Structural biology and regulation of the plant sulfation pathway

In plants, sulfur is an essential nutrient that must be converted into usable metabolic forms for the formation of sulfur-containing amino acids and peptides (primary route) and for the modification of diverse metabolites (secondary route). In plants, the fate of assimilated sulfate depends on the three enzymes – ATP sulfurylase, adenosine-5′-phosphate (APS) reductase, and APS kinase - that form a branchpoint in the pathway. ATP sulfurylase catalyzes the formation of the critical intermediate APS, which can either be used in the primary assimilatory route or be phosphorylated to 3′-phospho-APS (PAPS) for a variety of sulfation reactions. Recent biochemical and structural studies of the branchpoint enzymes in plant sulfur metabolism suggest that redox-regulation may control sulfur partitioning between primary and secondary routes. Disulfide-based redox switches differentially affect APS reductase and APS kinase. Oxidative conditions that promote disulfide formation increase the activity of APS reductase and decreases PAPS production by APS kinase. Here we review recent work on the ATP sulfurylase and APS kinase from plants that provide new insight on the regulation of PAPS formation, the structural evolution of these enzymes in different organisms, and redox-control of this key branchpoint in plant sulfur metabolism.