Classification of protein based on Structure (Fibrous and globular proteins)

Classification of protein based on Structure

Fibrous proteins

Structure: Polypeptide chains arranged in parallel along a single axis to yield long fibers or sheets.

Fibrous proteins have rod-like, thread-like or sheet-like structure.

Most of the fibrous proteins are extensively cross-linked to form fibrous structures.

Fibrous proteins usually contain a single type of secondary structure.

Abundance in nature: Fibrous proteins (the most abundant protein in the human body) have a higher total mass than globular proteins.

Solubility: Fibrous proteins are insoluble in water. Remain insoluble in acids and bases.

Sequences of amino acids: regular in fibrous protein

Strength of interactions: The peptide chains are held together by strong intermolecular hydrogen bonds

Function: They are the structural elements in the connective tissue of higher animals.

For example: Collagen of tendons and bone matrix

Elastin of elastic connective tissue

α – keratin of hair, skin, nails, feathers etc

Fibrous proteins mainly involve in supporting functions: providing tensile strength, elasticity, rigidity.

Structural functions: forming membrane structures, scaffold structures inside cells etc.

Stability: Less sensitive to change in pH and heat

Examples for fibrous proteins: F-actin, collagen, desmin, elastin, fibroin, keratin etc.

Globular proteins

 Structure: Polypeptide chains tightly folded into compact globular shapes.

Globular proteins have spherical shapes, which are three-dimensional (3D) in nature.

Globular proteins often have several types of secondary structure ( Complex with a mixture of alpha helices, beta sheets and loops)

The number of different types of globular proteins is higher than that of fibrous proteins.

Solubility:  Most globular proteins are soluble in aqueous solutions.

The water solubility of globular proteins enables them to transport through blood and other body fluids to various locations where their action is required.

Function: They have a mobile or dynamic function in the cell. Of the nearly 2000 different enzymes known to date, nearly all are globular proteins.

Globular proteins mainly involve in carrying many chemical reactions(catalysis)and metabolic functions(transport, regulation)

These proteins also act as catalysis for thousands of chemical reactions occurring in the body.

Eg: Globulin proteins involve in glucose metabolism, oxygen storage in muscles, oxygen transport in blood, immune responses etc.

Examples for globular proteins:  Insulin, myoglobin, hemoglobin, transferrin, and immunoglobulins. 

Solubility: Globular proteins are soluble in water, acids, and bases.

Intermolecular Forces: The peptide chains of globular proteins are held together by relatively weak intra molecular hydrogen bonds

Sequence of aminoacids: irregular in globular proteins

Stability: More sensitive to changes in pH and heat.

 Exceptions: Some proteins fall between the fibrous and globular types (resembling fibrous proteins in their long rod-like structures and the globular proteins in their solubility in aqueous salt solutions). Example:  myosin (structural element of muscle and fibrinogen)

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