FORMATE IS THE HYDROGEN DONOR FOR THE ANAEROBIC RIBONUCLEOTIDE REDUCTASE FROM ESCHERICHIA-COLI

During anaerobic growth Escherichia coli uses a specific ribonucleosid e triphosphate reductase (class III enzyme) for the production of deox yribonucleoside triphosphates. In its active form, the enzyme contains an iron-sulfur center and an oxygen-sensitive glycyl radical (Gly-681 ). The radical is generated in the inactive protein from S-adenosylmet hionine by an auxiliary enzyme system present in E. coli. By modificat ion of the previous purification procedure, we now prepared a glycyl r adical-containing reductase, active in the absence of the auxiliary re ducing enzyme system. This reductase uses formate as hydrogen donor in the reaction. During catalysis, formate is stoichiometrically oxidize d to CO2, and isotope from [H-3] formate appears in water. Thus E. col i uses completely different hydrogen donors for the reduction of ribon ucleotides during anaerobic and aerobic growth. The aerobic class I re ductase employs redox-active thiols from thioredoxin or glutaredoxin t o this purpose. The present results strengthen speculations that class III enzymes arose early during the evolution of DNA.