The crystal structure of fructose-1,6-bisphosphatase (EC 3.1.3.11) complexed with fructose 6-phosphate, AMP, and Mg2' has been solved by the molecular replacement method and refined at 2.5-A resolution to a R factor of 0.215, with root-mean-square deviations of 0.013 A and 3.5°for bond lengths and bond angles, respectively. No solvent molecules have been included in the refinement. This structure shows large quaternary and tertiary conformational changes from the structures of the unligated enzyme or its fructose 2,6-bisphosphate complex, but the secondary structures remain essentially the same. Dimer C3-C4 of the enzyme-fructose 6-phosphate-AMP-Mg2+ complex twists about 190 relative to the same dimer of the enzyme-fructose 2,6-bisphosphate complex if their C1-C2 dimers are superimposed on one another. Nevertheless, many interfacial interactions between dimers of C1-C2 and C3-C4 are conserved after quaternary structure changes occur. Residues of the AMP domain (residues 6-200) show large migrations of Ca atoms relative to barely significant positional changes of the FBP domain (residues 201-335).Fructose-1,6-bisphosphatase (Fru-1,6-Pase; D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11), a key regulatory enzyme in gluconeogenesis, catalyzes the hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate (F6P) and inorganic phosphate. Fru-1,6-Pase isolated from various sources consisted offour identical polypeptide chains that aggregate into a relatively flat tetramer ( Fig. 1). Seven complete amino acid sequences have been reported for Fru-1,6-Pases from various sources (1-6). Recently, threedimensional structures of the unligated Fru-1,6-Pase and of its complex with fructose 2,6-bisphosphate (Fru-2,6-P2) in the space group P3221 were described in detail (7, 8*).The catalytic and regulatory properties of the enzyme isolated from gluconeogenic tissues as well as other sources have been extensively studied (9, 10). Omitting those forms that also have a phosphorylation site, the enzyme activity is regulated in vivo by Fru-2,6-P2 and AMP (9, 10). Kinetic experiments controversially have suggested that Fru-2,6-P2 binds to the active site (11), to an allosteric site (12), or to both (13). On basis of the structures, we proposed that Fru-2,6-P2 binds to the active site (7,8). AMP is an allosteric inhibitor (14), and its inhibition is synergistic with Fru-2,6-P2 (15, 16).We have grown cocrystals of the enzyme complexed with F6P, AMP, and Mg2+ in the space group P21212. This structuret has been solved by the molecular replacement method and shows large quaternary and tertiary conformational changes from the unligated structure or the structure of the enzyme-Fru-2,6-P2 complex. Here, we describe the F6P and AMP binding sites and large structural differences between the enzyme-F6P-AMP-Mg2+ complex and the enzyme-Fru-2,6-P2 complex.
METHODSFru-1,6-Pase was purified from pig kidney as described (8). It has an optimal activity at neutral pH. The neutral form of Fru-1,6-Pase was cocrystallized with 1 mM...