Toward a mechanism for the allosteric transition of pig kidney fructose-1,6-bisphosphatase.

We examine structural aspects of the allosteric transition of pig kidney fructose-1,6-bisphosphatase (Fru-1,6-Pase) by analyzing the X-ray structures of the R and T form enzymes. The results show a hierarchical structural change during the R to T transition. Upon binding of AMP, a cascade of structural changes occurs starting from the AMP site: expansion of the AMP site, local conformational changes of helices H1 and H2, independent rotations and translation of helices H1, H2 and H3 (and loops connecting them), reorganization of the AMP domain as a whole and its 1.9 degrees rotation relative to the fructose-1,6-bisphosphate domain, and conformational changes at the C1-C2 and C1-C4 interfaces leading to the quaternary conformational change of a 17 degrees rotation between dimers. The AMP inhibition results from the relative movement between the AMP and FBP domains which distorts the active site during the transition by shifting the metal binding sites to unfavourable positions. Communication that ensures cooperativity during R to T transition relies on changes in positions of helices H1, H2 and H3, loops 127-131, 168-170 and 187-192, and on N-terminal residues. All of these features are close to the C1-C4 and symmetry equivalent C2-C3 interfaces and the relatively small C1-C3 interface of the T form. These secondary structures form the framework along which structural changes due to AMP binding can propagate to other parts of the monomers as well as across monomer interfaces. Future dynamics studies may be useful to analyze initiation, propagation and completion of the quaternary conformational change of Fru-1,6-Pase upon AMP binding. Also, site directed mutagenesis experiments are expected to provide more detailed descriptions of the importance of each of the residues that has been identified here in the proposed mechanisms.