This enzyme, coenzyme F420-dependent sulfite reductase, or Fsr, "uses an unusual coenzyme ?a deazaflavin molecule called Factor 420 -- as an electron carrier for the reduction of sulfite. None of the previously described sulfite reductases use F420," Johnson said.
By use of genome-sequence-driven proteomics techniques, they identified the gene for the enzyme. A search showed that this gene exists only in hydrothermal vent methanogens and their close relatives, but not in other microorganisms.
From the sequence of the fsr gene, Johnson and Mukhopadhyay discovered that the novel activity of Fsr comes from a unique structure; two previously known proteins with unrelated functions have been physically combined by use of a linker. Even after this linking, the two units retain their individual characteristics.
"We hypothesize that the NH2-terminal half of Fsr (named Fsr-N) collects electrons via F420 and the COOH-terminal half (Fsr-C) uses those electrons to reduce sulfite to sulfide," Johnson said.
In their experiments, the researchers detected both of these individual properties as well as the combined activity. "Fsr-N resembles a protein that introduces electrons into the membrane-based energy transduction systems of certain archaea. Such an energy transduction system is also found in E. coli and humans," Mukhopadhyay said. "Fsr-C is similar to the sulfite reductases that are found in certain bacteria and archaea. These previously described sulfite reductases do not use coenzyme F420 as the electron source and are also not tethered to their electron-donating partners."
"The existence of Fsr poses several questions that are important in the context of evolution of metabolism and enzyme mechanism," Mukhopadhyay said. "We do not know whether the splitting of the fsr gene gave rise to the sulfite reductases of the bacteria and energy transducers of certain archaea or if