Pyridoxal 5′-phosphate, the catalytically active form of vitamin B6, is an important cofactor for many enzymes involved in amino acid metabolism. Two pathways for its de novo synthesis are known. One of these, found in all Archaea, eukaryotes, and in some bacteria. The other pathway is specific for bacteria and consists of six enzymatic steps. The fourth step in this pathway is catalyzed by 4-hydroxythreonine-4-phosphate dehydrogenase (PdxA protein) which converts 4-phosphonooxy-L-threonine to (2S)-2-amino-3-oxo-4-phosphonooxybutanoate using NAD+ as a cofactor. The presented structure has unknown ligand bound in an interface of protein homodimers partially occupying an active center.