Access to the KpHpxA active site. (a) The structure of KpHpxA is shown (pale green, ribbon and ball-and-stick) with allantoin bound (blue, ball-and-stick). Glu53, which may serve as a gate for the active site, is shown in ball-and-stick with green carbon atoms and red oxygen atoms. (b) Cutaway surface representation of KpHpxA shown with allantoin bound (blue stick representation). The putative gating residue, Glu53, is shown in green stick representation. Fig. 8b is rotated 90° about the y-axis relative to Fig. 8a.