Proteins . Isoleucine* H C-CH -CH- Ile I | CH . Arginine* HN=C-NH-(CH ) - Arg R | NH . Lysine* H N-(CH ) - Lys K .
Glutamic acid HOOC-CH -CH - Glu E . Aspartic acid HOOC-CH - Asp D O | | . Glutamine H N-C-CH -CH - Gln Q O | | . Asparagine H N-C-CH - Asn N .
Threonine* H C-CHOH- Thr T . Serine HO-CH - Ser S . Cysteine HS-CH - Cys C . Methionine* H C-S-CH -CH - Met M .
Phenylalanine* C H -CH - Phe F . Tyrosine ( p )HO-C H -CH - Tyr Y . Tryptophan* –CH N Trp W . Histidine* His .
Proline Pro P * essential amino acid, a = entire structure 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 non- essential amino acids. On the other hand, those which cannot be synthesised in the body and must be obtained through diet, are known as essential amino acids (marked with asterisk in Table .
). . . Classification of Amino Acids 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 zwitter ion . This is neutral but contains both positive and negative charges.