I've found 4 distinct binding sites. 
Multiple ligand copies in the same crystal result in random sites being choosen.
The 2-3 sites that enrich best are on the outside of the protein. 
Niu choose the interior site.
PLP should be LLP instead.

1gfz-18.75(2.30), 3nc4-18.09(2.07), 3bcu-18.02(2.03)