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1 Department of Biochemistry, 2 Department of Biological Sciences, and 3 Cancer Center, Purdue University, West Lafayette, Indiana 47907, USA4 Walther Cancer Institute, Indianapolis, Indiana 46202, USA
Reprint requests to: Victor W. Rodwell, Department of Biochemistry, Purdue University, 175 South University Street, West Lafayette, Indiana 47907-2063; e-mail: vrodwell{at}purdue.edu; fax: (765) 494-7897.
(RECEIVED November 2, 2004; FINAL REVISION February 15, 2005; ACCEPTED February 15, 2005)
| Abstract |
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Keywords: phosphomevalonate kinase; isoprenoid biosynthesis; mevalonate pathway; 5-phosphomevalonate; mevalonate 5-phosphate; mevalonate 5-diphosphate; isopentenyl diphosphate; Enterococcus faecalis
Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.041210405.
| Introduction |
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Phosphomevalonate kinase, the fifth enzyme of the mevalonate pathway, catalyzes transfer of the
-phosphoryl group of ATP to the hydroxyl of the phosphoryl group of mevalonate 5-phosphate forming mevalonate 5-diphosphate. Phosphomevalonate kinase is a member of the family of GHMP kinases that participate in diverse metabolic processes (Yang et al. 2002) whose shared features include unique kinase folds and a conserved PX3GSSAA sequence that forms a phosphate-binding "p-loop" that contacts the
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-phosphates of ATP (Krishna et al. 2001). Here we report the cloning, expression, purification, and characterization of the phosphomevalonate kinase of E. faecalis.
| Results |
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Effect of temperature
Optimal phosphomevalonate kinase activity occurred at 37°C (Fig. S-2; see Supplemental Material). An activation energy (Ea) of 5.6 kcal/mol was calculated from the slope of 1.22 of an Arrhenius plot of selected data (Fig. S-2, inset; see Supplemental Material) and the relationship slope = Ea/2.303R = Ea/4.58.
Kinetic constants
Km values for phosphomevalonate kinase derived from Lineweaver-Burke plots were 0.19 mM for (R,S)-5-phosphomevalonate (Fig. S-3; see Supplemental Material) and 0.17 mM for ATP (Fig. S-4; see Supplemental Material).
Effect of divalent cations
As anticipated, phosphorylation of phosphomevalonate required a divalent cation. Significantly higher activity was detected using Mn++ than with Mg++. Optimal activity occurred at about 4 mM Mn++ (Fig. 3
). Ca++, 10 mM, could not replace Mn++ or Mg++.
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| Discussion |
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Antibiotics typically target a reaction unique to the pathogen. That humans and E. faecalis both use exclusively the mevalonate pathway for biosynthesis of IPP does not, however, preclude consideration of this pathway for the design of new antibiotics. Providing that significant structural differences characterize the human and bacterial isoforms, the enzymes of the mevalonate pathway could indeed represent appropriate targets for drug development. But do such differences exist? For HMG-CoA reductase, marked structural differences characterize the class I human enzyme and the class 2 enzyme of a bacterial nonpathogen (Tabernero et al. 2003; Hedl et al. 2004). While at present the only relevant crystal structures are those of HMG-CoA reductase, coordinates are on file for both E. faecalis and human HMG-CoA synthase, and crystallization trials for E. faecalis HMG-CoA reductase have been initiated. The monomers of the bacterial phosphomevalonate kinases are almost twice as large as those of the human and Caenorhabditis elegans. Size differences are most apparent at the N and C termini. Of the three glycine-rich motifs that characterize GHMP kinases (Tsay and Robinson 1991; Zhou et al. 2000), the mammalian and C. elegans enzymes lack motifs 1 and 3, and possibly also motif 2 (Fig. 4
). Additional differences will become apparent when native and binary complex crystal structures of the pathogen and human enzymes become available. The central metabolic roles of kinases dictate that a useful inhibitor of a specific kinase should be an analog of the substrate rather than of ATP. This parallels the inhibition HMG-CoA reductase by statins, which are analogs of the substrate, not of NADPH.
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| Materials and methods |
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DNA manipulations
PCR product purification, plasmid isolation, and DNA extraction from agarose gels kits were from Qiagen. Restriction enzyme-digested DNA was purified using a low melting point agarose gel (Invitrogen) prior to in-gel ligations. Standard protocols (Sambrook et al. 1989) were used for most other DNA manipulations.
Construction of the expression plasmid
E. faecalis strain 41 genomic DNA was used as a template to PCR amplify the 1107-bp mvaK2 open reading frame thought to encode phosphomevalonate kinase. The forward primer 5'-cgcgcgcatatgattgaagttactacg encoded an NdeI site at the N terminus (underlined) and the reverse primer 5'-cgcgcaagctttcatcttttcgattcatgct, a HindIII site at the C terminus (underlined). Ligation into the NdeI and HindIII sites of pET28b(+) (Novagen) yielded expression plasmid pET28EFK2. The DNA sequence of both strands of the insert, determined at the Purdue Genomics Core Facility, corresponded to that of the NCBI database sequence, and verified that no mutation had been introduced during PCR amplification.
Expression and purification of the gene product
Conditions optimal for expression of soluble protein were investigated in small-scale cultures and evaluated by SDS-PAGE. For optimal expression, Escherichia coli BL21(DE3) cells (Invitrogen) transformed with pET28EFK2 were grown initially at 37°C in Luria-Bertani broth (Sambrook et al. 1989) containing 50 µg/mL kanamycin to an A600 of 0.81.0, then cooled rapidly to 16°C. Following addition of 0.4 mM IPTG, growth at 16°C was continued for an additional 45 h. Cells were harvested by centrifugation; washed in 0.9% saline; suspended in buffer A (300 mM NaCl, 50 mM Tris [pH 8.0]) containing 10 mM imidazole, 1.0 mM phenylmethylsulfonyl fluoride, and 1 µg/mL each of pepstatin, leupeptin, and aprotinin; and ruptured in a French Pressure cell. The supernatant liquid obtained after centrifugation of the cell lysate was applied to a 25-mL column of Ni-NTA agarose equilibrated in buffer A containing 10 mM imidazole and washed in with 50 mL of the same buffer. Elution used successive 50-mL portions of 50 and 100 mM imidazole in buffer A. Migration of the eluted protein on SDS-PAGE (Fig. S-5; see Supplemental Material) was consistent with the observed solution and predicted calculated molecular weights of 48.0 (Table 1
, species 1) and 40.5 kDa, respectively. Protein yield, determined by the method of Bradford (1976), averaged 20 mg per liter.
MALDI mass spectrometry
Purified protein digested with trypsin was analyzed in positive reflector mode on an Applied Biosystems 4700 MALDI tandem time-of-flight mass spectrometer calibrated with standard peptides (Sigma). A list of monoisotopic peaks from the experimental sample spectra was submitted to Mascot (Matrix Science) for a peptide mass fingerprint analysis using the entire NCBI protein database and a peptide error tolerance of 100 ppm. This returned a single statistically significant result of the nearly two million protein entries in the database, E. faecalis phosphomevalonate kinase. Sequence coverage of the identified protein was 38%.
Assay of phosphomevalonate kinase activity
Measurement of phosphomevalonate kinase activity used pyruvate kinase and lactate dehydrogenase to couple mevalonate 5-phosphate-dependent production of ADP to the oxidation of NADH, monitored spectrophotometrically at 340 nm. Standard assay conditions at 25°C used a 200-µL reaction volume containing 50 mM Tris (pH 8.0), 75 mM KCl, 5 mM MgCl2, 5 mM MnCl2, 2.5 mM phosphoenolpyruvate, 2.0 mM ATP, 1.0 mM (R,S)-phosphomevalonate, 10 mM DTT, ~0.16 mM NADH, 5 units of L-lactate dehydrogenase, and 10 units of pyruvate kinase. Initial rates were calculated from the linear portion of the time course that followed a short initial lag due to the time required to accumulate sufficient ADP and pyruvate. Specific activity is expressed as micromol phosphomevalonate turned over per minute per milligram of protein and was determined to be 3.9 µmol substrate converted per minute per milligram of protein. In experiments where pH or temperature were varied, additional pyruvate kinase and lactate dehydrogenase were included to verify that the activity measured was that of phosphomevalonate kinase, not of the coupling enzymes.
Analytical ultracentrifugation
Sedimentation velocity studies used a Beckman XLI analytical ultracentrifuge and interference optics. Double-sector charcoal-filled epon centerpieces contained 420 µL of 20 mM HEPES (pH 8.0), 300 mM NaCl, and 1.0 mM DTT, plus or minus 1.0 mg/mL enzyme. Density, determined on an Anton-Paar DMA5000 density meter, was 1.016 g/cm3. Values for v (0.7398 mL/g) and viscosity (0.01031p) were calculated from the protein sequence and the buffer composition using SedNterp1.07 (http://www.rasmb.bbri.org/rasmb/windows/sednterp-philo). Following a brief low-speed centrifugation in an An60 Ti rotor at 20°C to sediment insoluble protein and a 60-min equilibration period at 0 rpm, the sample was sedimented at 45,000 rpm for 5 h. Fringe displacement profiles were acquired at 1-min intervals. Analysis of sedimentation velocity profiles used the procedures of Schuck (1998) and Schuck et al. (2002) to refine the positions of the meniscus and cell bottom for the continuous size-distribution analysis. Sedimentation coefficient distributions were calculated with the c(s) method by modeling with distributions of the Lamm equation solutions (Schuck et al. 2002). Analysis of the continuous sedimentation coefficient distribution used Sedfit 8.9 (http://www.analyticalultracentrifugation.com/sedfit.htm). Models with four discrete species were fit to the sedimentation velocity data with Sedphat 2.0b (http://www.analyticalultracentrifugation.com/sedphat/sedphat.htm) using several different weight average masses based on the enzyme sequence for the four largest peaks to insure a global minimum for both s and M.
| Footnotes |
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| Acknowledgments |
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