Bovine inositol monophosphatase: enzyme-metal-ion interactions studied by pre-equilibrium fluorescence spectroscopy.
Stopped-flow fluorescence spectroscopy has been used to determine the on-rate (kass) and the off-rate (kdiss) for the equilibrium between inositol monophosphatase and Mg2+ ions. The dissociation constant (Kd) for the equilibrium calculated from these constants suggests that the ions interact at site 1 on the enzyme with a Kd typically around 450 microM, close to values determined by equilibrium studies (270-300 microM). The affinity of this site on the wild-type enzyme for Mg2+ ions increases as the pH is increased. This is mediated almost entirely by change in the rate kdiss. A slow increase occurs in the fluorescence intensity of the pyrene-labelled enzyme after the initial, fast, increase in fluorescence caused by the binding of the Mg2+ ion. The rate of this change is independent of the concentration of the metal ion, implying that it may be a structural change in the enzyme-Mg2+ complex. Neither the fast nor the slow change in fluorescence intensity occurs when enzyme subjected to limited proteolysis by trypsin, which removes the N-terminal 36 residues, is mixed with Mg2+ ions. The data suggest that interaction with Mg2+ ions at a high-affinity site leads to a structural change in inositol monophosphatase. The data further confirm the importance of the presence of two metal ions in the structure/function of this enzyme, and show that the binding of the metal ions is not competitive with that of H+ ions and that the variation in Kd with pH is mediated almost totally by changes in kdiss.
Thorne
M R
MR
Department of Biochemistry, University of Southampton, UK.
Greasley
P J
PJ
Gore
M G
MG
D000818:Animals; D001665:Binding Sites; D002417:Cattle; D004926:Escherichia coli / Q000235:genetics; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D015394:Molecular Structure; D010744:Phosphoric Monoester Hydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013050:Spectrometry, Fluorescence
D011994:Recombinant Proteins; D010744:Phosphoric Monoester Hydrolases; C021822:myo-inositol-1 (or 4)-monophosphatase; D008274:Magnesium