Bovine testis acylphosphatase: purification and amino acid sequence

Acylphosphatase is a strong apoptosis inducer in HeLa cell line

Acylphosphatase (AcP) is a low-molecular-weight protein widely distributed in many vertebrate tissues with a yet unknown physiologic function. To study the in vivo behavior of AcP, HeLa cells were transiently transfected with a vector expressing the AcP/EGFP fusion protein. Analysis of the transfected cells showed a high level of cellular death in cells expressing the AcP/EGFP fusion protein with respect to control cells expressing EGFP alone. Flow cytometry and time lapse analysis of AcP/EGFP transfected cells evidenced a typical pattern of apoptosis. Surprisingly, cells transfected with a mutated form of AcP, with negligible in vitro acylphosphatase activity, undergo apoptosis as well as cells transfected with wild-type protein, suggesting that the physiologic role of AcP could be not related to this catalytic activity.

Common-type acylphosphatase: steady-state kinetics and leaving-group dependence

A number of acyl phosphates differing in the structure of the acyl moiety (as well as in the leaving-group pKa of the acids produced in hydrolysis) have been synthesized. The Km and Vmax values for the bovine common-type acylphosphatase isoenzyme have been measured at 25 degrees C and pH 5.3. The values of kcat differ widely in relation to the different structures of the tested acyl phosphates: linear relationships between log kcat and the leaving group pKa, as well as between log kcat/Km and the leaving-group pKa, were observed. On the other hand, the Km values of the different substrates are very close to each other, suggesting that the phosphate moiety of the substrate is the main chemical group interacting with the enzyme active site in the formation of the enzyme-substrate Michaelis complex. The enzyme does not catalyse transphosphorylation between substrate and concentrated nucleophilic acceptors (glycerol and methanol); nor does it catalyse H218O-inorganic phosphate oxygen exchange. It seems that no phosphoenzyme intermediate is formed in the catalytic pathway. Furthermore, during the enzymic hydrolysis of benzoyl phosphate in the presence of 18O-labelled water, only inorganic phosphate (and not benzoate) incorporates 18O, suggesting that no acyl enzyme is formed transiently. all these findings, as well as the strong dependence of kcat upon the leaving group pK1, suggest that neither a nucleophilic enzyme group nor general acid catalysis are involved in the catalytic pathway. The enzyme is competitively inhibited by Pi, but it is not inhibited by the carboxylate ions produced during substrate hydrolysis, suggesting that the last step of the catalytic process is the release of Pi. The activation energy values for the catalysed and spontaneous hydrolysis of benzoyl phosphate have been determined.

Characterization of multiple acid phosphatases in bovine liver cytosol and lysosome. Inactivation of cytosolic enzymes by disulfides and its redox regulation by thioltransferase

Cytosolic and lysosomal acid phosphatases have the ability to hydrolyze orthophosphoric monoesters below pH 5-6. However, it is thought they may have different intracellular roles. To clarify their properties, substrate specificity, inhibitor sensitivity and the modulation of enzyme by redox conditions were determined using bovine liver enzymes. DEAE-cellulose chromatography following (NH4)2SO4 fractionation revealed three forms of cytosolic acid phosphatases as in the KCl gradient (0-500 mM). After Sephadex G-75 gel filtration, the enzymes appeared as single bands on SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Their activities for D-erythrose 4-phosphate co-purified with p-nitrophenylphosphatase activities in all steps. In contrast the lysosomal enzyme was purified by Octyl-Sepharose column chromatography after n-butanol treatment, (NH4)2SO4 fractionation, Bio gel P-200 gel filtration and DE-52 chromatography. The relative molecular masses (M(r)) determined by SDS-PAGE indicated that M(r) of the cytosolic enzymes (16,000) was less that of lysosomal enzyme (160,000). The cytosolic enzymes were active against sugar phosphates and were inhibited by 1 mM Cu2+. In addition, the cytosolic enzymes were inactivated by 5 mM oxidized glutathione and protected by 10 mM reduced glutathione (in the presence or absence of thioltransferase), suggesting that sensitive cysteinyl residue(s) existed. The lysosomal enzyme was active against various substrates and was strongly inhibited by 1 mM Cu2+ and 2 mM fluoride. The results presented here suggest that cytosolic enzymes have different properties from those of lysosomal enzyme with respect to substrates, inhibitors and regulation of activity.

Structural and kinetic investigations on the 15-21 and 42-45 loops of muscle acylphosphatase: evidence for their involvement in enzyme catalysis and conformational stabilization

The structural and catalytic importance of the 15-21 and 42-45 loop residues of the acylphosphatase muscular isoenzyme has been investigated by oligonucleotide-directed mutagenesis. Seven mutants involving conserved residues of the two loops have been prepared and characterized for structural, kinetic, and stability features by using different spectroscopic techniques and compared to the wild-type enzyme. The results are discussed in light of the crystal structure of the highly homologous common type acylphosphatase [Thunnissen et al. (1997) Structure 5, 69-79]. A differential role of the two loops has emerged: the 15-21 and the 42-45 loops appear mainly involved in active site formation and enzyme structural stabilization, respectively. These conclusions are supported by a strong impairment of the catalytic efficiency, in terms of enzymatic activity and substrate binding capability, for most of the 15-21 loop mutants. In particular, the Gly15Ala mutant is completely inactive and displays a native-like overall fold, indicating that the correct geometry of the 15-21 loop is an essential requisite for optimal enzymatic catalysis. Instead, the Gly45Ala mutant, though revealing unchanged catalytic properties, shows a considerably reduced conformational stability, as judged by circular dichroism and 1H NMR spectroscopy. This finding confirms previous results relative to Thr42 and Thr46 residues [Taddei et al. (1996) Biochemistry 35, 7077-7083] underlining the structural importance of the 42-45 loop as a linker for the two beta alpha beta units constituting the overall enzyme structure.

Crystal structure of common type acylphosphatase from bovine testis

BACKGROUND

Acylphosphatase (ACP) is a low molecular weight phosphomonohydrolase catalyzing with high specificity the hydrolysis of the carboxyl-phosphate bond present in acylphosphates. The enzyme is thought to regulate metabolic processes in which acylphosphates are involved, such as glycolysis and the production of ribonucleotides. Furthermore the enzyme is capable of hydrolyzing the phospho-aspartyl intermediate formed during the action of membrane pumps such as (Ca2++Mg2+) ATPase. Although the tertiary structure of a muscle ACP has been determined by NMR spectroscopy, little is known about the catalytic mechanism of ACP and further structures might provide an increased understanding.

RESULTS

The structure of 'common type' ACP from bovine testis has been determined by X-ray crystallography to a resolution of 1.8 A. The structure has been refined to an R factor of 17.0 % using all data between 15 and 1.8 A. The binding of a sulphate and a chloride ion in the active centre allows a detailed description of this site. The overall protein folds of common type and muscle ACP are similar but their loops have very different conformations. These differences, in part, are probably caused by the binding of the ions in the active site of the common type form. The phosphate-binding loop of ACP shows some remarkable similarities to that of low molecular weight protein tyrosine phosphatase.

CONCLUSIONS

The active site of ACP has been located, enabling a reaction mechanism to be suggested in which the phosphate moiety bound to Arg23 acts as a base, abstracting a proton from a nucleophilic water molecule liganded to Asn41. The transition-state intermediate is stabilized by the phosphate-binding loop. We suggest the catalysis to be substrate assisted, which probably explains why this enzyme can only hydrolyze acylphosphates.

Looking for residues involved in the muscle acylphosphatase catalytic mechanism and structural stabilization: role of Asn41, Thr42, and Thr46

Asn41, Thr42, and Thr46 are invariant residues in both muscle and erythrocyte acylphosphatases isolated so far. Horse muscle acylphosphatase solution structure suggests their close spatial relationship to Arg23, the main substrate binding site. The catalytic and structural role of such residues, as well as their influence on muscle acylphosphatase stability, was investigated by preparing several gene mutants (Thr42Ala, Thr46Ala, Asn41Ala, Asn41Ser, and Asn41Gln) by oligonucleotide-directed mutagenesis. The mutated genes were cloned and expressed in Escherichia coli, and the mutant enzymes were purified by affinity chromatography and investigated as compared to the wild-type enzyme. The specific activity and substrate affinity of Thr42 and Thr46 mutants were not significantly affected. On the contrary, Asn41 mutants showed a residual negligible activity (about 0.05-0.15% as compared to wild-type enzyme), though maintaining an unchanged binding capability of both substrate and inorganic phosphate, an enzyme competitive inhibitor. According to the 1H nuclear magnetic resonance spectroscopy and circular dichroism results, all mutants elicited well-constrained native-like secondary and tertiary structures. Thermodynamic parameters, as calculated from circular dichroism data, demonstrated a significantly decreased stability of the Thr42 mutant under increasing temperatures and urea concentrations. The reported results strongly support a direct participation of Asn41 to the enzyme catalytic mechanism, indicating that Asn41 mutants may well represent a useful tool for the investigation of the enzyme physiological function by the negative dominant approach.

C-terminal region contributes to muscle acylphosphatase three-dimensional structure stabilisation

Ser-Ala and Ser-Ala-Ser-Ala C-terminus elongated (delta+2 and delta+4, respectively) and two C-terminus deleted (delta-2 and delta-3) muscle acylphosphatase mutants were investigated to assess the catalytic and structural roles of the C-terminal region. The kinetic analysis of these mutants shows that the removal of two or three C-terminal residues reduces the catalytic activity to 7% and 4% of the value measured for the wild-type enzyme, respectively; instead, the elongation of the C-terminus does not significantly change the enzyme behaviour. 1H Nuclear magnetic resonance spectroscopy indicates that all mutants display a native-like fold though they appear less stable, particularly delta-2 and delta-3 mutants, as compared to the wild-type enzyme. Such destabilisation of the C-terminal modified mutants is further confirmed by urea inactivation experiments. The results here presented account for an involvement of the C-terminal region in the stabilisation of the three-dimensional structure of acylphosphatase, particularly at the active-site level. Moreover, a participation of the C-terminal carboxyl group to the catalytic mechanism can be excluded.

Properties of Cys21-mutated muscle acylphosphatases

Cys21 is an invariant residue in muscle acylphosphatases, but is absent in the erythrocyte isozymes. To assess the importance of this residue in the muscle isozymes for catalytic, structural, and stability properties, two gene mutants have been prepared by oligonucleotide-directed mutagenesis and expressed in Escherichia coli cells; in these mutants, the codon for Cys21 was replaced by those for Ser and Ala, respectively. The two mutant enzymes, purified by immunoaffinity chromatography, showed kinetic and structural properties similar to those of the wild-type recombinant enzyme; however, the specific activity of the two mutants, especially that of the C21A mutant, was lower. The urea and thermal stabilities of the mutant enzymes were reduced with respect to those of the wild-type form, contrary to the susceptibility to inactivation by mercuric ions. The reported data support the possibility that Cys21 is involved in the stabilization of the enzyme active-site conformation.

Crystallisation and preliminary X-ray analysis of the 'common-type' acylphosphatase

Single crystals of a 'common-type' acylphosphatase from bovine testis have been grown. Crystals belong to space group C2 and have cell dimensions a = 64.6 A, b = 36.5 A, c = 45.2 A and beta = 104.8 and contain one monomer per asymmetric unit. The crystals diffract better than 2.0 A resolution and are well suited for an X-ray structure determination.

Properties of N-terminus truncated and C-terminus mutated muscle acylphosphatases

Enzymatic activity and structure of N-terminus truncated and C-terminus substituted muscle acylphosphatase mutants were investigated by kinetic studies under different conditions and 1H NMR spectroscopy, respectively. The N-terminus truncated mutant lacked the first six residues (delta 6), whereas arginine 97 and tyrosine 98 were replaced by glutamine giving two C-terminus substituted mutants (R97Q and Y98Q, respectively). All acylphosphatase forms were obtained by modifications of a synthetic gene coding for the human muscle enzyme which was expressed in E. coli. The delta 6 deletion mutant elicited a reduced specific activity and a native-like structure. The kinetic and structural properties of R97Q and Y98Q mutants indicate a possible role of Arg-97 in the stabilisation of the active site correct conformation, most likely via back-bone and side chain interactions with Arg-23, the residue involved in phosphate binding by the enzyme. This study also suggests a possible involvement of Tyr-98 in the stabilisation of the acylphosphatase overall structure.

2-Methoxybenzoyl phosphate: a new substrate for continuous fluorimetric and spectrophotometric acyl phosphatase assays

A new aromatic acyl phosphate, 2-methoxybenzoyl phosphate, has been synthesized. The compound shows an intrinsic fluorescence; it displays an intense emission band at 390 nm upon excitation in the near UV region. This band practically disappears after hydrolysis of the product. On the other hand, the product displays differences in the near UV absorption spectra measured before and after hydrolysis. The delta epsilon at 301 nm is 2720 M-1 cm-1, a value that is 4.3-fold higher than that of benzoyl phosphate (the usual substrate for acylphosphatase assay) at 283 nm. The main kinetic parameters of three different acylphosphatase molecular forms (the muscular isoenzyme and two subtypes of the organ common isoenzyme) were determined using both benzoyl phosphate and 2-methoxybenzoyl phosphate as substrates, and then compared. These kinetic data and the UV absorption and fluorescence properties of 2-methoxybenzoyl phosphate suggest that this compound has better substrate features than benzoyl phosphate, and can be used for both high sensitivity continuous fluorimetric and UV absorption spectrophotometric assays of acylphosphatase.

Arginine-23 is involved in the catalytic site of muscle acylphosphatase

Three mutants of human muscle acylphosphatase in which arginine-23 was replaced by glutamine, histidine and lysine, respectively, were prepared by oligonucleotide-directed mutagenesis of a synthetic gene coding for the enzyme. All mutants, purified by affinity chromatography, were almost completely unable to catalyze the hydrolysis of the substrate. 1H-NMR spectroscopy experiments showed the absence of any major conformational changes of the three mutants with respect to the wild-type recombinant enzyme. Equilibrium dialysis experiments demonstrated that the mutated proteins lost the ability of binding inorganic phosphate, a competitive inhibitor of the enzyme. These results strongly support an involvement of arginine-23 at the phosphate binding-site of acylphosphatase, confirming the hypothesis of the existence of a phosphate binding structural motif recently proposed by other authors.

Porcine liver low M(r) phosphotyrosine protein phosphatase: the amino acid sequence

Porcine low M(r) phosphotyrosine protein phosphatase has been purified and the complete amino acid sequence has been determined. Both enzymic and chemical cleavages are used to obtain protein fragments. FAB mass spectrometry and enzymic subdigestion followed by Edman degradation have been used to determine the structure of the NH2-terminal acylated tryptic peptide. The enzyme consists of 157 amino acid residues, is acetylated at the NH2-terminus, and has arginine as COOH-terminal residue. It shows kinetic parameters very similar to other known low M(r) PTPases. This PTPase is strongly inhibited by pyridoxal 5'-phosphate (Ki = 21 microM) like the low M(r) PTPases from bovine liver, rat liver (AcP2 isoenzyme), and human erythrocyte (Bslow isoenzyme). The comparison of the 40-73 sequence with the corresponding sequence of other low M(r) PTPases from different sources demonstrates that this isoform is highly homologous to the isoforms mentioned above, and shows a lower homology degree with respect to rat AcP1 and human Bfast isoforms. A classification of low M(r) PTPase isoforms based on the type-specific sequence and on the sensitivity to pyridoxal 5'-phosphate inhibition has been proposed.