Distances between Structural Metal Ion, Substrates, and Allosteric Modifier of Fructose Bisphosphatase

The binding of two paramagnetic probes within a subunit of fructose bisphosphatase, viz., Mn2+ at a structural site and a nitroxide spin-label at a sulfhydryl site, has permitted the measurement of NMR and electron paramagnetic resonance (EPR) relaxation rates to map the active and allosteric site topography. Distances from these loci to the phosphoryl of fructose 6-phosphate (Fru-6-P) and inorganic phosphate (Pi) and four nuclei of adenosine 5'-phosphate (AMP) (the phosphorus nucleus, H-8, FI-2, and H-V) were obtained. These measurements located the Mn2+ approximately equidistant from the two phosphoryl moieties of the product ligands Fru-6-P and Pi and in close proximity to the AMP. The adenosine moiety of the latter is oriented anti. Analysis of EPR data revealed that the nitroxide group is ~16Å from the structural Mn2+ site. Calculation of the residence times for the hydrolysis reaction products suggests that their dissociation should not be rate limiting in the overall reaction cycle.