The study will be conducted with Clostridium thermoaceticum and Clostridium formicoaceticum plus some other anaerobic bacteria, which synthesize acetate from CO2 via a pathway dependent on vitamin B12 and folic acid. The following enzymes involved in tetrahydrofolate metabolism will be studied: Formyltetrahydrofolate synthetase, methenyltetrahydrofolate cyclohydrolase, methylenetetrahydrafolate dehydrogenase and methylenetetrahydrofolate reductase. The research deals with the physical structures, the catalytic mechanisms and the control mechanisms of these enzymes. The acetate synthesis from CO2 also includes the transfer of a methyl group from methyltetrahydrofolate to a B12-protein as well as transcarboxylation of the latter with pyruvate. The elucidation of the mechanisms of these reactions is part of the project. Finally the role of selenium molybdenum and tungsten, in the reduction of CO2 to formate, which is the first step in the acetate synthesis, will be examined.