Predictions of histidine pKavalues in FocA wildtype and constriction mutants and solvation by water of the FocA F75A/F202A–L79A/L89A mutant.A, six histidine residues are present in the FocA protomer at positions 30, 155, 158, 159, 209, and 244 (Protein Data Bank ID: 3KCU). The DEPTH algorithm was used to predict respective pKa values (top panel). Effects of constriction-widening mutations on the pKa of the central His209 are shown in the lower panel. B, solvation simulations by water of the FocA F75A/F202A–L79A/L89A mutant indicates connecting water chains from the bulk to the central histidine (here from the cytoplasmic side) that may act at proton wires. FocA, FNT-type formate transporter from Escherichia coli.