Formyltetrahydrofolate hydrolase, a regulatory enzyme that functions to balance pools of tetrahydrofolate and one-carbon tetrahydrofolate adducts in Escherichia coli

The enzyme encoded by Escherichia coli purU has been overproduced, purified, and characterized. The enzyme catalyzes the hydrolysis of 10- formyltetrahydrofolate (formyl-FH₄) to FH₄ and formate. Formyl-FH₄ hydrolase thus generates the formate that is used by purT-encoded 5'- phosphoribosylglycinamide transformylase for step three of de novo purine nucleotide synthesis. Formyl-FH₄ hydrolase, a hexamer with 32-kDa subunits, is activated by methionine and inhibited by glycine. Heterotropic cooperativity is observed for activation by methionine in the presence of glycine and for inhibition by glycine in the presence of methionine. These results, along with previous mutant analyses, lead to the conclusion that formyl-FH₄ hydrolase is a regulatory enzyme whose main function is to balance the pools of FH₄ and C₁-FH₄ in response to changing growth conditions. The enzyme uses methionine and glycine to sense the pools of C₁- FH₄ and FH₄, respectively.