Page 12 - Lesson Notes - Biomolecules 1

health so, are need to be supplied through our diet. The dietary proteins are the
source of essential amino acids, e.g., Leucine, isoleucine, etc.
(ii) Non-essential Amino Adds These are those amino acids, which our body can
synthesise, e.g., Proline, serine.
Human adults require an additional essential amino acid named threonine while
children need two more arginine and histidine. These are. called semi-essential
amino acids.
Structure of Proteins
As mentioned earlier, proteins are heteropolymers containing strings of amino acids.
Biologists describe the protein structure at four different levels, i.e., primary,
secondary, tertiary and quaternary.

ii. Secondary Structure
The thread of the primary protein is folded in the form of a-helix. The a-helix is
stabilised by hydrogen bonds between oxygen of the carboxylic group of one amino
acid residue and —NH group of the next fourth amino acid residue, e.g., Keratin.
In β-pleated secondary structure, two or more polypeptide chains get interconnected
by hydrogen bonds. Adjacent strands of polypeptide may run in the same direction or
in opposite direction, e.g., Silk fibre.
In proteins, only right handed helices are observed.The polypeptide chain curls The
protein is more distended and longitudinally by the action of the hydrogen bond
forms a zig-zag hydrogen bonds forming a shaped protein structure called spiral or
helix. (which combines and forms p-sheet).
Differences between α-helix and β–pleated Structure of Proteins