examples for derived proteins. They are produced during the stepwise digestion of a protein.

Lipo-proteins  have lipids bound to proteins. Lipo-proteins are important constituents of all biological membranes : plasma membrane, membranes of the endoplasmic reticulum, lysosome, Golgi complex, mitochondria, nuclear envelope and the chloroplasts. They also occur as lipovitellin, a storage product in egg yolk.

The conjugated proteins are simple globular proteins linked to non-protein substances called prosthetic groups. Glycoproteins or Mucoproteins are proteins bound to carbohydrate groups and Phospho-proteins. They are proteins linked to phosphate groups; hence are acidic. E.g., casein of milk and vitellin of egg-yolk.

All are blood proteins.

They are naturally occurring proteins which yield only amino acids or their derivatives on hydrolysis. Examples are albumins, globulins and sclero-proteins.

Majority of the proteins are the most variable and highly specialized enzymes. Acting as organic catalysts, they accelerate metabolic reactions. Virtually, every type of metabolic reaction is driven by a particular or specific type of enzyme. Hence, metabolic reactions of thousands of different kinds in every cell require enzymes of thousands of different kinds. In fact, the basis of the phenotypic expression of every hereditary characteristic is a specific type of metabolic reaction which  in turn, is driven by a specific type of enzyme. Synthesis of defective or abnormal enzymatic proteins, therefore, forms the basis of genetic diseases. About 1400 such diseases are known in case of human beings.

Majority of proteins are the most variable and highly specialized enzymes. Acting as organic catalysts, they accelerate metabolic reactions. Virtually, every type of metabolic reaction is driven by a particular or specific type of enzyme. In fact, the basis of the phenotypic expression of every hereditary characteristic is a specific type of metabolic reaction which  in turn, is driven by a specific type of enzyme. Synthesis of defective or abnormal enzymatic proteins, therefore, forms the basis of genetic diseases.

They are protective proteins which protect the body from ill effects of poisonous (toxic) substances and pathogenic invaders (virus, bacteria and other pathogens) by destroying the toxins and invaders. Important defence proteins are antibodies or immuno-globulins produced by lymphocytes in vertebrates. Interferon protects cells from the attack of viruses. Thrombin and fibrinogen form blood clots to prevent loss of blood and infection at sites of injuries.

They are protective proteins which protect the body from ill effects of poisonous (toxic) substances and pathogenic invaders (virus, bacteria and other pathogens) by destroying the toxins and invaders. Important defence proteins are antibodies or immuno-globulins produced by lymphocytes in vertebrates. Interferon protects cells from the attack of viruses. Thrombin and fibrinogen form blood clots to prevent loss of blood and infection at sites of injuries. Certain proteins, called exploitive proteins, are normally toxic, but they may play a defensive role under certain conditions.

They are protective proteins which protect the body from ill effects of poisonous (toxic) substances and pathogenic invaders (virus, bacteria and other pathogens) by destroying the toxins and invaders. Important defense proteins are antibodies or immuno-globulins produced by lymphocytes in vertebrates. Interferon protects cells from the attack of viruses. Thrombin and fibrinogen form blood clots to prevent loss of blood and infection at the sites of the injuries. Certain proteins, called exploitive proteins, are normally toxic, but these may play a defensive role under certain conditions. These include snake venom, bacterial toxins and toxic plant protein called ricin.

Many proteins are stored to serve as reservoirs of amino acids and nitrogen. For instance, seeds of many plants store nutrient protein required for germination and growth of seedlings. Zeins of corn, phaseolin of peas and glutellins of wheat and rice are common storage proteins. Ferritin protein of bacteria and plant and animal cells stores iron. Myoglobin of muscle cells acts as a reservoir of oxygen.

They are protective proteins which protect the body from ill effects of poisonous (toxic) substances and pathogenic invaders (virus, bacteria and other pathogens) by destroying the toxins and invaders. Important defence proteins are antibodies or immuno-globulins produced by lymphocytes in vertebrates. Interferon protects cells from the attack of viruses. Thrombin and fibrinogen form blood clots to prevent loss of blood and infection at sites of injuries. Certain proteins, called exploitive proteins, are normally toxic, but they may play a defensive role under certain conditions. These include snake venom, bacterial toxins and toxic plant protein called ricin.

These proteins transport specific substances from one part of body to the others. Most familiar is haemoglobin of red blood cells (erythrocytes or RBCs) of vertebrates. Serum albumin transports fatty acids in blood from fatty tissues to other tissues. Transferrin protein transports iron, and certain other proteins transport steroid hormones, drugs, toxins, etc. in blood. Transport proteins of plasma membrane transport materials across the membrane. For instance, glucose and amino acids enter into all cells through membrane transport proteins.

These proteins transport specific substances from one part of body to the others. Most familiar is haemoglobin of red blood cells (erythrocytes or RBCs) of vertebrates. It transports oxygen from lungs to all tissues of the body. Likewise, lipo-proteins transport lipids in blood from liver to fatty tissues (fat deposits) and other tissues. Serum albumin transports fatty acids in blood from fatty tissues to other tissues.

In plants, P-protein is involved in transport of organic compounds through phloem. Many proteins are stored to serve as reservoirs of amino acids and nitrogen. For instance, seeds of many plants store nutrient protein required for germination and growth of seedlings. Similarly, ovalbumin of egg white provides nourishment to developing bird embryo and casein of milk provides nourishment to mammalian infants. Ferritin protein of bacteria and plant and animal cells stores iron.

These are filamentous proteins which enable cells and organisms to contract, to change shape, or to move about. For instance, actin and myosin are responsible for contraction and relaxation of muscle cells and occur in many other types of cells. Myosin also contains ATPase enzyme and is, therefore, both enzymatic and contractile. Tubulin protein is the major component of microtublules which form the locomotory organelles (flagella and cilia), basal bodies, centrioles and spindle apparatus in dividing cells.