Structural and biochemical characterization of fructose-1,6/sedoheptulose-1,7-bisphosphatase from the cyanobacterium Synechocystis strain 6803.

Cyanobacterial fructose-1,6/sedoheptulose-1,7-bisphosphatase (cy-FBP/SBPase) plays a vital role in gluconeogenesis and in the photosynthetic carbon reduction pathway, and is thus a potential enzymatic target for inhibition of harmful cyanobacterial blooms. Here, we describe the crystal structure of cy-FBP/SBPase in complex with AMP and fructose-1,6-bisphosphate (FBP). The allosteric inhibitor AMP and the substrate FBP exhibit an unusual binding mode when in complex with cy-FBP/SBPase. Binding mode analysis suggested that AMP bound to the allosteric sites near the interface across the up/down subunit pairs C1C4 and C2C3 in the center of the tetramer, while FBP binds opposite to the interface between the horizontal subunit pairs C1C2 or C3C4. We identified a series of residues important for FBP and AMP binding, and suggest formation of a disulfide linkage between Cys75 and Cys99. Further analysis indicates that cy-FBP/SBPase may be regulated through ligand binding and alteration of the structure of the enzyme complex. The interactions between ligands and cy-FBP/SBPase are different from those of ligand-bound structures of other FBPase family members, and thus provide new insight into the molecular mechanisms of structure and catalysis of cy-FBP/SBPase. Our studies provide insight into the evolution of this enzyme family, and may help in the design of inhibitors aimed at preventing toxic cyanobacterial blooms.