Classification of Proteins
Because of the complexity of structure and diversity in their function, it is very difficult to classify proteins in a single well defined fashion. However, according to their structure, proteins are classified as follows:
Fibrous proteins:
They consist of molecules having one or more polypeptide chains in the form of fibrils. Secondary structure is most important in them. They are insoluble in aqueous media. They are non-crystalline and are elastic in nature. They perform structural roles in cells and organisms. Examples are silk fiber (from silk worm, and spiders' web) myosin (in muscle cells), fibrin (of blood clot), and keratin (of nails and hair).
Globular proteins:
These are spherical or ellipsoidal due to multiple folding of polypeptide chains. Tertiary structure is most important in them. They are soluble in aqueous media such as salt solution, solution of acids or bases, or aqueous alcohol. They can be crystallized. They disorganize with changes in the physical and physiological environment. Examples are enzymes, antibodies, hormones and hemoglobin.
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