Proteins


3.3 Structure of Proteins  (Polypeptide chains)

A polypeptide is another example of a polymer and a macromolecule. A complete protein molecule may contain just one polypeptide chain or it may have two or more.

The particular amino acids contained in the chain, and the sequence in which they are joined is called the primary structure of the protein. The amino acids in a chain like this have an effect on each other even if they are not in direct contact to each other.

A polypeptide chain, or a part of it, often forms either a shape like a corkscrew, which is called Alpha-Helix, or in a straighter shape, and it’s called Beta-sheet or Beta-plated sheet. This shape is called the secondary structure.
This all due hydrogen bonding between the -CO group and the -NH group. I recommend to you to remember the look of a hydrogen bond that holds together the secondary structure.

secondary-protein-structure-bonds
Click to zoom

The secondary structure coils even more to form a 3D shape. Hydrogen bonds can form between a wide variety of R groups. Disulfide bonds form between sulphur atoms. Ionic bonds form between R-groups containing amine and carboxyl groups. This three-dimensional shape is called tertiary structure.

TO CLARIFY – The tertiary structure is held together by:

  • Hydrogen bonds
  • Ionic interactions between side chains (also called electrostatic attractions)
  • Van der Waals forces (this exist between non-polar parts of molecules, and they’re also called dipole-induced dipoles)
  • Disulfide bridges  (in proteins used outside the body) between cysteine molecules.

The association of different polypeptide chains is called the quaternary structure of the protein.

A protein whose molecules curl up into a “ball” shape is known as a globular protein. Globular proteins curl up so that their non-polar, hydrophobic R groups point into the centre of the molecule, away from the watery surroundings. Hydrophilic R groups remain on the outside of the molecule, making the globular proteins soluble.

Many proteins have roles in metabolic reactions. Their shape is the key to their functioning.

Many other protein molecules do not curl, but form long strands. These are known as fibrous proteins. They are usually not soluble in water and have structural roles ( E.g. keratin forms hair, or collagen which forms a waterproof structure).

 

GO FURTHER TO:

3.3 Haemoglobin

3.4 Collagen (To come)

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