Illustration of Fsr’s catalytic website the place sulfite will get lowered to sulfide. The siroheme (in pink) that binds and converts the sulfite is embedded in a cavity of the protein (grey floor) which is solvent accessible. This means, the sulfite can simply enter the protein and the produced sulfide can depart it. Credit: Max Planck Institute for Marine Microbiology
Researchers on the Max Planck Institute for Marine Microbiology have uncovered how a methane-producing microbe thrives on poisonous sulfite with out changing into poisoned.
Methanogens are tiny organisms that generate methane in an oxygen-deprived setting. Their manufacturing of methane, equivalent to within the digestive system of ruminants, performs a major function within the world carbon cycle as methane is a extremely potent greenhouse fuel. However, methane may also function an power supply for heating houses.
A poisonous base for progress
The object of the examine now revealed in Nature Chemical Biology are two marine heat-loving methanogens: Methanothermococcus thermolithotrophicus (lives in geothermally heated sediments at round 65 °C) and Methanocaldococcus jannaschii (prefers deep-sea volcanos with round 85 °C).
They receive their mobile power by producing methane and obtain sulfur for progress in type of sulfide, that’s current of their environments.
While sulfide is a poison for many organisms, it’s important for methanogens they usually can tolerate even excessive concentrations of it. However, their Achilles’ heel is the poisonous and reactive sulfur compound sulfite, which destroys the enzyme wanted to make methane.
In their environments, each investigated organisms are sometimes uncovered to sulfite, for instance, when oxygen enters and reacts with the lowered sulfide. Its partial oxidation leads to the formation of sulfite, and thus the methanogens want to guard themselves. But how can they do that?
Marion Jespersen with the purified F420-dependent sulfite reductase (Fsr). The black colour comes from all of the iron concerned within the response. Experiments are carried out in an anaerobic chamber and beneath synthetic gentle to guard the enzyme from oxygen and daylight. Credit: Tristan Wagner/Max Planck Institute for Marine Microbiology
A molecular snapshot of the method
Marion Jespersen and Tristan Wagner from the Max Planck Institute for Marine Microbiology in Bremen, Germany, along with Antonio Pierik from the University of Kaiserslautern, now present a snapshot of the enzyme detoxifying the sulfite. This butterfly-shaped enzyme is called the F420-dependent sulfite reductase or Fsr. It is able to turning sulfite into sulfide – a secure supply of sulfur that the methanogens require for progress.
In the present examine, Jespersen and her colleagues describe how the enzyme works. “The enzyme traps the sulfite and straight reduces it to sulfide, which may be included, for instance, into amino acids”, Jespersen explains, “As a result, the methanogen doesn’t get poisoned and even uses the product as its sulfur source. They turn poison into food!”
It sounds simple. But in fact, Jespersen and her colleagues found that they were dealing with a fascinating and complicated overlap. “There are two ways of sulfite reduction: dissimilatory and assimilatory”, Jespersen explains. “The organism under study uses an enzyme that is built like a dissimilatory one, but it uses an assimilatory mechanism. It combines the best of both worlds, one could say, at least for its living conditions.”
It is assumed that the enzymes from both the dissimilatory and the assimilatory pathways have evolved from one common ancestor. “Sulfite reductases are ancient enzymes that have a major impact on the global sulfur and carbon cycles”, adds Tristan Wagner, head of the Max Planck Research Group Microbial Metabolism at the Max Planck Institute in Bremen. “Our enzyme, the Fsr, is probably a snapshot of this ancient primordial enzyme, an exciting look back in evolution.”
Biotechnological applications in view
The Fsr not only opens up evolutionary implications but also allows us to better understand the fascinating world of marine microbes. Methanogens that can grow only on sulfite circumvent the need to use the dangerous sulfide, their usual sulfur substrate.
“This opens opportunities for safer biotechnological applications to study these important microorganisms. An optimal solution would be to find a methanogen that reduces sulfate, which is cheap, abundant, and a completely safe sulfur source”, says Wagner.
In fact, this methanogen already exists, it is Methanothermococcus thermolithotrophicus. The researchers hypothesized that Fsr orchestrates the last reaction of this sulfate reduction pathway because one of its intermediates would be sulfite.
“Our next challenge is to understand how it can transform sulfate to sulfite, to get a complete picture of the capabilities of these miracle microbes.”
Reference: “Structures of the sulfite detoxifying F420-dependent enzyme from Methanococcales” by Marion Jespersen, Antonio J. Pierik and Tristan Wagner, 19 January 2023, Nature Chemical Biology.
DOI: 10.1038/s41589-022-01232-y
