Elementary plant Biochemistry and Biotechnology (2+1)

b. Classification of proteins based on function

Proteins are classified based on their functions as
Catalytic proteins – Enzymes
 
• The most striking characteristic feature of these proteins is their ability to function within the living cells as biocatalysts. These biocatalysts are called as enzymes. Enzymes represent the largest class. Nearly 2000 different kinds of enzymes are known, each catalyzing a different kind of reaction. They enhance the reaction rates a million fold

Regulatory proteins - Hormones

• These are polypeptides and small proteins found in relatively lower concentrations in animal kingdom but play highly important regulatory role in maintaining order in complex metabolic reactions. e.g., growth hormone, insulin etc.

Protective proteins - Antibodies

• These proteins have protective defense function. These proteins combine with foreign protein and other substances and fight against certain diseases.e.g., immunoglobulin. These proteins are produced in the spleen and lymphatic cells in response to foreign substances called antigen. The newly formed protein is called antibody which specifically combines with the antigen which triggered its synthesis thereby prevents the development of diseases. Fibrin present in the blood is also a protective protein

Storage proteins

• It is a major class of proteins which has the function of storing amino acids as nutrients and as building blocks for the growing embryo. Storage proteins are source of essential amino acids, which cannot be synthesized by human beings. The major storage protein in pulses is globulins and prolamins in cereals. In rice the major storage protein is glutelins. Albumin of egg and casein of milk are also storage proteins.

Transport proteins

• Some proteins are capable of binding and transporting specific types of molecules through blood. Haemoglobin is a conjugated protein composed of colourless basic protein, the globin and ferroprotoporphyrin or haem. It has the capacity to bind with oxygen and transport through blood to various tissues. Myoglobin, a related protein, transports oxygen in muscle. Lipids bind to serum proteins like albumin and transported as lipoproteins in the blood

Toxic proteins

• Some of the proteins are toxic in nature. Ricin present in castor bean is extremely toxic to higher animals in very small amounts. Enzyme inhibitors such as trypsin inhibitor bind to digestive enzyme and prevent the availability of the protein.Lectin, a toxic protein present in legumes, agglutinates red blood cells. A bacterial toxin causes cholera, which is a protein. Snake venom is protein in nature.

Structural proteins

• These proteins serve as structural materials or as important components of extra cellular fluid. Examples of structural proteins are myosin of muscles, keratin of skin and hair and collagen of connective tissue. Carbohydrates, fats, minerals and other cellular components are organized around such structural proteins that form the molecular framework of living material

Contractile proteins

• Proteins like actin and myosin function as essential elements in contractile system of skeletal muscle

Secretary proteins

• Fibroin is a protein secreted by spiders and silkworms to form webs and cocoons.

Exotic proteins

• Antarctic fishes live in -1.9ºC waters, well below the temperature at which their blood is expected to freeze. These fishes are prevented from freezing by antifreeze glycoproteins present in their body.