Structure and Mechanism of Soybean ATP Sulfurylase and the Committed Step in Plant Sulfur Assimilation [Protein Structure and Folding]

February 28th, 2014 by Herrmann, J., Ravilious, G. E., McKinney, S. E., Westfall, C. S., Lee, S. G., Baraniecka, P., Giovannetti, M., Kopriva, S., Krishnan, H. B., Jez, J. M.

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 x-ray crystal structure of ATP sulfurylase isoform 1 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, but similar to mammalian PAPS synthetase. Steady-state kinetic analysis of twenty GmATPS point mutants suggests a reaction mechanism in which nucleophilic attack by sulfate on the alpha-phosphate of ATP involves transition state stabilization by Arg248, Asn249, His255, and Arg349. The structure and kinetic analysis suggest 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, which provides a molecular basis for understanding their effects on the sulfur assimilation pathway.