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1 Department of Biochemistry, Purdue University, West Lafayette, Indiana 47907-2063, USA
Reprint requests to: Victor W. Rodwell, Department of Biochemistry, Purdue University, 175 South University Street, West Lafayette, IN 47907-2063, USA; e-mail: vrodwell{at}purdue.edu; fax: (765) 494-7897.
(RECEIVED August 13, 2003; FINAL REVISION October 16, 2003; ACCEPTED November 10, 2003)
| Abstract |
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Ha was 22 kcal/mole. Kinetic analysis suggested that the reaction proceeds via a sequential mechanism. Km values were 0.33 mM (mevalonate), 1.1 mM (ATP), and 3.3 mM (Mg2+). Unlike mammalian mevalonate kinases, E. faecalis mevalonate kinase utilized all tested nucleoside triphosphates as phosphoryl donors. ADP, but not AMP, inhibited the reaction with a Ki of 2.7 mM. Keywords: Enterococcus faecalis; isoprenoid biosynthesis; isopentenyl diphosphate; mevalonate 5-phosphate; mevalonate pathway
Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.03367504.
| Introduction |
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Mevalonate kinase, the fourth enzyme of the mevalonate pathway, catalyzes a nucleophilic attack by a C5 anion of mevalonate on the
-phosphate of ATP forming mevalonate-5-phosphate:
![]() | (1) |
Mevalonate kinases have been purified and characterized from animal, plant, and archaeal sources (Tchen 1958; Potter and Miziorko 1997; Huang et al. 1999). A deficiency of mevalonate kinase in human subjects results in mevalonic aciduria and hyperimmunoglobulemia D (Houten et al. 2001).
Crystal structures of mevalonate kinase include a 2.4 Å structure of an ATP binary complex of the rat enzyme (Fu et al. 2002) and a 2.4 Å native structure of the thermostable enzyme from Methanococcus jannaschii. The overall fold of M. jannaschii mevalonate kinase resembles that of homoserine kinase from the same organism (Zhou et al. 2000; Yang et al. 2002). Site-directed mutagenesis has implicated several residues as important for catalysis. Glu 193H (subscripts on residue numbers refer to the enzymes from Enterococcus faecalis [E], human [H], and rat [R]) of the human enzyme and Lys 13R of the rat enzyme appear to be important for binding MgATP, and Ala 334H appears to participate in binding mevalonate (Hinson et al. 1997; Potter and Miziorko 1997; Potter et al. 1997). Asp 204H is thought to be the catalytic base that deprotonates the C-5 hydroxyl of mevalonate, generating the nucleophile that attacks the
-phosphate of ATP (Potter and Miziorko 1997).
We previously cloned, expressed, and characterized the E. faecalis enzymes that catalyze the first three reactions of the mevalonate pathway, acetoacetyl-CoA thiolase (Hedl et al. 2002), HMG-CoA synthase (Sutherlin et al. 2002), and HMG-CoA reductase (Hedl et al. 2002). We report here the cloning, expression, purification, and characterization of E. faecalis mevalonate kinase, the fourth enzyme of the mevalonate pathway, and the first characterized eubacterial mevalonate kinase.
| Results |
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Ha of 22 kcal/mole (Fig. 4
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Effect of adenosine di- and monophosphate
ADP and AMP structurally resemble ATP, the substrate for the reaction. Investigation of inhibition by ADP and AMP revealed that ADP acts an inhibitor of E. faecalis mevalonate kinase competitive with ATP with a Ki of 2.7 mM (Fig. 7
). By contrast, concentrations of AMP as high as 100 mM did not significantly inhibit activity.
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| Discussion |
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E. faecalis mevalonate kinase shares with both the hog enzyme (Beytia et al. 1970) and the yeast enzyme (Tchen 1958) a requirement for a divalent cation. Although it seems likely that the essential cation in vivo probably is Mg2+, preliminary investigations suggest that equal concentrations of Mn2+, Zn2+, or Ca2+ could each serve this function in vitro. Similar considerations apply to the phosphoryl donor. Although ATP, GTP, CTP, UTP, TTP, and ITP each served as the phosphoryl donor in vitro with similar Km and Vmax values, we consider ATP the most likely in vivo donor of the phosphoryl group. Broad specificity for the phosphoryl donor also characterizes yeast mevalonate kinase (Tchen 1958). By contrast, pig liver mevalonate kinase uses only ATP or ITP (Beytia et al. 1970). No data are available for the nucleotide specificity of the sole archaeal mevalonate kinase, possibly because the coupled spectrophotometric assay used to study this enzyme, which detects formation of ADP, does not lend itself to the investigation of phosphoryl donor specificity or of inhibition by ADP. ADP inhibits E. faecalis mevalonate kinase with a Ki of 2.7 mM, a value close to the 1.1 mM Km for ATP. By contrast, although similar in structure to ATP and ADP, AMP failed to inhibit, implying a requirement for a di- or tri-phosphate moiety for binding to the enzyme. Other nucleoside monophosphates were not investigated as potential inhibitors. These observations suggest that E. faecalis mevalonate kinase might be feedback-inhibited by ADP and that carbon flux through this enzyme might respond in vivo to ATP/ADP ratios.
Because a single monomer contains a complete active site, catalysis would not seem to require dimerization (Fu et al. 2002; Yang et al. 2002). The multimeric state of mevalonate kinase can, however, vary between monomer and dimer. Based on gel filtration data, rat and pig liver mevalonate kinases are homodimers (Beytia et al. 1970; Tanaka et al. 1990). The crystal structure of the rat holoenzyme revealed a dimer with a dimerization domain of two
-helices, one from each monomer. This domain comprises about 7.6% of the protein surface (Fu et al. 2002). By contrast, M. jannaschii kinase formed dimers in solution (Huang et al. 1999), but monomers in crystals (Yang et al. 2002). Our investigation of the mevalonate kinase of E. faecalis revealed that it exists primarily as a monomer in solution, with some dimer apparent (Fig. 3
). Differences between the apparent multimeric state of the E. faecalis kinase and other characterized mevalonate kinases may reflect factors such as protein concentration, the experimental method employed, and the lack of primary structure conservation between the region required for dimerization of the rat kinase and the corresponding sequence of the E. faecalis enzyme. Differences in multimeric states between bacterial and eukaryotic enzymes are also apparent in phosphomevalonate kinase, a member of a superfamily that includes mevalonate kinase. Although all known eukaryotic phosphomevalonate kinases exist as dimers, the S. pneumoniae enzyme is a monomer both in solution and in the crystalline states (Romanowski et al. 2002).
| Materials and methods |
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-D-thiogalactopyranoside (IPTG), T4 DNA ligase and restriction enzymes (New England Biolabs), shrimp alkaline phosphatase (Promega), 55 mCi/mmole 2-[14C]-(R,S)-mevalonate (Amersham), prestained low-range protein standards and Bradford reagent (Bio-Rad). NiNTA agarose (Qiagen) was used for Ni-affinity chromatography. Plasmid DNA preparations employed a QIAprep Spin Miniprep Kit (Qiagen) and agarose gel extractions a Qiagen Gel Extraction Kit. Synthetic oligonucleotides were prepared by IDT, Inc. of Coralville, Iowa. Unless otherwise specified, all other reagents were from Sigma.
Plasmids, bacterial strains, and culture media
Expression vector pET28b(+) was from Novagen. Bacterial strains used included E. coli strains DH5
and BL21(DE3) (Invitrogen). Genomic DNA from E. faecalis strain 41 (Glaxo SmithKline culture collection) was used for amplification of the mvaK1 open reading frame (PubMed locus number NP_814642
[GenBank]
) thought to encode mevalonate kinase. LuriaBertani (LB) medium and agar (Sambrook et al. 1989) supplemented with 50 µg of kanamycin per milliliter served for the growth of E. coli strains.
Construction of the expression plasmid
The mvaK1 gene that encodes a 314-residue polypeptide was PCR-amplified from E. faecalis genomic DNA using a forward primer (5'-CGT TCT TCA TAT GCA AGA AGG ACT TTT GTC C -3') and a reverse primer (5'-GTA TGC GCT CAA GCT TTT CCT GAA AGC -3') to introduce HindIII and BamHI restriction sites (underlined). The resulting 0.9-kb fragment was gel purified and ligated into Hind III and BamHI-digested pET28b(+) that had been dephosphorylated and gel purified (Sambrook et al. 1989). The resultant plasmid was termed EFK1-pET28b. Plasmid DNA was isolated from transformed E. coli DH5
cells and the insert was sequenced by the Iowa State University DNA Sequencing Facility to confirm the presence of unaltered mvaK1.
Expression and purification of the gene product
E. coli BL21(DE3) cells transformed with EFK1-pET28b were grown initially in 10 mL LBkan at 37°C for 15 h. The cultures were transferred to 100 mL LBkan, grown for an additional 6 h at 37°C, and transferred to 1 L of LBkan. Following addition of 0.5 mM IPTG at a cell density of 90 Klett units, growth was continued at 16°C for 48 h until the cells reached a density of about 300 Klett units. Cells were harvested by centrifugation, washed with 0.9% saline, and suspended in 500 mM KCl, 1 mM phenylmethylsulfonyl fluoride, 1 mM dithiothreitol, 100 mM 3-[cyclohexylamino]-1-propanesulfonic acid (CAPS), pH 10 (Buffer A). Lysis in a French pressure cell gave the cell lysate. The supernatant liquid obtained by centrifugation of the cell lysate (30,000 rpm, 60 min, 4°C) was applied to a NiNTA column. The column was washed with Buffer A, then eluted successively with 50 and 100 mM imidazole in Buffer A. Fractions with high activity were combined and stored at -70°C. Protein concentrations were determined by the method of Bradford (1976).
Assay of mevalonate kinase activity
Standard assay conditions for phosphorylation of mevalonate were modified from Popjak (1969). Briefly, the assays included 5 mM ATP, 1.0 mM (R,S)-mevalonate, 2.5 µCi 2-[14C]-(R,S)-mevalonate, 1.0 mM dithiothreitol (DTT), 10 mM MgCl2, 210 mM KCl, and 70 mM CAPS (pH 10), in a final volume of 130 µL. Reactions were initiated by the addition of 2-[14C]-(R,S)-mevalonate in 1 mM nonlabeled (R,S)-mevalonate. After a 10-min incubation at 37°C, the reaction was stopped by heating for 10 min at 94°C, then centrifuged (13,000 rpm, 15 min). Portions of the supernatant liquid were then applied either to a Kodak cellulose TLC sheet or to a Whatman 3 mm sheet and chromatographed in n-butanol:water: formic acid (77:13:10) in a small volume TLC chamber. One-centimeter sections were then removed and counted in 4 mL EcoLume scintillation liquid (ICN). Mevalonate phosphate migrated to an Rf of 0.2 to 0.3, and mevalonolactone to an Rf of 0.91.0. One enzyme unit (eu) represents the turnover, in 1 min, of 1 µmole of mevalonate. Reported results represent mean values for at least duplicate determinations.
Sedimentation velocity analytical ultracentrifugation
E. faecalis mevalonate kinase, 1.0 mg/mL or 0.4 mg/mL in 210 mM KCl, 70 mM CAPS (pH 10), was added to a Beckman analytical ultracentrifuge cell with sapphire windows and a charcoal-filled epoxy centerpiece. The cell was placed in a Beckman Model XL-1 centrifuge and allowed to come to thermal equilibrium at 20°C for 1 h. The sample was then spun at 50,000 rpm for 6 h. Rayleigh interference scans were taken at 3-min intervals.
| Acknowledgments |
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This work is journal paper 17095 from the Purdue University Agricultural Experiment Station.
The publication costs of this article were defrayed in part by payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.
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