The main focus of this special issue is on structural, functional, and biomedical studies on pyridoxal-5′-phosphate- (PLP-) dependent enzymes. The unparalleled catalytic versatility of PLP, the active form of vitamin B6, originates from its unique electron-sinking properties, which stabilize reaction intermediates, thus lowering the activation barrier during catalysis. At least five different protein scaffolds arose during evolution to bind PLP and harness its catalytic functionality. The role of the apoenzyme scaffolds is to assist in the proper orientation of the substrate's reacting groups relative to the π-electrons of PLP, to promote reactivity and control reaction specificity. In addition, the active site residues interacting with the leaving groups provide either stabilizing or destabilizing interactions to direct catalysis [1].