FIG 4.

Structural model of the FtpA A₁ domain. (A) Homology model of NPS7 using the crystal structure of PheA as the template molecule. (B) Close-up view of fumarate in the binding pocket. While PheA-D222 binds the amino group of the phenylalanine substrate in case of PheA, S225 forms an additional hydrogen bond with the C-1 carboxylate of fumarate. R229 positions the C-4 carboxylate and most likely defines a key residue for dicarboxylate specificity. (C) Due to the (E)-configuration, maleate does not hydrogen bond with R229. Hydrogen bonds are illustrated by dotted lines. Labels indicate residue type and number. For clarity, only key residues are shown.