Proteins are macro molecules made up of amino acids which are covalently linked by peptide bonds. A protein may contain one or more polypeptide chains. Peptide bonds are formed by the condensation of amino group and carboxylic group of adjacent amino acids. The first amino acid residue in the polypeptide chain retains its amino group while the last amino acid residue keeps its carboxylic acid group. Hence, the ends of a polypeptide chain are called N-terminus and C-terminus respectively. All newly synthesized polypeptides contain Methionine (Met) at the N-terminus, because the protein translation starts at ATG codon that encodes Met. In most of the proteins, it is often cleaved off post translationally.
Functions of proteins:
Proteins perform a vast variety of specialized and essential functions in the living systems. These functions can be structural or dynamic functions.

Structural functions: Several proteins act as building blocks of many cellular structures or they give strength to the existing structure.
e. Collagen, Elastin, Keratin etc..

Dynamic functions: Proteins can perform diverse functions. A few of them are listed below.
Enzymes: catalyze biochemical reactions
Hormones: involved in signaling cascades.
Clotting factors: Required for blood clotting
Immunoglobulin: Required for immunological response
Membrane receptors: Involved in signaling
Storage proteins: Energy storage function

Ex. Amylase is a salivary enzyme secreted in the mouth that catalyzes the initial step of digestion, i.e., It catalyzes the degradation of starch into amylase.

All the protein functions can be categorized into 6 types:

1. Repair and maintenance
2. Hormones
3. Enzymes
4. Energy
5. Storage and transportation
6. Protection

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