Tyrosine
(p)HO-C6H4-CH2-
Tyr
Y
18 Tryptophan*
–CH2
N
H
Trp
W
19 Histidine*
His
H
20 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