4-Phospho-hydroxy-L-threonine is an obligatory intermediate in pyridoxal 5'-phosphate coenzyme biosynthesis in Escherichia coli K-12.

We show that thrB-encoded homoserine kinase is required for growth of Escherichia coli K-12 pdxB mutants on minimal glucose medium supplemented with 4-hydroxy-L-threonine (synonym, 3-hydroxyhomoserine) or D-glycolaldehyde. This result is consistent with a model in which 4-phospho-hydroxy-L-threonine (synonym, 3-hydroxyhomoserine phosphate), rather than 4-hydroxy-L-threonine, is an obligatory intermediate in pyridoxal 5'-phosphate biosynthesis. Ring closure using 4-phospho-hydroxy-L-threonine as a substrate would lead to formation of pyridoxine 5'-phosphate, and not pyridoxine, as the first B6-vitamer synthesized de novo. These considerations suggest that E. coli pyridoxal/pyridoxamine/pyridoxine kinase is not required for the main de novo pathway of pyridoxal 5'-phosphate biosynthesis, and instead plays a role only in the B6-vitamer salvage pathway.