期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：5
页码：1362-1367
DOI：10.1073/pnas.1417252112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceHydrogenases are a source of environmentally benign bioenergy, catalyzing the reversible reduction of protons to form hydrogen. The most active subclass, the [FeFe]-hydrogenases, is dependent on a metallocofactor, the H cluster, which contains iron-bound CO and CN- ligands. Although the HydG maturase is known to catalytically form a CO- and CN--bound iron precursor to the H cluster, mechanistic insight into this complex process has been hampered by the lack of structural information about HydG. We now describe the high-resolution crystal structure and EPR analysis of HydG. These results reveal a previously unreported [5Fe-5S] cluster that features a labile iron center proposed to provide the site of formation for a labile Fe(CO)2CN synthon, the precursor of the diiron subcluster hydrogenase H cluster. Hydrogenases use complex metal cofactors to catalyze the reversible formation of hydrogen. In [FeFe]-hydrogenases, the H-cluster cofactor includes a diiron subcluster containing azadithiolate, three CO, and two CN- ligands. During the assembly of the H cluster, the radical S-adenosyl methionine (SAM) enzyme HydG lyses the substrate tyrosine to yield the diatomic ligands. These diatomic products form an enzyme-bound Fe(CO)x(CN)y synthon that serves as a precursor for eventual H-cluster assembly. To further elucidate the mechanism of this complex reaction, we report the crystal structure and EPR analysis of HydG. At one end of the HydG ({beta})8 triosephosphate isomerase (TIM) barrel, a canonical [4Fe-4S] cluster binds SAM in close proximity to the proposed tyrosine binding site. At the opposite end of the active-site cavity, the structure reveals the auxiliary Fe-S cluster in two states: one monomer contains a [4Fe-5S] cluster, and the other monomer contains a [5Fe-5S] cluster consisting of a [4Fe-4S] cubane bridged by a 2-sulfide ion to a mononuclear Fe2+ center. This fifth iron is held in place by a single highly conserved protein-derived ligand: histidine 265. EPR analysis confirms the presence of the [5Fe-5S] cluster, which on incubation with cyanide, undergoes loss of the labile iron to yield a [4Fe-4S] cluster. We hypothesize that the labile iron of the [5Fe-5S] cluster is the site of Fe(CO)x(CN)y synthon formation and that the limited bonding between this iron and HydG may facilitate transfer of the intact synthon to its cognate acceptor for subsequent H-cluster assembly.
关键词：radical SAM enzyme ; tyrosine lyase ; H-cluster biosynthesis
