UC Davis

Many enzymes utilize metallocofactors to catalyze chemical transformations important to creating a sustainable world. Our research focuses on using electron paramagnetic resonance (EPR) spectroscopy as our main tool to study these metallocofactors, and the transformations that they catalyze. My research has more specifically involved the bio-assembly of the active site found in [FeFe]-hydrogenase enzymes. [FeFe]-hydrogenases are the most active enzymes in the production of molecular hydrogen. Their energetic efficiency and capacity to reduce protons to form H2 makes hydrogenases important research targets in this context of renewable energy and sustainability. These [FeFe]-hydrogenase enzymes contain an active site H-cluster, consisting of a [4Fe-4S]H cluster linked to a [2Fe]H subcluster with CO, CN- and azadithiolate ligands. The research presented here focuses on understanding the biosynthesis of this H-cluster active site, and the role that the radical S-adenosyl-L-methionine (SAM) maturase enzyme HydG plays. The overall HydG reaction converts tyrosine, cysteine, and Fe(II) to the [Fe(II)(CN)(CO)2(cysteinate)]- product, Complex B. This HydG product is a diamagnetic (S = 0) low spin Fe(II) complex and is thus undetectable by EPR spectroscopy. We chose to use 57Fe Mössbauer spectroscopy to probe the chemical environment of the Complex B Fe, even in its diamagnetic state. I observed evidence of the product Fe species, during HydG turnover by 57Fe Mössbauer spectroscopy. With low isomer shifts and small quadrupolar splitting, the results are consistent with the expected low spin ferrous product of Complex B. I was able to observe this species in both fully and selectively 57Fe labeled HydG experiments. The product Complex B subsequently serves as the substrate for the following radical SAM enzyme HydE, which activates the inert low spin Fe(II) complex for dimerization, either within HydE or on the protein HydF. This dimerization results in the formation of an Fe2S2(CO)4(CN)2 species needing only the addition of the CH2NHCH2 component of the adt bridge to complete the binuclear subcluster. In addition to the work with radical SAM maturase enzymes, I have worked on several collaborations through EPR spectroscopy. I present work in collaboration with John Arnold’s group at UC Berkeley, examining the electronic structure of Actinide compounds. We successfully observed a Uranium(III) species that was reduced to an EPR silent integer spin Uranium(II). I also present recent work in collaboration with Alan Balch’s group here at UC Davis, where we observed and characterized the formation of DABCO radical species.