Ionizable residues and pK
determinants in the xylanase structure. (A) The figure shows four ionizable residues with the most shifted pKa values in the structure of Bacillus circulans xylanase [PDB:1XNB] displayed using the PROPKA GUI. Labels next to each residue give: one-letter residue code with its residue ID, the pKa value and the shift of the pKa from its model value. (B) When an ionizable residue is selected (in this case tyrosine 80), all of its pKa determinants are displayed together with their contributions to the pKa shift shown with the appropriate labels.