Nature shares a research paper examining the supercomplex’s structure using cryo-electron microscopy.
Nature shared a research paper titled “Cryo-EM structure of a methanogen nitrogenase–PII protein supercomplex.”
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Researchers have mapped how a methane-producing archaeon can switch its nitrogen-fixing machinery into an inactive state. In a study published in Nature on Oct. 7, the team used cryo-electron microscopy to examine a native nitrogenase complex from Methanosarcina acetivorans. Nitrogenase is the enzyme that reduces atmospheric dinitrogen to ammonia, a reaction central to the global nitrogen cycle.
The structure showed three NifDK heterotetramers bridged by six NifI1,2 heterotrimeric PII complexes. Those PII complexes physically block NifH from associating with NifDK, locking the enzyme in an inactive arrangement.
That architecture fills in a gap between bacterial and archaeal nitrogen fixation. Bacterial nitrogenase has been studied extensively, but the authors say the structure and regulation of archaeal nitrogenases had remained unclear despite long-standing evidence that methanogens can fix nitrogen.
The six PII complexes did not all carry the same molecules in the same way. Their asymmetric binding of ADP and 2-oxoglutarate links the inhibitory structure to signals about the cell’s energy and nitrogen status, according to the study.
When the researchers added 2-oxoglutarate and ATP, the NifI complexes detached and NifDK activity increased threefold in vitro. That result supports a model in which PII proteins regulate methanogen nitrogenase by driving it into, or releasing it from, a larger assembly.
The experiment establishes a structural mechanism in one methanogen, not a universal rule for all nitrogen-fixing organisms. The authors say the finding creates new lines of inquiry into nitrogenase evolution and regulation, as well as possible biotechnological uses.
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