Residues involved in the human FPPS C-terminal tail closure. (A) The structures of the FPPS-YS0470-Pi (green) and FPPS-YS0470-PPi (cyan) complexes are superimposed. The conformational changes that occur prior to the rigidification of the R351 side chain are indicated with black arrows. The residues Y349, F238, and Q242 are in the anchor-blocking conformation in the Pi-bound complex and in the anchor-accepting conformation in the PPi-bound complex. (B) A schematic representation of the Y349 switch activation: the K57 side chain rigidifies and attracts the C-terminal tail; N59 interacts with K347 via a water molecule; and the Y349 side chain rotates out due to the torsion created by these two forces.