PROTEINS:: PART 5

PROTEIN STRUCTURES

·         Proteins can be divided into four categories based upon their structure.

·         Primary structure: -

Ø  Primary structure refers to the linear sequence of amino acids from the N-terminal to the C-terminal.

·         Secondary structure: -

Ø  In this a polypeptide chain is folded about itself to form alpha-helices and beta-sheets.

Ø  These structures are stabilizes by hydrogen bonds.

Ø  Alpha-helices: -

      These are spiral structures.

      These have right hands or left handed orientation.

      Stabilized by hydrogen bond between amine and carboxyl group.

      These fibres are insoluble.

      Keratin is an example of alpha-helices.

      These form hair, nails, hooves and horns.

Ø  Beta-sheets: -

      These are sheet like structures where two or more sections of polypeptides come together.

      Formed by parallel or anti-parallel arrangement of polypeptides in a plane.

      When all N-terminals are on one edge they are called parallel beta-sheets.

      When alternative chains of N-terminal and C-terminal lie on one edge they are called anti-parallel beta-sheets.

      Silk is an example of beta-sheets.

                            

·         Tertiary structure: -

Ø  This involves folding of a single polypeptide into a globular structure called its tertiary structure.

Ø  Myoglobin and ribonuclease are proteins that have a tertiary structure.

·         Quaternary structure(protein-protein interaction): -

Ø  Association of number of peptide chains into a complex and large sized molecule is called quaternary structure.

Ø  Haemoglobin is the best example of protein with quaternary structure.