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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.
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