Allosteric transition of fructose-1,6-bisphosphatase.

Structural changes during the R-to-T transition of fructose-1,6-bisphosphatase (EC 3.1.3.11) form a hierarchy, in which structural changes at one level are supported by those at the other levels. The quaternary conformational changes involve a 17 degrees rotation between the upper and lower dimers, and a 3.4 degrees rotation between monomers in a dimer. Within monomers, the FBP domain, which remains rigid during the R-to-T transition, rotates 2.3 degrees relative to the AMP domain, which undergoes significant structural reorientations. The most important of these reorientations are the newly identified partially ordered loop residues 55-61 in the T state and reorientations of helices H1, H2, and H3. Supporting these structural changes are numerous readjustments of hydrogen bonding and van der Waals interactions throughout the entire tetrameric protein. Propagation of structural changes during the R-to-T transition relies primarily on helices H1, H2, H3, and loop 50-72. The change that begins at the AMP site causes reorientation of H1, H2, and H3 and changes of interactions across the C1-C4 (C2-C3) interface. These changes may propagate down H1, H2, H3, and loop 50-72 to affect interactions across the C1-C2 (C3-C4) and C1-C3 (C2-C4) interfaces. AMP inhibition is most probably caused by reduced metal binding affinity due to structural changes of metal ligands (Glu97, Asp118, and Asp121) in the active site. The eight-stranded beta-sheet, particularly the beta-strand B3, which connects Lys112 and Tyr113 of the AMP site with Asp118 and Asp121 of the metal site, may be responsible for communication between the AMP and active sites. Additional structural changes that support such communication include reorientation of the FBP domain and H1, H2, and H3 relative to the eight-stranded beta-sheet, and new conformations of loop 54-72 in the T state as AMP binds.