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The vitamin B6 derived cofactor pyridoxal-5’-phosphate (PLP) is the predominate coenzyme in metabolism and the wide variety of PLP catalyzed reactions has intrigued researchers for many decades. PLP is found in at least five protein fold types and is involved in a multitude of chemical transformations including decarboxylations, β and γ elimination, racemization, transamination, and dephosphorylation. A search of the RCSB Protein Data Bank for the PLP ligand indicate over 900 X-ray diffraction derived structures have been submitted. With the crystal structures providing positions of the heavier atoms, UV-Vis and NMR spectroscopy have provided evidence for the positions of hydrogens. Recently, we solved the complete structure of aspartate aminotransferase (AAT), in both the internal and the external aldimine forms, using joint refinement of X-ray and neutron diffraction data on perdeuterated enzyme …