In enzymology, a 4-hydroxythreonine-4-phosphate dehydrogenase (EC 1.1.1.262) is an enzyme that catalyzes the chemical reaction

4-phosphonooxy-L-threonine + NAD⁺ ${\displaystyle \rightleftharpoons }$ (2S)-2-amino-3-oxo-4-phosphonooxybutanoate + NADH + H⁺

Thus, the two substrates of this enzyme are 4-phosphonooxy-L-threonine and NAD⁺, whereas its 3 products are (2S)-2-amino-3-oxo-4-phosphonooxybutanoate, NADH, and H⁺.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD⁺ or NADP⁺ as acceptor. The systematic name of this enzyme class is 4-phosphonooxy-L-threonine:NAD⁺ oxidoreductase. Other names in common use include NAD⁺-dependent threonine 4-phosphate dehydrogenase, L-threonine 4-phosphate dehydrogenase, 4-(phosphohydroxy)-L-threonine dehydrogenase, PdxA, and 4-(phosphonooxy)-L-threonine:NAD⁺ oxidoreductase. This enzyme participates in vitamin B₆ metabolism.

As of late 2007, 6 structures have been solved for this class of enzymes, with PDB accession codes 1PS6, 1PS7, 1PTM, 1R8K, 1YXO, and 2HI1.

• ID; Hsiung, Yuju; Cornish, Julie A.; Robinson, J. Kenneth; Spenser, Ian D. (1998). "Biosynthesis of vitamine B6: The oxidation of L-threonine 4-phosphate by PdxA". J. Am. Chem. Soc. 120 (8): 1936–1937. doi:10.1021/ja9742085.
• Laber B, Maurer W, Scharf S, Stepusin K, Schmidt FS (1999). "Vitamin B6 biosynthesis: formation of pyridoxine 5'-phosphate from 4-(phosphohydroxy)-L-threonine and 1-deoxy-D-xylulose-5-phosphate by PdxA and PdxJ protein". FEBS Lett. 449 (1): 45–8. Bibcode:1999FEBSL.449...45L. doi:10.1016/S0014-5793(99)00393-2. PMID 10225425. S2CID 33542088.
• Sivaraman J, Li Y, Banks J, Cane DE, Matte A, Cygler M (2003). "Crystal structure of Escherichia coli PdxA, an enzyme involved in the pyridoxal phosphate biosynthesis pathway". J. Biol. Chem. 278 (44): 43682–90. doi:10.1074/jbc.M306344200. PMID 12896974.