Proteins are the most abundant macro-molecules, occurring in each cell and each part of a cell. They are generally formed by one or more long chain(s) of amino acid residues.
Classification of protein
1) Based on source
There are two types of protein.
a) Animal protein: They are derived from animal sources. These are usually high quality protein. E.g: eggs, milk, fish.
b) Plant protein: These are derived from plants and lower quantity protein. e.g: legumes.
2) Based on the shape of protein molecule
These are of two type.
a) Globular protein: Polypeptide chain folded into globular form, soluble in water. E.g: Insulin, albumin etc.
- Soluble in water.
b) Fibrous: Polypeptide chains arranged in a long strands. E.g: hair, skin etc.
- Insoluble in water.
- Also called scleroprotein.
- Resistant to proteolytic enzymes.
- Examples: Collagen, actin, myosin, keratin of hair, claws etc.
3) Based on shape of composition and solubility
Three major groups are present, such as: a)simple protein, b) conjugated protein, c) Derived protein.
a) Simple protein: These are of globular type and contain only amino acids as structural components. These are further classified on the basis of solubility into several types:
|Egg, milk, serum|
|Glutenin of wheat, oryzenine of rice|
|Zein of corn|
|Keratin of hair|
|The globin of hemoglobin|
|Clupeine from herring sperm.|
b) Conjugated protein: They are also globular protein combined with a non protein group, called prosthetic group. These are further classified on the basis of nature of prosthetic groups present.
|Nucleoprotein||Have nucleic acids as their prosthetic group.||Protamins|
|Proteoglycans||Are polysaccharides combined with small amount of protein.||Mucoproteins|
|Glycoprotein||Proteins are conjugated with carbohydrates.|
|Chromoproteins||The prosthetic group are color compound.||Flavoenzymes|
|Lipoprotein||Proteins are combined with phospholipids.||Egg yolk|
c) Derived proteins: They are products of denaturation or partial digestion of proteins.
- Proteans: Fibre from fibrinogen.
- Coagulated proteins: Coagulated egg white.