Science1 publisher2 min readPublished
A nitrogenase from a deep-sea archaeon makes ammonia only at high temperature
Tristan Wagner's group in Bremen purified the enzyme straight from Methanocaldococcus infernus, watched it hold together up to 98 C, and solved a near-atomic structure that mixes features of all three known nitrogenase families.
The Scientist · Science desk
What happened
- Tristan Wagner's laboratory at the Max Planck Institute for Marine Microbiology in Bremen forced the deep-sea archaeon Methanocaldococcus infernus to fix nitrogen gas at temperatures above 90 C.
- The nitrogenase they purified from it starts to fall apart only at 90 C, and some of the protein survived at 98 C, according to the study in Nature Communications.
- First author Nevena Maslac compared the conditions the enzyme tolerates to those in which most proteins would rapidly decay, like egg white cooked in hot water.
- Crystals studied at the Institut de Biologie Structurale in Grenoble gave a near-atomic-resolution view of the simplest nitrogenase known so far, mixing features of all three known families.
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Why it matters
- capability Maslac says the extreme stability let the group study states of the nitrogenase that are usually difficult to capture, so mechanism work now has a sample that tolerates handling instead of a short-lived state that has to be trapped during turnover.
- constraint Oxygen is a separate problem from heat: any biological or hybrid ammonia scheme still has to enclose a cofactor that air damages beyond repair.
- decision For a group choosing between engineering a room-temperature nitrogenase and starting from this archaeal one, the deciding number is an activity comparison, and this report does not supply it.
- precedent If ancestral nitrogenases resembled this archaeal enzyme more than the bacterial ones, evolutionary reconstructions built on bacterial molybdenum enzymes are working from the derived end of the family.
The enzyme has a lower temperature limit as well as an upper one. It begins to fall apart at 90 C, and some of it was still intact at 98 C [6], eight degrees past the point where unfolding starts [17]. The other end is room temperature. "It is not active at room temperature," first author Nevena Maslac said. "Rather, we show that it only produces ammonia at high temperatures." [7]
Heat is one way to destroy a nitrogenase. Oxygen is the other, and oxygen still destroys this one. Nitrogenase carries the most complex metallocofactor known in biology [9], and the Bremen group ran the physiology, the native purification, the biochemistry and the crystallography under strictly oxygen-free conditions to keep those cofactors from irreversible damage [11].
According to the phys.org account, the work does not report a rate of ammonia production or compare the enzyme with industrial synthesis [18]. It reports stability. An enzyme that had to be coaxed into fixing nitrogen above 90 C inside its own organism [4][5], and that has to be kept away from air [11], is material for mechanism work.
The structure is the simplest nitrogenase described so far, at near-atomic resolution, and it combines features of the molybdenum, vanadium and iron-only families [13]. "This type of nitrogenase could be similar to a common nitrogenase ancestor, the ancient system from which they all evolved," Wagner said [14]. The account presents the structure as support for that idea; Wagner said the enzyme could be similar to that ancestor, not that it is [13][14].
The cofactor turned out to be the molybdenum kind, the family the best-performing nitrogenases belong to [10][16]. Confirming that was the hard part. "Our search for the molybdenum was technically extremely challenging and required the experts at the synchrotron to push their instrument to its absolute limits," Wagner said [15]. The measurement at the Institut de Biologie Structurale in Grenoble returned the expected molybdenum signal plus a state that is a first for a molybdenum-containing nitrogenase, which the account calls a "turnover" state, potentially an intermediate step in the reaction [12][16].
What to watch
- A measured turnover rate at 90 C, alongside a molybdenum nitrogenase from a mesophile, would show whether the heat tolerance costs activity or comes free.
- Whether the 'turnover' state seen at Grenoble can be assigned to a specific intermediate, and whether anyone finds it in bacterial molybdenum nitrogenases.
- Whether the enzyme can be made in a laboratory host with an intact cofactor, since purification from a hyperthermophilic archaeon caps how much material other groups can get.