Thu, Jan. 21, 2016, 3:15pm - 4:15pm
Frick Chemistry Laboratory, Taylor Auditorium
Host: John Groves
Biosynthesis of the [FeFe]-Hydrogenase Active Site
[FeFe] hydrogenases reversibly interconvert protons and H2 and are the most rapid H2-producing enzymes. Their active sites are comprised of a conventional [4Fe–4S] subcluster coupled to an organometallic, diiron (“[2Fe]H”) subcluster that features multiple CO and CN– ligands. Although chemists are adept at synthesizing the [2Fe]H subcluster and related model complexes, how Nature performs this formidable task is largely unknown. Unraveling the mechanism of [2Fe]H-subcluster biosynthesis is important for developing biohydrogen fuel technologies and presents fresh challenges in the coordination chemistry, radical chemistry, and biochemistry of Fe-S enzymes. My seminar will focus on the reaction chemistry performed by the radical SAM enzyme HydG, one of three maturase enzymes that build the [2Fe]H subcluster. The role of HydG is particularly interesting because it synthesizes CO and CN–, and it also assembles the first organometallic precursors to the [2Fe]H subcluster. Characterization of intermediates using EPR (CW and pulse), time-resolved FTIR, and Mössbauer spectroscopies will be presented with an emphasis on the mechanisms of their formation and release. A primary topic of the seminar will be the surprising finding that free l-cysteine (Cys) functions as the ligand platform on which the first [Fe(CO)x(CN)y] intermediates are assembled. These results support a proposal in which HydG initially binds an inorganic [(Cys)Fe] moiety and releases an organometallic [(Cys)Fe(CO)2(CN)]– product. This model addresses and unifies the many contradictory reports regarding the metallocluster composition of HydG and the role of HydG in [2Fe]H subcluster biosynthesis.