Purification and properties of 5,10-methenyltetrahydrofolate cyclohydrolase from Clostridium formicoaceticum.

Methenyltetrahydrofolate cyclohydrolase (EC 3.5.4.9) from Clostridium formicoaceticum has been purified to a specific activity of 469 mumol min-1 mg-1 at 35 degrees C, pH 7.2. The purified enzyme is homogeneous as judged by polyacrylamide disc gel electrophoresis, sedimentation velocity, and gel filtration profiles. The molecular weight is 41,000 +/- 200 as determined by sedimentation equilibrium centrifugation. A subunit molecular weight of approximately 25,500 was obtained using sodium dodecyl sulfate-gel electrophoresis. The enzyme apparently is a dimer. The Stokes radius determined by gel filtration is 29.6 A. The apparent Km at pH 7.2 and 35 degrees C for 5,10-methenyltetrahydrofolate is 0.19 mM. The pure enzyme does not contain any 10-formyltetrahydrofolate synthetase or 5,10-methylenetetrahydrofolate dehydrogenase activities.