4-Phospho-hydroxy-l-threonine is an obligatory intermediate in pyridoxal 5′-phosphate coenzyme biosynthesis inEscherichia coliK-12

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Abstract
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 the formation of pyridoxine 5′-phosphate, and not pyridioxine, 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.