Phosphohistidine as a stoichiometric intermediate in reactions catalyzed by isoenzymes of wheat germ acid phosphatase

Structural and activity characterization of human PHPT1 after oxidative modification

Phosphohistidine phosphatase 1 (PHPT1), the only known phosphohistidine phosphatase in mammals, regulates phosphohistidine levels of several proteins including those involved in signaling, lipid metabolism, and potassium ion transport. While the high-resolution structure of human PHPT1 (hPHPT1) is available and residues important for substrate binding and catalytic activity have been reported, little is known about post-translational modifications that modulate hPHPT1 activity. Here we characterize the structural and functional impact of hPHPT1 oxidation upon exposure to a reactive oxygen species, hydrogen peroxide (H2O2). Specifically, liquid chromatography-tandem mass spectrometry was used to quantify site-specific oxidation of redox-sensitive residues of hPHPT1. Results from this study revealed that H2O2 exposure induces selective oxidation of hPHPT1 at Met95, a residue within the substrate binding region. Explicit solvent molecular dynamics simulations, however, predict only a minor effect of Met95 oxidation in the structure and dynamics of the apo-state of the hPHPT1 catalytic site, suggesting that if Met95 oxidation alters hPHPT1 activity, then it will do so by altering the stability of an intermediate state. Employing a novel mass spectrometry-based assay, we determined that H2O2-induced oxidation does not impact hPHPT1 function negatively; a result contrary to the common conception that protein oxidation is typically a loss-of-function modification.

Proteomics approach to identifying ATP-covalently modified proteins

This study aims to investigate functionally similar proteins based on their capacity to remain bound to ATP under stringent resolving conditions. Using two-dimensional gel electrophoresis and capillary liquid chromatography on-line mass spectrometry, we have identified several mammalian and E. coli proteins that appear to covalently bind ATP. To validate this approach, we obtained commercially purified forms of proteins identified from two-dimensional protein maps and tested their capacity to bind alpha 32P phosphate labeled ATP. This proteomics approach provides an initial screening method of identifying functionally similar proteins for further scrutiny by a more traditional analysis.

Catalytic domains of the LAR and CD45 protein tyrosine phosphatases from Escherichia coli expression systems: purification and characterization for specificity and mechanism

The cytoplasmic domains of two human transmembrane protein tyrosine phosphatases (PTPases), LAR and CD45, have been expressed in Escherichia coli, purified to near-homogeneity, and compared for catalytic efficiency toward several phosphotyrosine-containing peptide substrates. A 615-residue LAR fragment (LAR-D1D2) containing both tandemly repeated PTPase domains shows almost identical specific activity and high catalytic efficiency as the 40-kDa single-domain LAR-D1 fragment, consistent with a single functional active site in the 70-kDa LAR-D1D2 enzyme. A 90-kDa fragment of the human leukocyte CD45 PTPase, containing two similar tandemly repeated PTPase domains, shows parallel specificity to LAR-D1 and LAR-D1D2 with a high kcat/Km value for a phosphotyrosyl undecapeptide. Sufficient purified LAR-D1 and LAR-D1D2 PTPases were available to demonstrate enzymatic exchange of 18O from 18O4 inorganic phosphate into H2(16)O at rates of approximately 1 x 10(-2) s-1. The oxygen-18 exchange probably proceeds via a phosphoenzyme intermediate. Brief incubation of all three PTPase fragments with a [32P]phosphotyrosyl peptide substrate prior to quench with SDS sample buffer and gel electrophoresis led to autoradiographic detection of 32P-labeled enzymes. Pulse/chase studies on the LAR 32P-enzyme showed turnover of the labeled phosphoryl group.

Crystal structure of the H256A mutant of rat testis fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase. Fructose 6-phosphate in the active site leads to mechanisms for both mutant and wild type bisphosphatase activities

Fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase (Fru-6-P, 2-kinase/Fru-2,6-Pase) is a bifunctional enzyme, catalyzing the interconversion of beta-D-fructose- 6-phosphate (Fru-6-P) and fructose-2,6-bisphosphate (Fru-2,6-P2) at distinct active sites. A mutant rat testis isozyme with an alanine replacement for the catalytic histidine (H256A) in the Fru-2,6-Pase domain retains 17% of the wild type activity (Mizuguchi, H., Cook, P. F., Tai, C-H., Hasemann, C. A., and Uyeda, K. (1998) J. Biol. Chem. 274, 2166-2175). We have solved the crystal structure of H256A to a resolution of 2. 4 A by molecular replacement. Clear electron density for Fru-6-P is found at the Fru-2,6-Pase active site, revealing the important interactions in substrate/product binding. A superposition of the H256A structure with the RT2K-Wo structure reveals no significant reorganization of the active site resulting from the binding of Fru-6-P or the H256A mutation. Using this superposition, we have built a view of the Fru-2,6-P2-bound enzyme and identify the residues responsible for catalysis. This analysis yields distinct catalytic mechanisms for the wild type and mutant proteins. The wild type mechanism would lead to an inefficient transfer of a proton to the leaving group Fru-6-P, which is consistent with a view of this event being rate-limiting, explaining the extremely slow turnover (0. 032 s-1) of the Fru-2,6-Pase in all Fru-6-P,2-kinase/Fru-2,6-Pase isozymes.

Structure and function of the low Mr phosphotyrosine protein phosphatases

Phosphotyrosine protein phosphatases (PTPases) catalyse the hydrolysis of phosphotyrosine residues in proteins and are hence implicated in the complex mechanism of the control of cell proliferation and differentiation. The low Mr PTPases are a group of soluble PTPases displaying a reduced molecular mass; in addition, a group of low molecular mass dual specificity (ds)PTPases which hydrolyse phosphotyrosine and phosphoserine/threonine residues in proteins are known. The enzymes belonging to the two groups are unrelated to each other and to other PTPase classes except for the presence of a CXXXXXRS/T sequence motif containing some of the catalytic residues (active site signature) and for the common catalytic mechanism, clearly indicating convergent evolution. The low Mr PTPases have a long evolutionary history since microbial (prokaryotic and eukaryotic) counterparts of both tyrosine-specific and dsPTPases have been described. Despite the relevant number of data reported on the structural and catalytic features of a number of low Mr PTPases, only limited information is presently available on the substrate specificity and the true biological roles of these enzymes, in prokaryotic, yeast and eukaryotic cells.

From phosphatases to vanadium peroxidases: a similar architecture of the active site

We show here that the amino acid residues contributing to the active sites of the vanadate containing haloperoxidases are conserved within three families of acid phosphatases; this suggests that the active sites of these enzymes are very similar. This is confirmed by activity measurements showing that apochloroperoxidase exhibits phosphatase activity. These observations not only reveal interesting evolutionary relationships between these groups of enzymes but may also have important implications for the research on acid phosphatases, especially glucose-6-phosphatase-the enzyme affected in von Gierke disease-of which the predicted membrane topology may have to be reconsidered.

The crystal structure of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase reveals distinct domain homologies

BACKGROUND

Glucose homeostasis is maintained by the processes of glycolysis and gluconeogenesis. The importance of these pathways is demonstrated by the severe and life threatening effects observed in various forms of diabetes. The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase catalyzes both the synthesis and degradation of fructose-2,6-bisphosphate, a potent regulator of glycolysis. Thus this bifunctional enzyme plays an indirect yet key role in the regulation of glucose metabolism.

RESULTS

We have determined the 2.0 A crystal structure of the rat testis isozyme of this bifunctional enzyme. The enzyme is a homodimer of 55 kDa subunits arranged in a head-to-head fashion, with each monomer consisting of independent kinase and phosphatase domains. The location of ATPgammaS and inorganic phosphate in the kinase and phosphatase domains, respectively, allow us to locate and describe the active sites of both domains.

CONCLUSIONS

The kinase domain is clearly related to the superfamily of mononucleotide binding proteins, with a particularly close relationship to the adenylate kinases and the nucleotide-binding portion of the G proteins. This is in disagreement with the broad speculation that this domain would resemble phosphofructokinase. The phosphatase domain is structurally related to a family of proteins which includes the cofactor independent phosphoglycerate mutases and acid phosphatases.

Isolation and characterization of a homogeneous isoenzyme of wheat germ acid phosphatase

An acid phosphatase (orthophosphoric monoester phosphohydrolase, acid optimum; EC 3.1.3.2) isoenzyme from wheat germ was purified 7000-fold to homogeneity. The effect of wheat germ sources and their relationship to the isoenzyme content and purification behavior of acid phosphatases was investigated. Extensive information about the purification and stabilization of the enzyme is provided. The instability of isoenzymes in the latter stages of purification appeared to be the result of surface inactivation together with a sensitivity to dilution that could be partially offset by addition of Triton X-100 during chromatographic procedures. Added sulfhydryl protecting reagents had no effect on activity or stability, which was greatest in the pH range 4-7. The purified isoenzyme was homogeneous by polyacrylamide gel electrophoresis and exhibited the highest specific activity and turnover number reported for any acid phosphatase. The molecular weights of the pure isoenzyme and of related isoenzymes from wheat germ were found to be identical (58,000). The pure isoenzyme contained a single polypeptide chain and had a negligible carbohydrate content. The amino acid composition was determined. Of the various reasons that were considered to explain isoenzyme occurrence, a genetic basis was considered most likely. The enzyme was found to exhibit substrate inhibition with some substrates below pH 6, while above pH 8 it exhibited downwardly curving Lineweaver-Burk plots of the type that are generally described as "substrate activation". The observation of a phosphotransferase activity was consistent with the formation of a covalent phosphoenzyme intermediate, while inactivation by diethyl pyrocarbonate was consistent with the presence of an active site histidine.

Multiple forms of barley root acid phosphatase: purification and some characteristics of the major cytoplasmic isoenzyme

The major acid phosphatase form (orthophosphoric-monoester phosphohydrolase (acid optimum), EC 3.1.3.2) was purified from the soluble extract of barley roots. The enzyme is homogeneous on polyacrylamide gel electrophoresis and moves as a single band of Mr approximately 38,000 in the presence of sodium dodecyl sulphate. The molecular weight of the native enzyme was Mr 77,600 and 79,000 as determined, respectively, by gel filtration on a Sephadex G-100 column and by density gradient ultracentrifugation. The isoelectric point was about 6.28. The enzyme is competitively inhibited by molybdate (Ki = 9 x 10(-7) M). NaF, Ag(+), Hg(2+), Pb(2+) and Zn(2+) are also inhibitors, while other cations showed no effect. The enzyme hydrolyzes a wide variety of natural and synthetic phosphate esters. In particular, the enzyme seems to be active on ATP, o-phosphotyrosine, o-phosphoserine and glucose 1-phosphate. The pH dependence studies between pH 4-8 using p-nitrophenylphosphate as substrate and diethylpyrocarbonate inactivation indicate the presence of essential histidine residue at the active site.

Phosphorylation of proteins in rat ovarian plasma membranes by [gamma-32P]GTP: evidence for the formation of a high energy phosphoprotein

Incubation of rat ovarian plasma membranes with [gamma-32P]guanosine 5'-triphosphate (GTP) in the presence of an adenosine triphosphate (ATP)-trapping system results in the labeling of a single protein, Mr 33,000 +/- 3000 designated 'a' (Amir-Zaltsman, Y., Ezra, E., Walker, M., Lindner, H. R. and Salomon, Y. (1980) FEBS Lett. 122, 166-170). Based on competition with other nucleotides it is concluded that protein 'a' is preferentially phosphorylated by [gamma-32P]GTP (Km = 0.28 microM). Phosphorylation of protein 'a' does not occur at pH less than 5 and progressively increases to plateau levels at pH 7-9. Phosphorylation of protein 'a' is absolutely dependent on the presence of divalent cations 1 mM Mg2+, Ca2+, or Cd2+. At higher concentrations, 5-20 mM, Mg2+ or in the presence of 1 mM Mn2+ ions other proteins are also phosphorylated. While vanadate ions selectively prevent the labeling of protein 'a', molybdate ions were found to inhibit phosphorylation of all the membrane proteins including protein 'a'. In contrast to molybdate ions, vanadate ions were found to accelerate the dephosphorylation of phosphoprotein 'a'. We suggest that phosphoprotein 'a' is a high energy protein intermediate in which the phosphate is present as a phosphoramidate for the following reasons: (i) Guanosine diphosphate (GDP) but not guanosine 5'-O-(2-thiodiphosphate) selectively accelerated the dephosphorylation of phosphoprotein 'a' but only in the presence of Mg2+ ions. (ii) The phosphoprotein intermediate is hydrolyzed in the presence of hydroxylamine. (iii) Phosphoprotein 'a' is labile in the presence of 1 N HCl but stable in 1 N NaOH at 37 degrees C. (iv) Phosphoprotein 'a' is heat labile. Phosphoprotein 'a' is readily digested by several proteolytic enzymes and a single cleavage peptide is generated upon treatment with Staphylococcus aureus V8 protease. The properties of protein 'a' were compared and found different from another phosphoprotein Mr 90,000 +/- 1000, designated 'b' that was selected arbitrarily. We propose that protein 'a' is a GTP requiring enzyme intermediate, of yet unidentified function.

Catalysis by enzymes entrapped into hydrated surfactant aggregates having lamellar or cylindrical (hexagonal) or ball-shaped (cubic) structure in organic solvents

Instead of aqueous solutions, universally recognized in enzymology, ternary systems of the water/organic solvent/surfactant type are suggested as liquid-crystalline media for enzymatic reactions. Two systems, water/octane/Aerosol OT and water/cyclohexane/Brij 96, have been used to solubilize acid and alkaline phosphatases and peroxidase. The enzymes under study do function in liquid-crystalline mesophases having lamellar, cylindrical (reversed hexagonal) and ball-shaped (reversed cubic) packing of the surfactant molecules. A significant result is that the phase transition from one liquid-crystalline structure to another entails, as a rule, a reversible change in the catalytic activity of the solubilized enzyme.

Biochemical studies on a novel vanadate- and molybdate-sensitive acid phosphatase from human epidermis

A novel vanadate- and molybdate-sensitive human skin epidermal acid phosphatase was purified and characterized. The enzyme was extracted from epidermal sheets with a 0.1% Triton X-100 solution buffered at pH 7.0. The purification procedure consisted of molecular permeation chromatography on Sephadex G-200 followed by chromatography on hydroxylapatite using an ammonium sulfate gradient. The molecular weight of the enzyme was 82,000 and the isoelectric point was at pH 5.6. At the optimum pH (5.1) the enzyme hydrolyzed most rapidly 1-naphthyl phosphate (Km = 0.28 mM) and 4-nitrophenyl phosphate (Km = 0.28 mM). In general, the best substrates had an aromatic leaving group. Fluoride (Ki = 39 microM; noncompetitive) and phosphate (competitive) inhibited by binding to different binding sites of the enzyme. The most potent inhibitors were vanadate (Ki = 1.9 X 10(-6)M), tungstate (Ki = 1.4 X 10(-7)M), and molybdate (Ki = 2.0 X 10(-9)M). Chemical modification and kinetic experiments suggested that the activity of the enzyme is based on imidazole, tyrosyl, and carboxyl groups. Benzoyl peroxide was a relatively potent inhibitor (Ki = 5.0 X 10(-5)M; noncompetitive). This enzyme resembled the prostatic acid phosphatase with regard to substrate specificity, inhibition characteristics, and functional groups.

Unambiguous stereochemical course of rabbit liver fructose bisphosphatase hydrolysis

The stereochemical course of rabbit liver fructose bisphosphatase (EC 3.1.3.11) was determined by hydrolyzing the substrate analogue (Sp)-[1-18O]fructose 1-phosphorothioate 6-phosphate in H(2)17O, incorporating the chiral, inorganic phosphorothioate product into adenosine 5'-O-(2-thiotriphosphate) (ATP beta S), and analyzing the isotopic distribution of 18O in ATP beta S by 31P NMR. The result indicates that the 1-phosphoryl group is transferred with inversion of configuration. A series of single-turnover experiments ruled out an acyl phosphate intermediate in the hydrolysis. Consequently, fructose bisphosphatase catalyzes the hydrolysis of fructose 1,6-bisphosphate via a direct transfer of the phosphoryl moiety to water.

Isolation and characterization of a homogeneous acid phosphatase from catfish liver

A homogeneous, tartrate-inhibitable acid phosphatase (AcPase) was obtained from the liver of channel catfish (Ictalurus punctatus) by the use of Affi Gel-10-coupled aminohexyltartramic acid affinity chromatography. The enzyme has a molecular weight of 82,500 and is a dimer consisting of two apparently equivalent subunits with subunit weights of 35,000 +/- 3000. Amino acid composition data are presented and compared with those of mammalian acid phosphatases. Data suggest that the enzyme is a metalloacid phosphatase. Catfish liver AcPase exhibits two molecular forms with pI 5.66 and 5.37 which were separated by chromatofocusing. A spontaneous conversion of the less acidic form to a more acidic form was observed and this conversion was accompanied by a decreased sensitivity towards tartrate inhibition.

An essential carboxylic acid group in human prostate acid phosphatase

Treatment of homogenous human prostatic acid phosphatase (orthophosphoric-monoester phosphohydrolase (acid optimum), EC 3.1.3.2) with low concentrations of Woodward's reagent K (N-ethyl-5-phenylisoxazolium-3'-sulfonate) leads to a rapid loss of enzymic activity. The rate of inactivation of the enzyme is reduced in the presence of the competitive inhibitors phosphate and L-(+)-tartrate, but not in the presence of non-inhibitory D-tartrate. Measurement of the ethylamine produced upon hydrolysis of enzyme modified in the presence of D- and of L-tartrate permitted the quantitative estimation of the number of carboxylic acid residues at the active site. The data indicate that two carboxyl groups per (dimeric) enzyme molecule are essential for catalytic activity. It is proposed that one function of the active site carboxyl group may be to protonate the leaving alcohol or phenol portion of the phosphomonoester substrate during the formation of the covalent phosphoenzyme intermediate.

Dimeric nature and amino acid compositions of homogeneous canine prostatic human liver and rat liver acid phosphatase isoenzymes. Specificity and pH-dependence of the canine enzyme

Two isoenzymes of rat liver acid phosphatase (orthophosphoric-monoester phosphohydrolase (acid optimum) EC 3.1.3.2) have been purified to homogeneity, at least one of these for the first time. Both of the rat liver isoenzymes have identical specific activities towards p-nitrophenyl phosphate. Molecular weights of the native enzymes are 92 000 for rat liver isoenzyme I and 93 000 for isoenzyme II, while the subunit molecular weights are 51 000 and 52 000 respectively. Data on substrate specificity and pH dependence are presented for the homogeneous canine prostatic enzyme, which is also isolated as a dimeric enzyme of (native) molecular weight 89 000. Carbohydrate analysis data are presented for canine prostatic acid phosphatase and it is further noted that both isoenzymes of rat liver acid phosphatase are also glycoproteins. The amino acid compositions of the two rat liver isoenzymes are presented together with those of the similar dimeric acid phosphatase of human liver and of canine prostate. Comparison of these results with published data for the amino acid composition of human prostatic acid phosphatase shows substantial similarities. However, significant differences are seen in the amino acid composition of rat liver acid phosphatase isoenzyme I as compared to a previous literature report. Most notably, 17 histidine residues are found per mol of isoenzyme I and 18 for isoenzyme II.

An essential active-site histidine residue in human prostatic acid phosphatase. Ethoxyformylation by diethyl pyrocarbonate and phosphorylation by a substrate

Human prostatic acid phosphatase (orthophosphoric-monoester phosphohydrolase (acid optimum), EC 3.1.3.2) is a dimeric (alpha 2) protein that catalyses the hydrolysis of phosphomonoesters. Several reports suggest that a phosphoenzyme intermediate is involved in the mechanism of acid phosphatase. Chemical modification studies and trapping experiments were therefore undertaken in order to ascertain the identity of the amino acid residue(s) involved in the formation of this intermediate. Human prostatic acid phosphatase is inactivated by diethyl pyrocarbonate (second-order rate constant of 7 M-1. min-1 at pH 6.2) with an accompanying increase in absorbance at 242 nm due to formation of ethoxyformylhistidyl derivatives. In the presence of competive inhibitors the rate of inactivation is decreased. Inactivation can be partially reversed by hydroxylamine. The pH curve of inactivation indicates the involvement of a residue having a pK alpha of 6.5. Direct evidence for the involvement of a histidine residue in the mechanism was obtained by trapping a covalent phosphohistidyl-enzyme intermediate. Incubation of the enzyme with p-nitrophenyl [32 P] phosphate leads to incorporation of 0.44 mol 32P/mol enzyme. The denatured phosphoenzyme,which was acid labile but base stable, was hydrolyzed in 3 M KOH and the radioactivity was found to cochromatograph with synthetic tau-phosphohistidine on Dowex-1 ion-exchange resin. These results are consistent with a catalytic mechanism involving histidine as a nucleophile in the formation of a covalents phosphoenzyme intermediate.