The role of molybdenum in the formate dehydrogenase of methanobacterium formicicum
An examination of oxidation products of the pterin cofactor in the formate dehydrogenase of Methanobacterium formicicum revealed that the cofactor is a 6-substituted pterin similar to the molybdopterin of xanthine oxidase. In contrast to the molybdopterin from xanthine oxidase the formate dehydrogenase cofactor was unable to complement the cofactor-deficient nitrate reductase of Neurospora crassa mutant nit-1 and possessed two phosphate groups.
Incubation of oxidized formate dehydrogenase with cyanide resulted in an irreversible loss of enzyme activity which could not be restored by treatment with sulfide. Equimolar amounts of thiocyanate were released from cyanide-treated formate dehydrogenase suggesting the loss of one terminal sulfur ligand to molybdenum. These results along with electron paramagnetic resonance spectroscopy of the cyanide-inactivated formate dehydrogenase suggest that the the molybdenum ligands of the enzyme are similar to those of xanthine oxidase.
The concentration of molybdenum and the level of formate dehydrogenase activity in extracts of hydrogen-grown Methanobacterium formicicum decreased by at least 10- fold when the organism was grown for several transfers in molybdenum-deficient media or with added tungstate. Immunochemical analysis showed that both subunits of the formate dehydrogenase were produced regardless of the growth condition. However the amount of formate dehydrogenase protein decreased more than 10-fold when the amount of molybdenum in the cell was low. The pterin cofactor was present in the inactive enzyme from tungstategrown cells; however the protein contained less than 0.05 molecules of molybdenum or tungsten per formate dehydrogenase. Messenger RNA specific for fdh gene was detected in high amount in cells grown without added molybdenum and in low amount in cells that contained high amounts of molybdenum. These results suggest that molybdenum is required for the synthesis of a stable formate dehydrogenase and that a molybdenum-dependent repressor may be required for the termination of fdh transcription.