Structural characterization of myo-inositol monophosphatase from bovine brain by secondary structure prediction, fluorescence, circular dichroism and Raman spectroscopy

Inhibition of myo-inositol monophosphatase isoforms by aromatic phosphonates

alpha-Hydroxyphosphonates are moderately potent (Ki = 6-600 microM) inhibitors of the enzyme myo-inositol monophosphatase (McLeod et al., Med. Chem. Res. 1992, 2, 96). Hydroxy-[4-(5,6,7,8-tetrahydronaphtyl-1-oxy)phenyl]methyl phosphonate (3) was resynthesized and its inhibitory potency towards the recombinant bovine brain enzyme confirmed (Ki = 20 microM). Similar aromatic difluoro-, keto-, and ketodifluorophosphonates (5, 7, 9) were inactive. Compound 3 was 15-fold less active on the human as compared to the bovine enzyme. Molecular modeling suggested that the hydrophobic part of the inhibitor interacts with amino acid side chains that are located at the interface between the enzyme subunits in an area (amino acids 175-185) with low similarity between the two isozymes. Phe-183 in the human enzyme was replaced with leucine, the corresponding residue in the bovine isoform. The three isozymes (human wild-type, bovine wild-type and human F183L) had similar kinetic properties, except that the bovine enzyme was less effectively inhibited by high concentrations of the activator Mg2+. The F183L mutant enzyme had a twofold increased affinity for compound 3 as compared to the human wild-type form. We conclude that residue 183 contributes to the binding of aromatic hydroxyphosphonates to IMPase, but it is not the only determining factor for inhibitor specificity with respect to different isozymes.

Detection of metal binding to bovine inositol monophosphatase by changes in the near and far ultraviolet regions of the CD spectrum

Mg2+ ions, essential for the catalytic activity of mammalian inositol monophosphatase, increase the ellipticity in the near-ultraviolet region of the CD spectrum of the enzyme. These spectral changes are not affected by the additional presence of substrate and are reversed if EDTA is added to the solution of enzyme and metal ions. Titration of the spectral perturbation at 275 nm shows that this binding occurs with a dissociation constant (Kd) around 275 microM, 292 microM and 302 microM for the wild-type, [Gln217]inositol monophosphatase and [Phe219]inositol monophosphatase enzymes respectively. The source of the spectroscopic change at 275 nm is not Trp219. The addition of Mg2+ also causes a decrease in ellipticity over most of the far-ultraviolet region of the spectrum (between 205-240 nm). The Kd values describing the binding of Mg2+ ions are 3.9 mM, 6.8 mM and 29.1 mM for the wild-type, [Gln217]inositol monophosphatase and [Phe219]inositol monophosphatase enzymes, respectively, each showing an approximate 12% change in ellipticity. In the additional presence of 10 mM Pi, there is a fourfold increase in the affinity of wild-type enzyme for Mg2+. It is concluded that CD spectral changes at wavelengths around 275 nm are indicative of metal ions interacting with a high-affinity metal-binding site (site 1). The spectral changes around 225 nm are associated with interactions at a lower-affinity site normally occupied by the Mg2+ ion which is reflected by the Km value for this metal ion. Other metal ions such as Ca2+ and Tb3+ (but not Mn2+ or Zn2+) also perturb the CD spectrum of the enzyme in both regions of the spectrum. The amplitudes of these signal changes are greater for Mg2+ or Tb3+ (25%) ions than for Ca2+ (8.5%), although two Ca2+-binding sites with Kd values of 20 microM and 100 microM have been identified. The uncompetitive inhibitor Li+ causes little change in the near-ultraviolet spectrum in the absence or presence of either substrate or Pi. However, in contrast to other metal ions, Li+ ions elicit a 10% increase in ellipticity at 220 nm with a Kd of 0.8 mM.

Reversible denaturation of myo-inositol monophosphatase. The stability of the metal-binding loop

The unfolding of bovine brain myo-inositol monophosphatase by guanidine. HCl (Gdn. HCl) has been investigated. The recovery of circular dichroism, emission spectra, and catalytic activity after dilution of Gdn.HCl-treated samples indicate that the overall process is reversible. The steepness of the spectroscopic changes between 3 M and 5 M Gdn.HCl, and the lack of any discernible plateau suggest that unfolding of the protein is a cooperative process. The sensitized luminescence of bound Tb(III) was used as a probe of conformational changes of the metal-binding loop. Denaturation of the enzyme by Gdn.HCl does not abolish sensitized luminescence. A 50% decrease in sensitized luminescence was observed in 5 M Gdn.HCl. Under this set of experimental conditions, the protein binds terbium with an association constant of 1 x 10(6)M-1. It is suggested that a residual structure of denatured myo-inositol monophosphatase is responsible for the binding of terbium ions. The kinetics of unfolding and refolding as a function of Gdn.HCl concentration were monitored by protein fluorescence in a stopped-flow instrument. The monophosphatase unfolded in a single kinetic phase with rate constants in the range 80-65 s-1 at 25 degrees C. The refolding kinetics fit monoexponential functions with rate constants in the range 120-65 s-1 depending on the Gdn.HCl concentration. Substantial refolding of the protein occurs within the dead time of mixing.

The contribution of lysine-36 to catalysis by human myo-inositol monophosphatase

The role of lysine residues in the catalytic mechanism of myo-inositol monophosphatase (EC 3.1.3.25) was investigated. The enzyme was completely inactivated by amidination with ethyl acetimidate or reductive methylation with formaldehyde and cyanoborohydride. Activity was retained when the active site was protected with Mg2+, Li+, and D,L-myo-inositol 1-phosphate. Using radiolabeling, peptide mapping, and sequence analysis, Lys-36 was shown to be the protected residue, which is responsible for inactivation. Replacing Lys-36 with glutamine produced a mutant protein, K36Q, with similar affinities for the substrate and the activator Mg2+, but a 50-fold lower turnover number as compared to the wild-type enzyme. Crystallographic studies did not indicate any gross structural changes in the mutant as compared to the native form. Initial velocity data were best described by a rapid equilibrium ordered mechanism with two Mg2+ binding before and a third one binding after the substrate. Inhibition by calcium was unaffected by the mutation, but inhibition by lithium was greatly reduced and became noncompetitive. The pH dependence of catalysis and the solvent isotope effect on kcat are altered in the mutant enzyme. D,L-myo-Inositol 1-phosphate, 4-nitrophenyl phosphate, and D-glucose 6-phosphate are cleaved at different rates by the wild-type enzyme, but with similar efficiency by K36Q. All data taken together are consistent with the hypothesis that modifying or replacing the lysine residue in position 36 decreases its polarizing effect on one of the catalytic metal ions and prevents the efficient deprotonation of the metal-bound water nucleophile.

7Li nuclear-magnetic-resonance study of lithium binding to myo-inositolmonophosphatase

The interaction of Li+ with myo-inositol monophosphatase was studied by 7Li-NMR spectroscopy. Li+ binding to the enzyme induces a downfield shift and broadening of the 7Li-NMR signal. Changes of the chemical shift were used to follow the titration of the enzyme with lithium and to determine a dissociation constant, Kd = (1.0 +/- 0.1) mM. Only one major binding site/enzyme subunit was inferred. The complex forms independently of the presence of inorganic phosphate. Metals from the group IIa of the periodic table compete with Li+ binding with the affinity increasing in the order Mg2+ < Ca2+ < Be2+. In contrast to lithium, their binding is enhanced by phosphate.

Kinetic characterization of enzyme forms involved in metal ion activation and inhibition of myo-inositol monophosphatase

Activation and inhibition of recombinant bovine myo-inositol monophosphatase by metal ions was studied with two substrates, D,L-inositol 1-phosphate and 4-nitrophenyl phosphate. Mg2+ and Co2+ are essential activators of both reactions. At high concentrations, they inhibit hydrolysis of inositol 1-phosphate, but not 4-nitrophenyl phosphate. Mg2+ is highly selective for inositol 1-phosphate (kcat. = 26 s-1) compared with the aromatic substrate (kcat. = 1 s-1), and follows sigmoid activation kinetics in both cases. Co2+ catalyses the two reactions at similar rates (kcat. = 4 s-1), but shows sigmoid activation only with the natural substrate. Li+ and Ca2+ are uncompetitive inhibitors with respect to inositol 1-phosphate, but non-competitive with respect to 4-nitrophenyl phosphate. Both metal ions are competitive inhibitors with respect to Mg2+ with 4-nitrophenyl phosphate as the substrate. With inositol 1-phosphate, Ca2+ is competitive and Li+ non-competitive with respect to Mg2+. Multiple inhibition studies indicate that Li+ and Pi can bind simultaneously, whereas no such complex was detected with Ca2+ and Pi. Several sugar phosphates were also characterized as substrates of myo-inositol monophosphatase. D-Ribose 5-phosphate is slowly hydrolysed (kcat. = 3 s-1), but inhibition by Li+ is very efficient (Ki = 0.15 mM), non-competitive with respect to the substrate and competitive with respect to Mg2+. Depending on the nature of the substrate, Li+ inhibits by preferential binding to free enzyme (E), the enzyme-substrate (E.S) or the enzyme-phosphate (E.Pi) complex. Ca2+, on the other hand, inhibits by binding to E and E.S, in competition with Mg2+. The results are discussed in terms of a catalytic mechanism involving two metal ions.

Biochemical and molecular properties of lithium-sensitive myo-inositol monophosphatase

Myo-inositol monophosphatase is a pivotal enzyme of the inositol second messenger system which is specifically inhibited by therapeutic levels of lithium salts, implicating inhibition of this enzyme as a potential site of its action in bipolar disease. This enzyme has a native molecular weight of 59,000, and has traditionally been found in the cytosolic fraction, although a membrane-bound form has also been identified. Possessing two identical subunits, this enzyme hydrolyzes those monophosphates which are equatorially located within the inositol ring, and several nucleoside monophosphates phosphorylated at the 2-position. Each subunit of the native enzyme contains an active site with unusually large caverns as revealed by crystallographic studies, which may explain the accommodation of these structurally unrelated substrates. We have suggested that the uncompetitive inhibition of this phosphatase by lithium ions may prevent the formation of an enzyme-bound non-isomeric (meso) intermediate, Mg(2+)-inositol 1,3 or 4,6 cyclic monophosphate when this enzyme hydrolyzes its respective isomeric substrates.