The text on this page establishes that amino acids are amphoteric molecules—they can act as both acids (proton donors) and bases (proton acceptors). This behavior is dictated by the ionizable groups attached to the $\alpha$-carbon:
Subject: Acid-Base Properties of Amino Acids and Titration Curves Source: Fondamenti di Biochimica (Lehninger) Context: Chapter 3 - Amino Acids, Peptides, and Proteins fondamenti di biochimica lehninger pdf 114
R: Sì, spesso l’anteprima mostra le prime 120 pagine. Cercate "Fondamenti di biochimica Lehninger Zanichelli" su Google Books, quindi navigate fino a pagina 114. The text on this page establishes that amino