`=>` Proteins are the most abundant biomolecules of the living system.

`=>` Chief sources of proteins are milk, cheese, pulses, peanuts, fish, meat, etc.

`=>` They occur in every part of the body and form the fundamental basis of structure and functions of life.

`=>` They are also required for growth and maintenance of body.

`=>` The word protein is derived from Greek word, “proteios” which means primary or of prime importance.

`=>` All proteins are polymers of `color{red}(α)`-amino acids.

`=>` Amino acids contain amino `color{red}(–NH_2)` and carboxyl `color{red}(–COOH)` functional groups.

`=>` Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be classified as `color{red}(α, β, γ, δ)` and so on.

`=>` Only `color{red}(α)`-amino acids are obtained on hydrolysis of proteins. They may contain other functional groups also. See fig.1.

`=>` All `color{red}(α)`-amino acids have trivial names, which usually reflect the property of that compound or its source.

`=>` Glycine is so named since it has sweet taste (in Greek glykos means sweet) and tyrosine was first obtained from cheese (in Greek, tyros means cheese).

`=>` Amino acids are generally represented by a three letter symbol, sometimes one letter symbol is also used.

`=>` Structures of some commonly occurring amino acids along with their 3-letter and 1-letter symbols are given in Table 14.2.

`=>` Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule.

`=>` Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic.

`=>` The amino acids, which can be synthesised in the body, are known as nonessential amino acids.

`=>` The amino acids, which cannot be synthesised in the body and must be obtained through diet, are known as essential amino acids (marked with asterisk in Table 14.2).

`=>` Amino acids are usually colourless, crystalline solids.

`=>` These are water-soluble, high melting solids and behave like salts rather than simple amines or carboxylic acids.

`=>` This behaviour is due to the presence of both acidic (carboxyl group) and basic (amino group) groups in the same molecule.

`=>` In aqueous solution, the carboxyl group can lose a proton and amino group can accept a proton, giving rise to a dipolar ion known as `color{green}("zwitter ion")`. This is neutral but contains both positive and negative charges.

● In zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases.

`=>` Except glycine, all other naturally occurring `color{red}(α)`-amino acids are optically active, since the `color{red}(α)`-carbon atom is asymmetric.

● These exist both in `‘color{red}(D)’` and `color{red}(L)` forms.

`=>` Most naturally occurring amino acids have `color{red}(L)`-configuration.

`=>` `color{red}(L)`-aminoacids are represented by writing the `color{red}(–NH_2)` group on left hand side.