The pKa of an amino acid depends upon its type, group and side chains. For example, when lysine is part of the carboxylic acid group, it has a pKa of 2.18, compared to a pKa of 8.95 when it is part of the amino group and 10.53 with side chains.
When in the carboxylic acid group, most amino acids have pKa values of just over 2, with only histidine falling below with a pKa of 1.77. An acid with a pKa value below 2 is considered a strong acid, while those above are considered weak acids. Amino acids in the amino group have pKa values ranging from the high 8s to the low 10s.