Linked from
The 92 pages that link to Amino acid, each with the reason it gives.
Carboxylic acidRelated: Its carboxyl group participates in peptide formation and acid–base behavior.
pKaRelated: Its ionizable groups have pKa values that govern charge across pH conditions.
Weak acidRelated: Its carboxyl group commonly behaves as a weak acid in water.
HomochiralityRelated: Nearly all amino acids in ribosomally made proteins have the L configuration.