With regard to amino acids in general, there's only a few amino acids with -R groups that have a pKa worth considering, and these are: (3) basic amino acids: Arginine, Lysine, Histidine; (2) acidic amino acids (Glutamic acid, Aspartic acid); Also, Serine and Cysteine (although they are less often presented).
Any other amino acid -R group, you don't need to concern yourself with its acidity/bascity. This includes threonine, which also has an -OH group. So you may think it too can be deprotonated and obtain a negative charge, but for all intents and purposes we disregard it.