AMP transforms fructose-1,6-bisphosphatase from its active R-state to its inactive T-state. This quaternary transition involves movements of different regions on the molecule to affect an R- to T-state transition. The mechanism of transformation is poorly understood. The mutation of Ala54[right pointing arrow] Leu destabilizes the T-state of fructose-1,6-bisphosphatase. In the absence of AMP, the Leu54 enzyme adopts an R-state conformation, but the mutant enzyme grows in two crystal forms in the presence of AMP, a T-like and an R-like conformation. The latter structure is the first example of an AMP bound R-like quaternary state providing insight into the initial changes in helices H1 and H2 brought about by the sequential binding of AMP to the R-state. The structure enabled us to propose a model for R- to T-state transition and AMP cooperativity. The mutation of Ile10[right pointing arrow] Asp destabilizes the hydrophobic packing in the centre of R-state and the disengaged loop conformation of T-state fructose-1,6-bisphosphatase. Crystals of AMP/Mg2+ and AMP/Zn2+ complexes of the Asp10 enzyme are in T-like conformations, the latter providing the first example of an engaged dynamic loop in a T-like quaternary state. The AMP molecules push into helix H1 moving it toward the center of the tetramer and forces loop 190 into possible contact with the dynamic loop from a neighboring subunit. Movement of this dynamic loop triggers a conformational change in residues 264--274 transferring an unfavorable nonbonded contact within the R-state subunit to the distal end of the engaged loop. The aforementioned contact could be a significant driving force in the displacement of the dynamic loop from its engaged conformation resulting in complete inhibition. The Ile10 structure reveals a concerted model for transition from R- to T-state.