Fundamental chemical transformations for biogeochemical cycling of sulfur and nitrogen are catalyzed by sulfite and nitrite reductases. The crystallographic structure of Escherichia coli sulfite reductase hemoprotein (SiRHP), which catalyzes the concerted six-electron reductions of sulfite to sulfide and nitrite to ammonia, was solved with multiwavelength anomalous diffraction (MAD) of the native siroheme and Fe4S4 cluster cofactors, multiple isomorphous replacement, and selenomethionine sequence markers ...
Fundamental chemical transformations for biogeochemical cycling of sulfur and nitrogen are catalyzed by sulfite and nitrite reductases. The crystallographic structure of Escherichia coli sulfite reductase hemoprotein (SiRHP), which catalyzes the concerted six-electron reductions of sulfite to sulfide and nitrite to ammonia, was solved with multiwavelength anomalous diffraction (MAD) of the native siroheme and Fe4S4 cluster cofactors, multiple isomorphous replacement, and selenomethionine sequence markers. Twofold symmetry within the 64-kilodalton polypeptide generates a distinctive three-domain alpha/beta fold that controls cofactor assembly and reactivity. Homology regions conserved between the symmetry-related halves of SiRHP and among other sulfite and nitrite reductases revealed key residues for stability and function, and identified a sulfite or nitrite reductase repeat (SNiRR) common to a redox-enzyme superfamily. The saddle-shaped siroheme shares a cysteine thiolate ligand with the Fe4S4 cluster and ligates an unexpected phosphate anion. In the substrate complex, sulfite displaces phosphate and binds to siroheme iron through sulfur. An extensive hydrogen-bonding network of positive side chains, water molecules, and siroheme carboxylates activates S-O bonds for reductive cleavage.
Related Citations:
Structures of the Siroheme-and Fe4S4-Containing Active Center of Sulfite Reductase in Different States of Oxidation: Heme Activation Via Reduction-Gated Exogenous Ligand Exchange Crane, B.R., Siegel, L.M., Getzoff, E.D. (1997) Biochemistry 36: 12101
Multiwavelength Anomalous Diffraction of Sulfite Reductase Hemoprotein: Making the Most of MAD Data Crane, B.R., Bellamy, H., Getzoff, E.D. (1997) Acta Crystallogr D Biol Crystallogr 53: 8
Determining Phases and Anomalous-Scattering Models from the Multiwavelength Anomalous Diffraction of Native Protein Metal Clusters. Improved MAD Phase Error Estimates and Anomalous-Scatterer Positions Crane, B.R., Getzoff, E.D. (1997) Acta Crystallogr D Biol Crystallogr 53: 23
Characterization of the Cysjih Regions of Salmonella Typhimurium and Escherichia Coli B. DNA Sequences of Cysi and Cysh and a Model for the Siroheme-Fe4S4 Active Center of Sulfite Reductase Hemoprotein Based on Amino Acid Homology with Spinach Nitrite Reductase Ostrowski, J., Wu, J.Y., Rueger, D.C., Miller, B.E., Siegel, L.M., Kredich, N.M. (1989) J Biol Chem 264: 15726
The Heme and Fe4S4 Cluster in the Crystallographic Structure of Escherichia Coli Sulfite Reductase Mcree, D.E., Richardson, D.C., Richardson, J.S., Siegel, L.M. (1986) J Biol Chem 261: 10277
Organizational Affiliation:
Department of Molecular Biology, Scripps Research Institute, La Jolla, CA 92037, USA.
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