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Structure and mechanism of soybean ATP sulfurylase and the committed step in plant sulfur assimilation

Jonathan Herrmann, Geoffrey E. Ravilious, Samuel E. McKinney, Corey S. Westfall, Soon Goo Lee, Patrycja Baraniecka, Marco Giovannetti, Stanislav Kopriva, Hari B. Krishnan, Joseph M. Jez
Journal of Biological Chemistry 2014 v.289 no.15 pp. 10919-10929
Arabidopsis, Glycine max, X-radiation, adenosine triphosphate, catalytic activity, crystal structure, dimerization, mutants, mutation, nutrient content, soybeans, sulfate adenylyltransferase, sulfur
Enzymes of the sulfur assimilation pathway are potential targets for improving nutrient content and environmental stress responses in plants. The committed step in this pathway is catalyzed by ATP sulfurylase, which synthesizes adenosine-5'-phosphosulfate (APS) from sulfate and ATP. To better understand the molecular basis of this energetically unfavorable reaction, the 2.48 Å resolution x-ray crystal structure of ATP sulfurylase from soybean (GmATPS) in complex with APS was determined. This structure revealed several highly conserved substrate-binding motifs in the active site and a distinct dimerization interface compared to other ATP sulfurylases. Steady-state kinetic analysis of twenty GmATPS point mutants suggests a reaction mechanism in which nucleophilic attack by sulfate on the a-phosphate of ATP requires bending of the nucleotide phosphate backbone into a "U"-shape and stabilization of the transition state by Arg248, Asn249, His255, and Arg349. This analysis indicates that ATP sulfurylase overcomes the energetic barrier of APS synthesis by distorting nucleotide structure and identifies critical residues for catalysis. Mutations that alter sulfate assimilation in Arabidopsis were mapped to the structure and provide a molecular basis for understanding their effects on the sulfur assimilation pathway.