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Enzymology

A Green Answer to Heavy Metal

In the ongoing fight against pollution, plants offer a valuable contribution by collecting, transforming, or removing pollutants from soil or water—something called phytoremediation.
To counter heavy metal waste, for example, vegetal tools rely on molecules like nicotianamine (NA), a small molecule crucial for transporting the ionic forms of metals such as iron, zinc, and copper, throughout plant organs. But mystery surrounding how NA—which has proven aptitude for nickel detoxification—is synthesized, has so far prevented its potential from being fully explored. A team from IBEB1 working at the CEA's2 Cadarache research center, and from INRA3 in Montpellier has now unlocked the mechanism that produces NA, paving the way for new biological tools.4

green answer

© CEA

3D structure of the NA-Synthase from Methanothermobacter thermautotrophicus.



The key to NA synthesis lies in nicotianamine synthase (NA-Synthase), a plant enzyme that is extremely difficult to isolate. To overcome this obstacle, the researchers examined NA-Synthase-like enzymes in the archaea5 Methanothermobacter thermautotrophicus after noticing that “NA-Synthase in eukaryotes finds distant counterparts in certain archaea,” explains Pascal Arnoux from the CEA. They have thus uncovered the secret to NA's synthesis in the reaction chamber buried at the enzyme's heart.
So how exactly is NA made?
3D structure analysis of the NA-Synthase-like enzyme at work reveals that one by one, three molecules of the substrate S-adenosylmethionine penetrate a tiny opening to the enzyme's reaction chamber, to reach the active site where the reaction takes place. Once the first molecule is processed, it is pushed deeper into the chamber by the second molecule that replaces it in the active site. The two molecules then link together and move further into the chamber while the third molecule takes its turn in the active site. It is finally fused to the duo, completing the NA molecule.
With this finding, the design of NA-reliant biosensors for detecting heavy metals in the environment can now be optimized. “This result also paves the way for developing molecules related to NA as novel actors in the field of bio- and phyto-remediation,” indicates Arnoux.

Fui Lee Luk

Notes :

1. Institut de biologie environnementale et de biotechnologie (CNRS / CEA / Université Aix-Marseille II).
2. Commissariat à l'énergie atomique.
3. Institut national de la recherche agronomique.
4. C. Dreyfus et al., “Crystallographic snapshots of iterative substrate translocations during nicotianamine synthesis in archaea,” PNAS, 2009. 106: 16180-4.
5. Unicellular micro-organisms that thrive in extreme conditions.

Contacts :

Pascal Arnoux,
CEA, St Paul lez Durance.
pascal.arnoux@cea.fr


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