Classification of proteins : According to chemical composition, proteins are divided into two classes
(i) Simple proteins : Simple proteins are composed of chains of amino acid units only joined by peptide linkages. These proteins on hydrolysis yield only mixture of amino acids. Examples are :
Egg albumin, serum globulins, glutenin in wheat, coryzenin in rice, tissue globulin, etc.
(ii) Conjugated proteins : The molecules of conjugated proteins are composed of simple proteins and non protein material. The non-protein material is called prosthetic group or cofactor. These proteins on hydrolysis yield amino acids and non-protein material. Examples are
Mucin in saliva (prosthetic group, carbohydrate), casein in milk (prosthetic group, phosphoric acid), haemoglobin in blood (prosthetic group, iron pigment), etc.
According to molecular shape, proteins are divided into two types
(i) Fibrous proteins : These are made up of polypeptide chains that run parallel to the axis and are held together by strong hydrogen and disulphide bonds. They can be stretched and contracted like a thread. These are usually insoluble in water. Examples are : a-keratin (hair, wool, silk and nails); myosin (muscles); collagen (tendons, bones), etc.
(ii) Globular proteins : These have more or less spherical shape (compact structure). a-helics are tightly held up by weak attractive forces of various types: Hydrogen bonding, disulphide bridges, salt or ionic bridges. These are usually soluble in water. Examples are: Insulin, pepsin, haemoglobin, cytochromes, albumins, etc.
Proteins can also be divided on the basis of their function
Table :
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Protein
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Function
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Examples
|
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Enzymes
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Biological catalysts, vital to all living systems.
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Trypsin, pepsin.
|
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Structural proteins
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Proteins that hold living systems together.
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Collagen.
|
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Harmones
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Act as messengers.
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Insulin.
|
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Transport proteins
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Carry ions or molecules from place to another in the living system.
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Haemoglobin.
|
|
Protective proteins (antibiotics)
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Destroy any foreign substance released into the living system.
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Gamma globulin.
|
|
Toxins
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Poisonous in nature.
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Snake venom.
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