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Siroheme is the central cofactor in a conserved class of sulfite and nitrite reductases that catalyze the six-electron reduction of sulfite to sulfide and nitrite to ammonia. In Salmonella enterica serovar Typhimurium, siroheme is produced by a trifunctional enzyme, siroheme synthase (CysG). A bifunctional active site that is distinct from its methyltransferase activity catalyzes the final two steps, NAD + -dependent dehydrogenation and iron chelation. How this active site performs such different chemistries is unknown. Here, we report the structures of CysG bound to precorrin-2, the initial substrate; sirohydrochlorin, the dehydrogenation product/chelation substrate; and a cobalt-sirohydrochlorin product. We identified binding poses for all three tetrapyrroles and tested the roles of specific amino acids in both activities to give insights into how a bifunctional active site catalyzes two different chemistries and acts as an iron-specific chelatase in the final step of siroheme synthesis.
Organizational Affiliation:
Department of Biological Science and Institute of Molecular Biophysics, Florida State University, 91 Chieftan Way, Tallahassee, FL, 32306, USA.
Department of Molecular Medicine, University of South Florida College of Medicine, 12901 Bruce B. Downs Blvd, MDC 3522, Tampa, FL, 33612, USA.
Department of Biological Science and Institute of Molecular Biophysics, Florida State University, 91 Chieftan Way, Tallahassee, FL, 32306, USA. mestroupe@bio.fsu.edu.
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