#14. Globular and fibrous proteins - haemoglobin and collagen

Globular and Fibrous are 2 main types of proteins with a 3D structure.

1. Haemoglobin, a globular protein 

• Composed of 2 α + 2 β polypeptide chains + 1 inorganic prosthetic haem group.

Hb's function: carry O₂ from lungs to respiring tissues.

2. Collagen - a fibrous protein

• Composed of 3 polypeptide chains wound around each other. Each chain is a coil itself of around 1000 amino acids. 

• Structure strength is increased by forming: 

                 - H bonds between the 3 polypeptide chains.
                 - Collagen molecules wrapped around each other ---> Collagen Fibrils 

                 - Collagen Fibrils ---> Collagen Fibres.

Collagen's function: support and elasticity 

in many animal tissues (human skin, bone and tendons).

3. Test for proteins

The Biuret Test shows the presence of peptide bonds, which are the basis for the formation of proteins. These bonds will make the blue Biuret reagent turn purple.

Add biuret solution. A purple colour indicates the presence of protein.

 

Syllabus 2015 (h) describe the molecular structure of haemoglobin as an example of a globular protein, and of collagen as an example of a fibrous protein and relate these structures to their functions (the importance of iron in the haemoglobin molecule should be emphasised. A haemoglobin molecule is composed of 2 alpha (α) chains and 2 beta (β) chains, although when describing the chains the terms α-globin and β-globin may be used. There should be a distinction between collagen molecules and collagen fibres);

Syllabus 2016 - 2018 c)   describe the molecular structure of haemoglobin as an example of a globular protein, and of collagen  as an example of a fibrous protein  and relate  these structures to their functions (The importance of iron in the haemoglobin molecule should be emphasised. A haemoglobin molecule is composed of two alpha (α) chains  and two beta  (β) chains, although when describing the chains  the terms α-globin and β-globin may be used. There should  be a distinction between collagen molecules and collagen  fibres)