How can amino acids be enabled as buffers in biological tissue?

Amino acids form Zwitterions which means that the -NH₂ group becomes -NH₃⁺ that will remove alkali OH⁻⁻ ions to form water and -NH₂

The -COOH becomes a -COO⁻ which reacts with acidic H⁺ ions to reform the  -COOH group.

Because of this fact and the fact that the zwitterions can be in equilibrium means that it can act as a pH buffer in tissue as the body contains many amino acids.

Hope that helps :)