Spontaneous subunit exchange in porcine liver fructose-1,6-bisphosphatase

No evidence to date suggests the possibility of subunit exchange between tetramers of mammalian fructose-1,6-bisphosphatase. An engineered fructose-1,6-bisphosphatase, with subunits of altered electrostatic charge, exhibits spontaneous subunit exchange with wild-type enzyme in the absence of ligands. The exchange process reaches equilibrium in approximately 5 h at 4 degrees C, as monitored by non-denaturing gel electrophoresis and anion exchange chromatography. Active site ligands, such as fructose 6-phosphate, abolish subunit exchange at the level of the monomer, but permit dimer-dimer exchanges. AMP, alone or in the presence of active site ligands, abolishes all exchange processes. Exchange phenomena may play a role in the kinetic mechanism of allosteric regulation of fructose-1,6-bisphosphatase.