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

Structural aspects of myo-inositol monophosphatase were examined by spectroscopic techniques and empirical prediction methods. The enzyme belongs to the alpha/beta class of proteins, with approx. 33% alpha-helix and 29% beta-sheet, as shown by circular dichroism (CD), Raman spectroscopy and prediction based on the amino-acid sequence. The Raman spectrum also suggests that the three tryptophan residues in myo-inositol monophosphatase are not exposed to solvent. This was confirmed by a blue shift of 25 nm in the fluorescence emission spectrum, as compared to tryptophan in water, and by quenching studies with acrylamide. The enzyme shows a transition temperature of 87 degrees C for the CD signal at 222 nm. This remarkable heat stability is not due to the presence of disulfide bonds, since both the Raman spectrum and chemical modification studies clearly indicate that all six cysteine residues are in the reduced state.

Monoaryl- and bisaryldihydroxytropolones as potent inhibitors of inositol monophosphatase.

The first successful preparation of mono- and disubstituted 3,7-dihydroxytropolone involves a four-step synthetic scheme. Thus, bromination of 3,7-dihydroxytropolone (8) followed by permethylation of the resultant products furnished gram quantities of intermediates 13-18. Single or double Suzuki coupling reactions between these permethylated monobromo- and dibromodihydroxytropolone derivatives and a variety of boronic acids delivered the expected products whose deprotection yielded the desired compounds 1a-u and 26a-n, usually in fair to good yields. Tropolones 1 and 26 were found to be potent inhibitors of inositol monophosphatase with IC50 values in the low-micromolar range. The results are discussed in the context of the recently described novel mode of inhibition of the enzyme by 3,7-dihydroxytropolones.

Selective mechanism-based inactivation of peptidylglycine alpha-hydroxylating monooxygenase in serum and heart atrium vs. brain.

Peptidylglycine alpha-hydroxylating monooxygenase (PHM; EC 1.14.17.3) catalyses the rate-limiting step in the post-translational activation of substance P, among other neuropeptides, from its glycine-extended precursor. Comparative kinetic studies were performed, using trans-styrylacetic acid or trans-styrylthioacetic acid as known mechanism-based inhibitors, of PHM isolated from rat, horse or human blood serum. Distinctive species differences with respect to PHM inactivation were observed: the efficiency of inactivation decreased in the order of horse >> rat > human. Trans-styrylacetic acid was more active than its thioether derivative. Moreover, we studied the differential sensitivity towards mechanism-based inactivation, of soluble PHM from rat blood serum and rat brain by trans-styrylacetic acid or benzylhydrazine, as well as the membrane-associated enzymes from rat brain and heart atrium. For the heart atrium membrane PHM or the soluble PHM from blood serum, inactivation rate constants k(inact)/K(I) of approximately 100 M(-1)sec(-1) were found with trans-styrylacetic acid. However, neither of the two tested compounds, at 100 microM or 12 mM, respectively, could inactivate the soluble or membranous PHMs from rat brain during a 15-min pre-incubation period. Instead, under conditions of reversible inhibition, trans-styrylacetic acid competitively inhibited the soluble or membrane-associated brain PHM with inhibition constants K(I) = 0.6 microM and 1.0 microM, respectively. Organ-selective, time-dependent inactivation of PHM with compounds of the above types might be an important pharmacological tool to control peripheral neuropeptide activation.

Catalysis by yeast alcohol dehydrogenase.

Table 7 presents a brief summary of the effects of various mutations on some of the relevant kinetic constants. The results illustrate several important features of the use of site-directed mutagenesis in exploring structure and function of enzymes. Note that most of the mutations affect a given step or kinetic parameter in the mechanism, such as the binding of NAD+ or the turnover number with ethanol. Furthermore, one mutation can affect many steps in the mechanism. Thus, it is difficult to ascribe a particular role to an amino acid residue. It is also difficult to quantify the function of a residue, since the magnitudes of the effects on kinetic parameters will be modulated by the other amino acid residues that participate in the reaction. Comprehensive and quantitative kinetic studies of many mutant enzymes are required if we are to understand catalysis and specificity. We are reluctant to describe any residue as "essential" for activity since substitution with some amino acid can probably produce an enzyme with some residual activity. (Maybe the Thr48Gly enzyme would be active, as a water molecule could substitute for the hydroxyl of the threonine.) Likewise, when substitution of a residue partially, but not totally, decreases activity, it does not necessarily mean that the residue is "not essential". The change in activity can reflect the contribution of that residue to catalysis. On the other hand, if various substitutions of a residue do not change activity, it would be reasonable to conclude that the residue is not essential (Plapp et al., 1971). Most of the amino acid residues at the active site are involved in the catalytic mechanism, either by contacting the substrates directly or by participating in the chemistry. Some of the residues that are outside of the active site are indirectly involved, by affecting the structure of the protein. Substitution of an important amino acid residue should significantly affect activity, and studies on the kinetics and structure should allow one to distinguish among the various explanations.

Kinetic studies with myo-inositol monophosphatase from bovine brain.

The kinetic properties of myo-inositol monophosphatase with different substrates were examined with respect to inhibition by fluoride, activation or inhibition by metal ions, pH profiles, and solvent isotope effects. F- is a competitive inhibitor versus 2'-AMP and glycerol 2-phosphate, but noncompetitive (Kis = Kii) versus DL-inositol 1-phosphate, all with Ki values of approximately 45 microM. Activation by Mg2+ follows sigmoid kinetics with Hill constants around 1.9, and random binding of substrate and metal ion. At high concentrations, Mg2+ acts as an uncompetitive inhibitor (Ki = 4.0 mM with DL-inositol 1-phosphate at pH 8.0 and 37 degrees C). Activation and inhibition constants, and consequently the optimal concentration of Mg2+, vary considerably with substrate structure and pH. Uncompetitive inhibition by Li+ and Mg2+ is mutually exclusive, suggesting a common binding site. Lithium binding decreases at low pH with a pK value of 6.4, and at high pH with a pK of 8.9, whereas magnesium inhibition depends on deprotonation with a pK of 8.3. The pH dependence of V suggests that two groups with pK values around 6.5 have to be deprotonated for catalysis. Solvent isotope effects on V and V/Km are greater than 2 and 1, respectively, regardless of the substrate, and proton inventories are linear. These results are consistent with a model where low concentrations of Mg2+ activate the enzyme by stabilizing the pentacoordinate phosphate intermediate. Li+ as well as Mg2+ at inhibiting concentrations bind to an additional site in the enzyme-substrate complex. Hydrolysis of the phosphate ester is rate limiting and facilitated by acid-base catalysis.

Carboxyl groups near the active site zinc contribute to catalysis in yeast alcohol dehydrogenase.

The importance of carboxyl groups near the active site zinc for the catalytic function of alcohol dehydrogenase I from Saccharomyces cerevisiae was examined by directed mutagenesis and steady state kinetics. Asp-49 was changed to asparagine and Glu-68 to glutamine (residue numbering as for horse liver enzyme). The catalytic efficiencies (V/Km) for ethanol oxidation and acetaldehyde reduction were decreased by factors of 1000 with the Asn-49 mutant and 100 with the Gln-68 enzyme. For the Asn-49 mutant, dissociation constants for coenzymes increased 7-fold, and Michaelis and inhibition constants for substrates and substrate analogs increased by factors of 20-50. The turnover numbers were reduced 50-fold for ethanol oxidation and 15-fold for acetaldehyde reduction. Product and dead-end inhibition studies and kinetic isotope effects showed that the mechanism with NAD+ and ethanol was rapid equilibrium random, in contrast to the ordered mechanism of wild-type enzyme. Alcohol dehydrogenase I and the Asn-49 mutant had similar CD spectra and 2 zinc atoms/subunit, but slightly different UV absorption and fluorescence spectra. The Gln-68 mutant resembled the wild-type enzyme in most kinetic constants, but the turnover number for ethanol oxidation decreased 35-fold, and Kd for NAD+ and Km for acetaldehyde increased by factors of 4 and 50, respectively. The pK values for V1 and V1/Km for ethanol oxidation were shifted from 7.7 (wild-type) to 6.8 in the Gln-68 and 6.2 in the Asn-49 mutant. The altered electrostatic environment near the active site zinc apparently decreases activities by hindering isomerizations of enzyme-substrate complexes.

The effect of histidine modification on the activity of myo-inositol monophosphatase from bovine brain.

The pH dependence of myo-inositol monophosphatase may indicate a role for histidine residues in the catalytic mechanism (Ganzhorn, A. J., and Chanal, M.-C. (1990) Biochemistry 29, 6065-6071). This possibility was investigated by chemical modification. At pH 6.0 and 25 degrees C, the enzyme was inactivated by diethylpyrocarbonate in a pseudo-first order reaction with a bimolecular rate constant of 0.37 M-1 s-1. Two histidines were modified rapidly with no effect on enzyme activity, while 3 residues were modified at a slower rate corresponding to the rate of inactivation. No noticeable changes in the secondary structure of the enzyme were observed by comparison of circular dichroic spectra before and after modification. Treatment of myo-inositol monophosphatase with diethylpyrocarbonate in the presence of inositol 1-phosphate, Mg2+, and Li+ protected 2 residues from modification and decreased the inactivation rate by about 5-fold. Spectrophotometric analysis, the restoration of enzyme activity by hydroxylamine, and the lack of any inhibitory effect with alkylating agents suggest that inactivation is due solely to modification of histidine. We conclude that a histidine residue is essential for activity and may act as a base catalyst during hydrolysis of the substrate.

Peptidylglycine alpha-amidating mono-oxygenase: neuropeptide amidation as a target for drug design.

1. Peptidylglycine alpha-amidating mono-oxygenase (PAM) is a bifunctional key enzyme in the bioactivation of neuropeptides. Its biosynthesis, distribution, functional role, and pharmacological manipulation are discussed. 2. PAM biosynthesis from a single gene precursor is characterized by alternative splicing and endoproteolytic events, which control intracellular transport, targeting, and enzyme activity. 3. The enzyme is mainly stored in secretory vesicles of many neuronal and endocrine cells with high abundance in the pituitary gland. Its functional role has been studied using enzyme inhibitors. Thus selective, peripheral PAM inhibition reduces substance P along with an anti-inflammatory action. 4. PAM-related pathologies are characterized by an increased relative abundance of alpha-amidated neuropeptides. To attenuate such hormone overproduction, novel, specific, and disease-targeted PAM inhibitors may be developed based on enzyme polymorphism.

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.

Stimulation of cGMP-dependent protein kinase I alpha by a peptide from its own sequence. An investigation by enzymology, circular dichroism and 1H NMR of the activity and structure of cGMP-dependent protein kinase I alpha-(546-576)-peptide amide.

The structure of cGMP-dependent protein kinase I alpha-(546-576)-peptide amide (peptide-546) and its effects on cGMP-dependent protein kinase I alpha (G-kinase) have been studied. By primary sequence analysis and analogy to a peptide that stimulates protein kinase C, peptide-546 was predicted to form part of the protein/peptide binding site of G-kinase, and it was proposed that it would stimulate the enzyme by interaction with an autoinhibitory site. The portion of cAMP-dependent protein kinase analogous to peptide-546 forms part of the peptide substrate binding site, interacting with the peptide inhibitor residues Argp-2 and Phep-11 (where p is the pseudophosphorylation site), through residues at positions corresponding to Glu4, Pro10 and Ser13 in peptide-546. Peptide-546 is a reasonably potent G-kinase activator, increasing the turnover number with the peptide substrate Arg-Lys-Arg-Ser-Arg-Lys-Glu by about threefold with an activation constant that is about fivefold lower than the Km value of this peptide substrate. Peptide-546 does not appear to change the affinity of the enzyme for the above substrate, ATP or cGMP and does not affect the binding of [3H]cGMP to G-kinase. The activation does not seem to result from an interaction between peptide-546 and peptide substrates, and a kinetic scheme is proposed which is compatible with an action of peptide-546 on G-kinase independent of substrates. The activation is additive with that given by cGMP and causes the enzyme to enter a hitherto unrecognised superactive state. Peptide conformation has been monitored in mixed 2,2,2-trifluoroethanol/H2O solvents by circular dichroism: helical structure is observed in these mixtures when the 2,2,2-trifluoroethanol content is above 25%. The structure is lost only gradually on raising the temperature to 80 degrees C with no clear melting transition. Assignment of the resonances in the 1H-NMR spectrum has allowed the identification of elements of secondary structure from detected nuclear Overhauser effects. In particular, a helical segment from Met18 to Arg26 is observed. The four proline residues (Pro10, Pro11, Pro15 and Pro17) are all seen to be in the trans conformation, although additional, weaker peaks in the spectra may correspond to a minor conformer in which one or more of the prolines is in a cis conformation. The N-terminal residues are less structured but show some helical character.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.

Structure and function in yeast alcohol dehydrogenases.

The structure and kinetics of the isozymes from Saccharomyces cerevisiae (ADH I, II, III) have been compared, and the ADH I gene was specifically mutagenized in order to substitute amino acid residues of particular interest. A model of the yeast enzyme was constructed on the basis of the structure of the homologous horse liver enzyme. Steady state kinetic studies, at pH 7.3 and 30 degrees C, showed that the enzymes follow the Ordered Bi Bi mechanism. ADH II has a Michaelis constant for ethanol that is 10-fold smaller than the constants found for ADH I or III. Replacement of Met-294 (liver numbering) in the substrate binding pocket of ADH I with Leu, as found in ADH II, could be responsible for the different kinetics. However, the mutant enzyme, ADH I-Leu, had constants with ethanol that were similar to those of ADH I. Nevertheless, the Leu enzyme had better catalytic activity with longer chain alcohols than did the Met enzyme. Other substitutions must account for the differences between ADH I and II. His-47 binds the pyrophosphate of coenzyme, and replacement with Arg (as in the liver enzyme) decreases turnover numbers by 6-fold and dissociation constants for NAD+ and NADH by only 2 to 4-fold. The lower turnover number explains why yeast harboring the mutant Arg enzyme are resistant to poisoning by allyl alcohol. ADH I and ADH I-Arg have maximal activity on ethanol at pH values above a pK of about 7. Replacement of His-51 with Gln reduces activity 12-fold and abolishes the pK value.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic characterization of yeast alcohol dehydrogenases. Amino acid residue 294 and substrate specificity.

A three-dimensional model of yeast alcohol dehydrogenase, based on the homologous horse liver enzyme, was used to compare the substrate binding pockets of the three isozymes (I, II, and III) from Saccharomyces cerevisiae and the enzyme from Schizosaccharomyces pombe. Isozyme I and the S. pombe enzyme have methionine at position 294 (numbered as in the liver enzyme, corresponding to 270 in yeast), whereas isozymes II and III have leucine. Otherwise the active sites of the S. cerevisiae enzymes are the same. All four wild-type enzymes were produced from the cloned genes. In addition, oligonucleotide-directed mutagenesis was used to change Met-294 in alcohol dehydrogenase I to leucine. The mechanisms for all five enzymes were predominantly ordered with ethanol (but partially random with butanol) at pH 7.3 and 30 degrees C. The wild-type alcohol dehydrogenases and the leucine mutant had similar kinetic constants, except that isozyme II had 10-20-fold smaller Michaelis and inhibition constants for ethanol. Thus, residue 294 is not responsible for this difference. Apparently, substitutions outside of the substrate binding pocket indirectly affect the interactions of the alcohol dehydrogenases with ethanol. Nevertheless, the substitution of methionine with leucine in the substrate binding site of alcohol dehydrogenase I produced a 7-10-fold increase in reactivity (V/Km) with butanol, pentanol, and hexanol. The higher activity is due to tighter binding of the longer chain alcohols and to more rapid hydrogen transfer.

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.