Chemistry Proteins, Amino Acids and Their Classification
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### Topics Covered :

● Proteins
● Amino Acids
● Classification of Amino Acids

### Proteins :

=> 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 :

=> 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.

### Classification of Amino Acids :

=> 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.