The Last Piece in the Vitamin B1 Biosynthesis Puzzle: STRUCTURAL AND FUNCTIONAL INSIGHT INTO YEAST 4-AMINO-5-HYDROXYMETHYL-2-METHYLPYRIMIDINE PHOSPHATE (HMP-P) SYNTHASE*

Vitamin B₁ is essential for all organisms being well recognized as a necessary cofactor for key metabolic pathways such as glycolysis, and was more recently implicated in DNA damage responses. Little is known about the enzyme responsible for the formation of the pyrimidine moiety (4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate (HMP-P) synthase). We report a structure-function study of the HMP-P synthase from yeast, THI5p. Our crystallographic structure shows that THI5p is a mix between periplasmic binding proteins and pyridoxal 5′-phosphate-dependent enzymes. Mutational and yeast complementation studies identify the key residues for HMP-P biosynthesis as well as the use of pyridoxal 5′-phosphate as a substrate rather than as a cofactor. Furthermore, we could show that iron binding to HMP-P synthase is essential for the reaction.