Structural analysis of the siroheme biosynthetic enzyme CysG, a central player in sulfur metabolism

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Heme is the iron-containing molecule that allows hemoglobin to carry oxygen in blood. Heme also plays many other important biochemical functions in cells. With this award, the Chemistry of Life Processes Program in the Chemistry Division is funding Dr. Elizabeth Stroupe at Florida State University to study a special form of heme (called siroheme). Siroheme helps chemically transform environmental sulfur or nitrogen compounds to biologically usable forms using microbes. This research topic is of particular importance because environmental pollution in the form of toxic sulfur byproducts from commercial pig farms has become problematic due to recent flooding in the southern United States. Dr. Stroupe is studying a key enzyme (CysG) made from two bacteria in order to determine the basic science of siroheme formation. By knowing how siroheme is formed, scientists may improve upon the bio-remediation of toxic sulfur compounds. The project trains diverse groups of undergraduate and graduate students in molecular structure determinations and follows the biochemical activity of enzymes at various stages of reaction. Dr. Stroupe and her students impart an appreciation for the role that organic chemistry plays in biology to middle and high school students through the SCIGirls and Florida's Next Generation Science Standards outreach programs.Proteobacteria use a single gene product (CysG) to synthesize siroheme. CsyG is the product of a gene fusion event between a methyltransferase (CysG-A) and a bifunctional dehydrogenase/ferrochelatase (CysG-B). The mechanisms for CysG-B's bifunctionality and its alternative function as a cobaltchelatase are unknown. Other organisms use three distinct enzymes for siroheme synthesis, including the CysG-B homolog SirC, which is a monofunctional dehydrogenase. X-ray crystallography, biochemical analysis, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) are applied to dissect the underlying mechanisms for the last two steps of siroheme synthesis. The questions addressed by this project include how bifunctional active sites in CysG-B catalyze dehydrogenation and metal chelation and how chelatase discriminates between iron and cobalt to make siroheme. Researchers also ask how CysG-B functions as a bifunctional dehydogenase/ferrochelatase when SirC, a homologous enzyme, only functions as a dehydrogenase.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.