Structural enzymology of Li(+)-sensitive/Mg(2+)-dependent phosphatases

Li(+)-sensitive/Mg(2+)-dependent phosphatases have attracted considerable attention since they have been proposed as targets for lithium therapy in the treatment of manic-depressive patients. The members of this enzyme superfamily display low levels of sequence identity while possessing a common fold and active site. Extensive structural and biochemical data demonstrate the direct involvement of two metal ions in catalysis, and show that lithium exerts its inhibitory action by blocking the products at the active site. By exploiting the different inhibitory properties of magnesium and calcium, we have been able to solve the X-ray structures of the Li(+)-sensitive/Mg(2+)-dependent 3'-phosphoadenosine-5'-phosphatase in complex with its substrate and with its products. The structural comparison of these complexes provides a 3D picture of the different stages of the catalytic cycle. This gives new insights into the understanding of the biological function of this group of enzymes and their lithium inhibition, and should assist in the design of improved inhibitors of therapeutic value.     

Colorimetric determination of pure Mg(2+)-dependent phosphatidate phosphatase activity

The malachite green-molybdate reagent was used for a colorimetric assay of pure Mg2(+)-dependent phosphatidate phosphatase activity. This enzyme plays a major role in fat metabolism. Enzyme activity was linear with time and protein concentration, and with the concentration of water-soluble dioctanoyl phosphatidate. The colorimetric assay was used to examine enzyme inhibition by phenylglyoxal, propranolol, and dimethyl sulfoxide. Pure enzyme and a water-soluble phosphatidate substrate were required for the assay, which should be applicable to a well-defined large-scale screen of Mg2(+)-dependent phosphatidate phosphatise inhibitors (or activators).     

Acid phosphatases in mammalian tissues. Evidence for the existence of a 57 kDa Zn(2+)-dependent acid phosphatase form.

1. A comparative study of multiple forms of acid phosphatase (AcPase) in various organs of mammals was carried out. 2. These studies indicated that the high-molecular weight AcPase is preferentially expressed by tissues which undergo cell proliferation such as epithelial tissues; on the contrary, the low-molecular weight enzyme seems to be characteristic of highly differentiated tissues such as nervous, muscle and blood erythrocytes. 3. The existence of a new AcPase activated by Zn2+ ions was observed in all tissues studied with the exception of erythrocytes. 4. The enzyme shows a molecular weight of 57 kDa, is insensitive to NaF, hydrolyzes p-nitro-phenylphosphate and o-c-phenylphosphate; ATP, a-naphthyl-phosphate and beta-glycerolphosphate are also dephosphorylated.     

Identification of coenzyme M biosynthetic phosphosulfolactate synthase: a new family of sulfonate-biosynthesizing enzymes

The hyperthermophilic euryarchaeon Methanococcus jannaschii uses coenzyme M (2-mercaptoethanesulfonic acid) as the terminal methyl carrier in methanogenesis. We describe an enzyme from that organism, (2R)-phospho-3-sulfolactate synthase (ComA), that catalyzes the first step in coenzyme M biosynthesis. ComA catalyzed the stereospecific Michael addition of sulfite to phosphoenolpyruvate over a broad range of temperature and pH conditions. Substrate and product analogs moderately inhibited activity. This enzyme has no significant sequence similarity to previously characterized enzymes; however, its Mg(2+)-dependent enzyme reaction mechanism may be analogous to one proposed for enolase. A diverse group of microbes and plants have homologs of ComA that could have been recruited for sulfolactate or sulfolipid biosyntheses.     

Selection of novel Mg(2+)-dependent self-cleaving ribozymes.

Four RNA motifs are known that catalyse site-specific cleavage in the presence of Mg2+ ions, all discovered in natural RNAs. In a single in vitro selection experiment we have isolated representatives of five novel classes of Mg(2+)-dependent ribozymes. Small versions of three of these showed that a very simple internal loop type of secondary structure is responsible for the activity. One of these was synthesized in a bimolecular form, and compared directly with the hammerhead ribozyme; for the new ribozyme, the cleavage step of the reaction is much faster than the spontaneous rate of phosphodiester bond cleavage, yet substantially slower than that for the hammerhead. The results suggest that many more Mg(2+)-dependent self-cleaving RNA sequences can be found.     

Interconversion of Mn(2+)-dependent and -independent protein phosphatase 2A from human erythrocytes: role of Zn(2+) and Fe(2+) in protein phosphatase 2A.

Human erythrocyte Mn(2+)-dependent (C'A') and -independent (CA) protein-serine/threonine phosphatase (PP) 2A are composed of 34-kDa catalytic C' and C subunits, in which the metal dependency resides, and 63-kDa regulatory A' and A subunits, respectively. Each catalytic and regulatory subunit gave the same V8- and papain-peptide maps, respectively. Stoichiometric zinc and substoichiometric iron were detected in CA but not in C'A' [Nishito et al. (1999) FEBS Lett. 447, 29-33]. The Mn(2+)-dependent protein-tyrosine phosphatase (PTP) activity of C'A' was about 70-fold higher than that of CA. Pre-incubation of CA with 25 mM NaF changed CA to a Mn(2+)-dependent form with higher PTP activity. The same NaF treatment had no effect on C'A'. Pre-incubation of C'A' with ZnCl(2), zinc-metallothionein, or FeCl(2) activated the Mn(2+)-independent PP activity, but pre-incubation with FeCl(3) did not. Ascorbate in the pre-incubation and assay mixture significantly stimulated the effect of FeCl(2). Pre-incubation of C'A' with 5 microM ZnCl(2) and 15 microM FeCl(2) in the presence of 1 mM ascorbate synergistically stimulated the Mn(2+)-independent PP activity, with concomitant suppression of the Mn(2+)-dependent PP and PTP activities. The PP and PTP activities of CA were unaffected by the same zinc and/or iron treatment. Micromolar concentrations of vanadate strongly inhibited the Mn(2+)-dependent PP activity of C'A' but only slightly inhibited the PP activity of CA. Using the distinct effect of vanadate as an indicator, the interconversion between CA and C'A' with the above mentioned treatments was proved. These results support the notion that Mn(2+)-independent CA is a Zn(2+)- and Fe(2+)-metalloenzyme, whose apoenzyme is Mn(2+)-dependent C'A'.     

Characterization of a phosphatase secreted by Staphylococcus aureus strain 154, a new member of the bacterial class C family of nonspecific acid phosphatases

An acid phosphatase, designated SapS, hydrolyzing p-nitrophenyl phosphate (pNPP), was identified and characterized from the culture supernatant of a Staphylococcus aureus strain isolated from vegetables. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the protein indicated an estimated molecular mass of 30 kDa. The enzyme displayed optimum activity at 40 degrees C and pH 5. Characterization of the phosphatase in a reconstitution assay showed that MgCl2 and Triton X-100, respectively, restored maximal activity, but not CaCl2 The phosphatase activity was affected by EDTA and sodium molybdate. The DNA sequence encoding SapS was cloned and sequenced. The putative acid phosphatase gene encodes a protein of 296 amino acids with a 31-residue signal peptide. Database searches revealed significant structural homology of SapS to several proteins belonging to the bacterial class C family of nonspecific acid phosphatases. Comparison of the sequences indicated that despite a low level of overall conservation between the proteins, four conserved sequence motifs could be identified.     

Identification of the Ca(2+)-dependent calmodulin-binding region of chromogranin A.

Chromogranin A (CGA), the most abundant protein in bovine adrenal chromaffin granules, is a high-capacity, low-affinity Ca(2+)-binding protein found in most neuroendocrine cells, and binds calmodulin (CaM) in a Ca(2+)-dependent manner. The binding of chromogranin A to calmodulin was determined by measuring the intrinsic tryptophan fluorescence of chromogranin A in the presence and absence of Ca2+. Binding was specifically Ca(2+)-dependent; neither Mg2+ nor Mn2+ could substitute for Ca2+. Chelation of Ca2+ by EGTA completely eliminated the chromogranin A-calmodulin interaction. CaM binding was demonstrated by a synthetic CGA peptide representing residues 40-65. When the CGA peptide and CaM were mixed in the presence of 15 mM CaCl2, the intrinsic tryptophan fluorescence emission underwent a substantial blue-shift, shifting from 350 to 330 nm. Like the intact CGA, the peptide-CaM binding was specifically Ca(2+)-dependent, and neither Mg2+ nor Mn2+ could induce the binding. Calmodulin bound both to CGA and to the synthetic CGA peptide with a stoichiometry of one to one. The dissociation constants (Kd) determined by fluorometric titration were 13 nM for the peptide-CaM binding and 17 nM for intact CGA-CaM binding. The Kd values are comparable to those (approximately 10(-9) M) of other CaM-binding proteins and peptides, demonstrating a tight binding of CaM by CGA. The CaM-binding CGA residues 40-65 are 100% conserved among all the sequenced CGAs in contrast to 50-60% conservation found in the entire sequence, implying essential roles of this region.     

Zn(2+)-dependent tyrosine phosphorylation of a 68-kDa protein and its differentiation from Mg(2+)-dependent tyrosine phosphorylation in sheep platelets.

A 68-kDa protein that was tyrosine phosphorylated in the presence of Zn2+ and two proteins of 52 and 46 kDa that were tyrosine phosphorylated in the presence of Mg2+ were separated by column chromatography of a sheep platelet high speed supernatant on poly(Glu, Tyr)4:1 copolymer-Sepharose or tyrosine-Sepharose. Phosphorylation of the 68-kDa protein occurred maximally in the presence of Zn2+ while Mg2+ was ineffective. The kinases responsible for the Zn(2+)- and Mg(2+)-dependent tyrosine phosphorylation could also tyrosine phosphorylate poly(Glu, Tyr)4:1, histone, and angiotensin II with the same metal ion specificity. The two tyrosine kinase activities could be also distinguished by their differential response to polyamines and quercetin. Zn2+ stimulation did not appear to be due to the inhibition of a protein tyrosine phosphatase. Sephadex G-100 gel filtration of the fraction showing Zn(2+)-dependent tyrosine phosphorylation of the 68-kDa protein showed that the tyrosine kinase activity corresponded to a molecular mass of 68,000 and it showed a protein band of 68 kDa as detected by silver staining on sodium dodecyl sulfate-polyacrylamide gel.     

A monoclonal antibody to the calmodulin-binding (Ca2+ + Mg2+)-dependent ATPase from pig stomach smooth muscle inhibits plasmalemmal (Ca2+ + Mg2+)-dependent ATPase activity.

A monoclonal antibody (2B3) directed against the calmodulin-binding (Ca2+ + Mg2+)-dependent ATPase from pig stomach smooth muscle was prepared. This antibody reacts with a 130,000-Mr protein that co-migrates on SDS/polyacrylamide-gel electrophoresis with the calmodulin-binding (Ca2+ + Mg2+)-ATPase purified from smooth muscle by calmodulin affinity chromatography. The antibody causes partial inhibition of the (Ca2+ + Mg2+)-ATPase activity in plasma membranes from pig stomach smooth muscle, in pig erythrocytes and human erythrocytes. It appears to be directed against a specific functionally important site of the plasmalemmal Ca2+-transport ATPase and acts as a competitive inhibitor of ATP binding. Binding of the antibody does not change the Km of the ATPase for Ca2+ and its inhibitory effect is not altered by the presence of calmodulin. No inhibition of (Ca2+ + Mg2+)-ATPase activity or of the oxalate-stimulated Ca2+ uptake was observed in a pig smooth-muscle vesicle preparation enriched in endoplasmic reticulum. These results confirm the existence in smooth muscle of two different types of Ca2+-transport ATPase: a calmodulin-binding (Ca2+ + Mg2+)-ATPase located in the plasma membrane and a second one confined to the endoplasmic reticulum.     

Identification of Ca(2+)-activated Mg(2+)-dependent ATP-hydrolase activity of membrane microsomal fraction of loach embryo (Misgurnus fossilis L.)

The functional confirmation of availability of Ca2+ transport initially-active systems in the embryo cells of loach Misgurnus fossilis L. has been obtained. Using thapsigargin, the specific inhibitor of endoplasmic reticulum of Ca2+, Mg(2+)-ATPase, this enzyme activity was divided into thapsigargin-sensitive (actually endoplasmic reticulum Ca2+, Mg(2+)-ATPase) and thapsigargin-insensitive (plasma membrane Ca2+, Mg(2+)-ATPase) constituents. The Ca(2+)-independent Mg(2+)-dependent ATPase activity makes above 39.7% of the common Ca2+, Mg(2+)-ATPase activity of embryo loach. The periodic changes of Ca2+, Mg(2+)-ATPase activity (except for the changes of plasma membrane Ca2+, Mg(2+)-ATPase activity) were found out, which coincide with periodic [Ca2+]i oscillations during the synchronous divisions of loach blastomers embryos.     

A novel Mg(2+)-dependent O-methyltransferase in the phenylpropanoid metabolism of Mesembryanthemum crystallinum

Upon irradiation with elevated light intensities, the ice plant (Mesembryanthemum crystallinum) accumulates a complex pattern of methylated and glycosylated flavonol conjugates in the upper epidermal layer. Identification of a flavonol methylating activity, partial purification of the enzyme, and sequencing of the corresponding peptide fragments revealed a novel S-adenosyl-l-methionine-dependent O-methyltransferase that was specific for flavonoids and caffeoyl-CoA. Cloning and functional expression of the corresponding cDNA verified that the new methyltransferase is a multifunctional 26.6-kDa Mg(2+)-dependent enzyme, which shows a significant sequence similarity to the cluster of caffeoyl coenzyme A-methylating enzymes. Functional analysis of highly homologous members from chickweed (Stellaria longipes), Arabidopsis thaliana, and tobacco (Nicotiana tabacum) demonstrated that the enzymes from the ice plant, chickweed, and A. thaliana possess a broader substrate specificity toward o-hydroquinone-like structures than previously anticipated for Mg(2+)-dependent O-methyltransferases, and are distinctly different from the tobacco enzyme. Besides caffeoyl-CoA and flavonols, a high specificity was also observed for caffeoylglucose, a compound never before reported to be methylated by any plant O-methyltransferase. Based on phylogenetic analysis of the amino acid sequence and differences in acceptor specificities among both animal and plant O-methyltransferases, we propose that the enzymes from the Centrospermae, along with the predicted gene product from A. thaliana, form a novel subclass within the caffeoyl coenzyme A-dependent O-methyltransferases, with potential divergent functions not restricted to lignin monomer biosynthesis.     

Protein tyrosine phosphatase stimulates Ca(2+)-dependent amylase secretion from pancreatic acini.

Eukaryotic cells respond to various stimuli by an increase or decrease in levels of phosphoproteins. Phosphotyrosine levels on eukaryotic cellular proteins are tightly regulated by the opposing actions of protein-tyrosine kinases and protein-tyrosine phosphatases (PTPases, EC 3.1.3.48). Studies on permeabilized mast cells suggest that the enabling reaction for exocytosis might involve protein dephosphorylation. In the present studies, a recombinant form of rat brain PTPase (rrbPTP-1) has been used to examine the potential role of PTPases in Ca(2+)-dependent amylase secretion from permeabilized rat pancreatic acini. Additionally, the concentrations and subcellular distributions of endogenous PTPase activity in rat pancreas were determined. The results from these experiments indicate that addition of exogenous PTPase stimulated Ca(2+)-dependent amylase secretion from pancreatic acinar cells and that endogenous PTPase activity was associated with the postgranule supernatant, zymogen granules, and in particular zymogen granule membranes. Our data suggest that protein tyrosine dephosphorylation is potentially involved in regulated secretion at a site(s) distal to receptor-mediated elevation of intracellular second messengers.     

Identification and characterization of Mg2+-dependent phosphotyrosyl protein phosphatase from rat liver cytosol

Although highly purified preparations of Mg2+-dependent phosphoseryl protein phosphatase (also designated phosphatase IA or phosphatase 2C) dephosphorylated phosphotyrosyl histone, the activity has been resolved from phosphatase IA by polyacrylamide gel electrophoresis at pH 9.5. This novel phosphotyrosyl-specific protein phosphatase absolutely requires Mg2+ or Mn2+ for activity, is inhibited by Zn2+, vanadate and fluoride, and has an optimal pH of 9.0 and Mr = 50,000. Certain properties of this phosphatase so closely resemble those of phosphatase IA that the two enzymes tend to be copurified through various separation procedures.     

Characterization of a Mg2+-dependent phosphatase from turkey gizzard smooth muscle

A Mg2+-dependent phosphatase has been purified to apparent homogeneity from turkey gizzard smooth muscle. The enzyme has a Mr = 43,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and 44,500 as determined by sedimentation equilibrium centrifugation under nondenaturing conditions. Using polyacrylamide gel electrophoresis in the absence of sodium dodecyl sulfate all of the phosphatase activity was found to migrate as a single band, subsequently shown to have an Mr = 43,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme is inactive in the absence of Mg2+ and maximum activity is reached at a free concentration of 12 mM Mg2+. Mn2+ can replace Mg2+, but the activity is only about one-fifth of that found with 12 mM Mg2+. NaF and the nucleotides ATP, ADP, and AMP inhibit phosphatase activity. This inhibition appears to be independent of their ability to bind Mg2+. The phosphatase purified from turkey smooth muscle appears to be identical with that purified from canine heart (Binstock, J. F., and Li, H. C. (1979) Biochem. Biophys. Res. Commun. 87, 1226-1234) and rat liver (Hiraga, A., Kikuchi, K., Tamura, S., and Tsuiki, S. (1981) Eur. J. Biochem. 119, 503-510).     

Calmodulin affinity chromatography yields a functional purified erythrocyte (Ca+ + Mg2+)-dependent adenosine triphosphatase.

The (Ca2+ + Mg2+)-dependent ATPase of human erythrocyte membranes was solubilized with deoxycholate and purified by calmodulin affinity chromatography to yield a functional enzyme. The method gave an enzyme purified 207-fold as compared with that of the erythrocyte membranes. The molecular weight of the ATPase was in the range 135 000-150 000, as revealed by a single major band after electrophoresis on dodecyl sulphate/polyacrylamide gels. The isolated enzyme was highly sensitive to calmodulin, since the activity was increased about 9-fold. At 37 degrees C and in the presence of calmodulin the purified ATPase had a specific activity of 10.1 mumol/min per mg of protein. Triton X-100 or deoxycholate stimulated the calmodulin-deficient enzyme in a concentration-dependent fashion whereby the calmodulin-sensitivity was lost. The purification method is suitable for studying the lipid-sensitivity of the ATPase, since the lipids can easily be exchanged without a significant loss of activity. A purification procedure described by Niggli, Penniston & Carafoli [(1979) J. Biol. Chem. 254, 9955-9958] resulted in an enzyme that indeed was pure but was lacking a predominant feature, namely the modulation by calmodulin.     

Transport of coenzyme M (2-mercaptoethanesulfonic acid) in Methanobacterium ruminantium.

A system for transport of coenzyme M, 2-mercaptoethanesulfonic acid (HS--CoM), in Methanobacterium ruminatium strain M1 required energy, showed saturation kinetics, and concentrated the coenzyme against a gradient. The process was sensitive to temperature and was maximally active at pH 7.1. Cells took up HS--CoM at a linear rate, with a Vmax of 312 pmol/min per mg (dry weight) and an apparent Km of 73 nM. An intracellular pool of up to 5 mM accumulated which was not exchangeable with the medium. Uptake required both hydrogen and carbon dioxide; it was inhibited by O2. Bromoethanesulfonic acid (BrCH2CH2SO3-), a potent inhibitor of methanogenesis in cell-free extracts, inhibited both uptake and methane production. Results of inhibitor studies with derivatives and analogs of the coenzyme showed that the specificity of the carrier is restricted to a limited range of thioether, thioester, and thiocarbonate derivatives. 2-(Methylthio)ethanesulfonic acid (CH3--S--CoM) showed an apparent Ki for HS--CoM uptake of 15 nM, being taken up itself with a Vmax of 320 pmol/min per mg (dry weight) and an apparent Km of 50 nM. An analysis of intracellular pools after HS--CoM uptake indicated that the predominant forms are a heterodisulfide of unknown composition and CH3--S--CoM.