How metals work together to use nitrogenase to weaken nitrogen

Nitrogen is an essential element for all living cells, accounting for about 78% of the earth's atmosphere. However, most organisms cannot use this nitrogen until it is converted to ammonia. Before the industrial process of ammonia synthesis was invented by human beings, almost all ammonia on the earth was produced by microorganisms using nitrogenase. Nitrogenase is the only enzyme that can destroy the nitrogen nitrogen bond found in gaseous dinitrogen or N2.
These enzymes contain metal and sulfur clusters that contribute to this key reaction, but the mechanism by which they do so is unclear. MIT chemists have now determined for the first time the structure of the complexes formed when N 2 binds to these clusters, and they have found that these clusters can weaken the N-N bond to a surprising extent.
"This study gives us insight into the mechanisms that enable you to activate this truly inert molecule, which has very strong bonds that are hard to break," said Daniel suess, 48, assistant professor of career development chemistry. MIT and senior author of the study.
Alex McSkimming is a former postdoctoral fellow at MIT and an assistant professor at Durham University. He is the main author of the paper, which is published in nature chemistry today.
Nitrogen fixation is an important part of protein, DNA and other biological molecules. In order to extract nitrogen from the atmosphere, early microorganisms evolved nitrogenase, which converts nitrogen into ammonia (NH3) through a process called nitrogen fixation. Cells can then use this ammonia to build more complex nitrogen compounds.
"The ability to capture fixed nitrogen on a large scale helps to promote the proliferation of life," sous said“ Dinitrogen has very strong bonds and is very inactive, so chemists basically think it is an inert molecule. It's a problem that life has to solve: how to turn these inert molecules into useful chemicals. "
All nitrogenases contain a cluster of iron and sulfur atoms, some of which also contain molybdenum. Dinitrogen is believed to bind to these clusters to initiate the conversion to ammonia. However, the nature of this interaction is not clear. Until now, scientists have not been able to characterize the binding of N2 with iron sulfur clusters.
To illustrate how nitrogenase binds to N 2, chemists have designed simpler iron sulfur clusters, which they can use to mimic natural clusters. The most active nitrogenase uses an iron sulfur cluster with seven iron atoms, nine sulfur atoms, one molybdenum atom and one carbon atom. In this study, the MIT team created a method with three iron atoms, four sulfur atoms, one molybdenum atom and no carbon atom.
One challenge in trying to simulate the natural binding of dinitrogen to iron sulfur clusters is that when these clusters are in solution, they can react with themselves rather than with substrates such as dinitrogen. To overcome this problem, sous and his students created a protective environment around the clusters by connecting chemical groups called ligands.
In addition to one iron atom, the researchers attached a ligand to each metal atom, where N2 binds to the cluster. These ligands prevent unwanted reactions and allow dinitrogen to enter the cluster and bind to one of the iron atoms. Once this combination occurs, researchers can use X-ray crystallography and other techniques to determine the structure of the complex.
They also found that the triple bond between the two nitrogen atoms of N 2 was weakened to a surprising degree. When iron atoms transfer most of the electron density to the nitrogen nitrogen bond, this weakening occurs, which greatly reduces the stability of the bond.
Cluster cooperation
Another surprising finding is that all the metal atoms in the cluster contribute to this electron transfer, not just the iron atoms bound to dinitrogen.
"This suggests that these clusters can electronically activate this inert bond," sous said“ The nitrogen nitrogen bond can be weakened by the iron atom, otherwise it will not be weakened. Because they are in a cluster, they can work together. "
Theodore betley, Dean of the Department of chemistry and chemical biology at Harvard University, who was not involved in the study, said the findings represent "an important milestone in iron sulfur cluster chemistry.".
"Although it is known that the nitrogenase that immobilizes atmospheric nitrogen consists of fused iron sulfur clusters, it is only now that synthetic chemists have been able to demonstrate the absorption of dinitrogen using synthetic analogues," bateley said“ This work is a great progress for the iron sulfur cluster and the whole bioinorganic chemist. Most importantly, this progress indicates that there are abundant chemical reactions in iron sulfur clusters, which have not been found yet
The researchers' findings also confirm that simpler iron sulfur clusters, such as the ones they created for this study, can effectively weaken nitrogen bonds. The first microbes to develop nitrogen fixation capabilities may have evolved similar types of simple clusters, suess said.
Sue and his students are now studying how more complex, naturally occurring iron sulfur clusters interact with dinitrogen, funded by the MIT research support Council.
MIT chemists have identified the structure of the complex formed when gaseous dinitrogen or N2 combines with iron sulfur clusters, providing clues to how microorganisms (yellow) use nitrogenase to break nitrogen nitrogen bonds.