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Radical S-Adenosyl-l-Methionine Carbon Monoxide and Cyanide Synthase HydG

Yvain Nicolet


Crystal structures of [FeFe]-hydrogenases have revealed complex coordination of their active site [FeFe] motif. Besides a dithiomethylamine (DTMA) ligand that bridges the two iron ions through its thiolate groups, each metal has one CN− and one CO terminal ligand in addition to a metal-bridging CO. It is known that, in spite of its complexity, only three protein maturases called HydE HydF and HydG are both necessary and sufficient to assemble the hydrogenase active site. Previous work has also shown that HydF has GTPase activity and may be the scaffold where the Fe2(CO)3(CN)2(DTMA) unit is put together. Conversely, HydE is likely to be the site of DTMA synthesis because it is known that HydG makes the triplebonded ligands CO and CN−. Similar to HydE, HydG is a radical S-adenosyl, l-methionine (SAM) enzyme. HydG, the subject of this chapter, has l-tyrosine lyase activity and, as shown by electron paramagnetic resonance (EPR) spectroscopy, it cleaves this free aromatic amino acid into a p-cresyl radical and dehydroglycine (DHG). In addition to the expected radical, SAM-associated [4Fe-4S] cluster HydG has a second [FeS]s cluster located in its C-terminal domain where CO and CN− synthesis from DHG takes place. This cluster appears to be a regular [4Fe-4S] cuboid unusually connected to a catalytically relevant fifth iron. We and others have investigated the catalytic mechanism of HydG and found that, in vitro, some DHG can be hydrolyzed to glyoxylate. It was also shown that although HydG deprived of its [FeS]s cluster cannot synthesize CO it still makes some CN−; this shows that the cluster is not absolutely required for the synthesis of this ligand (but it may contribute to the stability of the site where it is produced). We also found that in variants lacking the [FeS]s cluster a significant amount of formate was detected in in vitro experiments. Taken together, these results have allowed us to proposed a catalytic mechanism where DHG, CN−, and CO are made at three different active sites.
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hal-01644311 , version 1 (22-11-2017)



Yvain Nicolet, Juan-Carlos Fontecilla-Camps. Radical S-Adenosyl-l-Methionine Carbon Monoxide and Cyanide Synthase HydG. Encyclopedia of Inorganic and Bioinorganic Chemistry, 2017, ⟨10.1002/9781119951438.eibc2506⟩. ⟨hal-01644311⟩
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