Structure of the Methanococcus jannaschii mevalonate kinase, a member of the GHMP kinase superfamily

Gonadotropin receptors: role of post-translational modifications and post-transcriptional regulation

This review focuses on the post-translational modifications of LH and FSH receptors and recent studies on the regulation of LH receptor expression mediated by an RNA binding protein. Both LH and FSH receptors undergo extensive post-translational modifications. N-linked glycosylation occurs co-translationally and plays a role in the maturation and processing of the receptor, while palmitoylation is involved in receptor endocytosis and post-endocytic trafficking. A third type of post-translational modification is phosphorylation and its function has been reviewed. Finally, the regulation of LH receptor at the mRNA level by an RNA binding protein is discussed in the context of ovarian function.

Enterococcus faecalis phosphomevalonate kinase

The six enzymes of the mevalonate pathway of isopentenyl diphosphate biosynthesis represent potential for addressing a pressing human health concern, the development of antibiotics against resistant strains of the Gram-positive streptococci. We previously characterized the first four of the mevalonate pathway enzymes of Enterococcus faecalis, and here characterize the fifth, phosphomevalonate kinase (E.C. 2.7.4.2). E. faecalis genomic DNA and the polymerase chain reaction were used to clone DNA thought to encode phosphomevalonate kinase into pET28b(+). Double-stranded DNA sequencing verified the sequence of the recombinant gene. The encoded N-terminal hexahistidine-tagged protein was expressed in Escherichia coli with induction by isopropylthiogalactoside and purified by Ni(++) affinity chromatography, yield 20 mg protein per liter. Analysis of the purified protein by MALDI-TOF mass spectrometry established it as E. faecalis phosphomevalonate kinase. Analytical ultracentrifugation revealed that the kinase exists in solution primarily as a dimer. Assay for phosphomevalonate kinase activity used pyruvate kinase and lactate dehydrogenase to couple the formation of ADP to the oxidation of NADH. Optimal activity occurred at pH 8.0 and at 37 degrees C. The activation energy was approximately 5.6 kcal/mol. Activity with Mn(++), the preferred cation, was optimal at about 4 mM. Relative rates using different phosphoryl donors were 100 (ATP), 3.6 (GTP), 1.6 (TTP), and 0.4 (CTP). K(m) values were 0.17 mM for ATP and 0.19 mM for (R,S)-5-phosphomevalonate. The specific activity of the purified enzyme was 3.9 micromol substrate converted per minute per milligram protein. Applications to an immobilized enzyme bioreactor and to drug screening and design are discussed.

Classification of common functional loops of kinase super-families

A structural classification of loops has been obtained from a set of 141 protein structures classified as kinases. A total of 1813 loops was classified into 133 subclasses (9 betabeta(links), 15 betabeta(hairpins), 31 alpha-alpha, 46 alpha-beta and 32 beta-alpha). Functional information and specific features relating subclasses and function were included in the classification. Functional loops such as the P-loop (shared by different folds) or the Gly-rich-loop, among others, were classified into structural motifs. As a result, a common mechanism of catalysis and substrate binding was proved for most kinases. Additionally, the multiple-alignment of loop sequences made within each subclass was shown to be useful for comparative modeling of kinase loops. The classification is summarized in a kinase loop database located at http://sbi.imim.es/archki.

Molecular structure of human galactokinase: implications for type II galactosemia

Galactokinase functions in the Leloir pathway for galactose metabolism by catalyzing the MgATP-dependent phosphorylation of the C-1 hydroxyl group of alpha-D-galactose. The enzyme is known to belong to the GHMP superfamily of small molecule kinases and has attracted significant research attention for well over 40 years. Approximately 20 mutations have now been identified in human galactokinase, which result in the diseased state referred to as Type II galactosemia. Here we report the three-dimensional architecture of human galactokinase with bound alpha-D-galactose and Mg-AMPPNP. The overall fold of the molecule can be described in terms of two domains with the active site wedged between them. The N-terminal domain is dominated by a six-stranded mixed beta-sheet whereas the C-terminal motif contains six alpha-helices and two layers of anti-parallel beta-sheet. Those residues specifically involved in sugar binding include Arg37, Glu43, His44, Asp46, Gly183, Asp186, and Tyr236. The C-1 hydroxyl group of alpha-D-galactose sits within 3.3 A of the gamma-phosphorus of the nucleotide and 3.4 A of the guanidinium group of Arg37. The carboxylate side chain of Asp186 lies within approximately 3.2 A of the C-2 hydroxyl group of alpha-D-galactose and the guanidinium group of Arg37. Both Arg37 and Asp186 are strictly conserved among both prokaryotic and eukaryotic galactokinases. In addition to providing molecular insight into the active site geometry of the enzyme, the model also provides a structural framework upon which to more fully understand the consequences of the those mutations known to give rise to Type II galactosemia.

Substrate specificity and mechanism from the structure of Pyrococcus furiosus galactokinase

Galactokinase (GalK) catalyses the first step of the Leloir pathway of galactose metabolism, the ATP-dependent phosphorylation of galactose to galactose-1-phosphate. In man, defects in galactose metabolism can result in disorders with severe clinical consequences, and deficiencies in galactokinase have been linked with the development of cataracts within the first few months of life. The crystal structure of GalK from Pyrococcus furiosus in complex with MgADP and galactose has been determined to 2.9 A resolution to provide insights into the substrate specificity and catalytic mechanism of the enzyme. The structure consists of two domains with the active site in a cleft at the domain interface. Inspection of the substrate binding pocket identifies the amino acid residues involved in galactose and nucleotide binding and points to both structural and mechanistic similarities with other enzymes of the GHMP kinase superfamily to which GalK belongs. Comparison of the sequence of the Gal3p inducer protein, which is related to GalK and which forms part of the transcriptional activation of the GAL gene cluster in the yeast Saccharomyces cerevisiae, has led to an understanding of the molecular basis of galactose and nucleotide recognition. Finally, the structure has enabled us to further our understanding on the functional consequences of mutations in human GalK which cause galactosemia.

Isolation and Characterization of a Novel trans-Factor for Luteinizing Hormone Receptor mRNA from Ovary

Post-transcriptional mechanisms play a major role in regulating luteinizing hormone (LH) receptor mRNA expression in the ovary. An ovarian cytosolic protein that we have identified in rats and humans, which binds to a polypyrimidine-rich bipartitate sequence in the coding region of LHR mRNA, acts as a trans-acting factor in this process. In the present study, we isolated and characterized this LH receptor mRNA-binding protein (LRBP) from rat ovary. LRBP was purified to homogeneity by cation exchange chromatography followed by Northwestern analysis and subsequent elution of the single protein band from SDS-polyacrylamide gel. Purified LRBP was subjected to N-terminal microsequencing followed by homology search, which revealed its identity as mevalonate kinase. Purified rat mevalonate kinase antibody recognized the gel-purified LRBP on Western blots performed with one- and two-dimensional SDS-polyacrylamide gels. When recombinant mevalonate kinase produced in human embryonic kidney cells (293 cells) was tested, it showed all of the characteristics of LRBP with respect to specificity of LHR mRNA binding sequence, as examined by gel mobility shift analysis. Inhibition of LHR mRNA binding activity of mevalonate kinase in the presence of ATP and mevalonate indicates that the RNA recognition site of mevalonate kinase might involve the ATP/mevalonate binding region of the protein. Treatment of 293 cells with mevastatin to deplete cellular mevalonate resulted in an increase in LHR mRNA binding activity of mevalonate kinase. Collectively, the data support the novel function of rat mevalonate kinase as a LHR mRNA-binding protein in the post-transcriptional regulation of LH receptor expression in the ovary.

Enterococcus faecalis mevalonate kinase

Gram-positive pathogens synthesize isopentenyl diphosphate, the five-carbon precursor of isoprenoids, via the mevalonate pathway. The enzymes of this pathway are essential for the survival of these organisms, and thus may represent possible targets for drug design. To extend our investigation of the mevalonate pathway in Enterococcus faecalis, we PCR-amplified and cloned into pET-28b the mvaK1 gene thought to encode mevalonate kinase, the fourth enzyme of the pathway. Following transformation of the construct EFK1-pET28b into Escherichia coli BL21(DE3) cells, the expressed C-terminally hexahistidine-tagged protein was purified on a nickel affinity support to apparent homogeneity. The purified protein catalyzed the divalent ion-dependent phosphorylation of mevalonate to mevalonate 5-phosphate. The specific activity of the purified kinase was 24 micromole/min/mg protein. Based on sedimentation velocity data, E. faecalis mevalonate kinase exists in solution primarily as a monomer with a mass of 32.2 kD. Optimal activity occurred at pH 10 and at 37 degrees C. Delta H(a) was 22 kcal/mole. Kinetic analysis suggested that the reaction proceeds via a sequential mechanism. K(m) values were 0.33 mM (mevalonate), 1.1 mM (ATP), and 3.3 mM (Mg(2+)). 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 K(i) of 2.7 mM.

Staphylococcus aureus mevalonate kinase: isolation and characterization of an enzyme of the isoprenoid biosynthetic pathway

It has been proposed that isoprenoid biosynthesis in several gram-positive cocci depends on the mevalonate pathway for conversion of acetyl coenzyme A to isopentenyl diphosphate. Mevalonate kinase catalyzes a key reaction in this pathway. In this study the enzyme from Staphylococcus aureus was expressed in Escherichia coli, isolated in a highly purified form, and characterized. The overall amino acid sequence of this enzyme was very heterologous compared with the sequences of eukaryotic mevalonate kinases. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and analytical gel filtration chromatography suggested that the native enzyme is a monomer with a molecular mass of approximately 33 kDa. The specific activity was 12 U/mg, and the pH optimum was 7.0 to 8.5. The apparent K(m) values for R,S-mevalonate and ATP were 41 and 339 micro M, respectively. There was substantial substrate inhibition at millimolar levels of mevalonate. The sensitivity to feedback inhibition by farnesyl diphosphate and its sulfur-containing analog, farnesyl thiodiphosphate, was characterized. These compounds were competitive inhibitors with respect to ATP; the K(i) values were 46 and 45 micro M for farnesyl diphosphate and its thio analog, respectively. Parallel measurements with heterologous eukaryotic mevalonate kinases indicated that S. aureus mevalonate kinase is much less sensitive to feedback inhibition (K(i) difference, 3 orders of magnitude) than the human enzyme. In contrast, both enzymes tightly bound trinitrophenyl-ATP, a fluorescent substrate analog, suggesting that there are similarities in structural features that are important for catalytic function.

Sites for interaction between Gal80p and Gal1p in Kluyveromyces lactis: structural model of galactokinase based on homology to the GHMP protein family

The induction of transcription of the galactose genes in yeast involves the galactose-dependent binding of ScGal3p (in Saccharomyces cerevisiae) or KlGal1p (in Kluyveromyces lactis) to Gal80p. This binding abrogates Gal80's inhibitory effect on the activation domain of Gal4p, which can then activate transcription. Here, we describe the isolation and characterization of new interaction mutants of K.lactis GAL1 and GAL80 using a two-hybrid screen. We present the first structural model for Gal1p to be based on the published crystal structures of other proteins belonging to the GHMP (galactokinase, homoserine kinase, mevalonate kinase and phosphomevalonate kinase) kinase family and our own X-ray diffraction data of Gal1p crystals at 3A resolution. The locations of the various mutations in the modelled Gal1p structure identify domains involved in the interaction with Gal80p and provide a structural explanation for the phenotype of constitutive GAL1 mutations.

Functionally important amino acids in Saccharomyces cerevisiae aspartate kinase

Saccharomyces cerevisiae aspartate kinase (AK(Sc)) phosphorylates L-Asp as the first step in the aspartate pathway responsible for the biosynthesis of L-Thr, L-Met, and L-Ile in microorganisms and plants. Using site-directed mutagenesis, we have evaluated the importance of residues in AK(Sc) that are strongly conserved among aspartate kinases or in other small molecule kinases. Steady state kinetic analysis of the purified AK(Sc) variants reveals that several of the targeted amino acids, particularly K18 and H292, have important roles in the enzymatic reaction. These results provide the first identification of amino acid residues crucial to the action of this important metabolic enzyme.

Molecular structure of galactokinase

Galactokinase plays a key role in normal galactose metabolism by catalyzing the ATP-dependent phosphorylation of alpha-D-galactose to galactose 1-phosphate. In humans, mutations in the galactokinase gene can lead to the diseased state referred to as Type II galactosemia. Here we describe the three-dimensional structure of galactokinase from Lactococcus lactis determined to 2.1-A resolution. As expected from amino acid sequence alignments, galactokinase adopts a similar topology to that observed for members of the GHMP superfamily. The N-terminal domain is characterized by a five-stranded mixed beta-sheet while the C-terminal motif is dominated by two distinct four-stranded anti-parallel beta-sheets. The structure was solved in the presence of alpha-D-galactose and inorganic phosphate. These ligands are wedged between the N- and C-terminal domains. Amino acid side chains responsible for anchoring the sugar ligand to the protein include Arg36, Glu42, Asp45, Asp183, and Tyr233. Both Arg36 and Asp183 are strictly conserved in the amino acid sequences available in the literature thus far for galactokinases. Interestingly, the carboxylate side chain of Asp183 is positioned within 3.5 A of the C-1 hydroxyl group of galactose, whereas the guanidinium group of Arg36 is situated between both the C-1 hydroxyl group and the inorganic phosphate. Most likely these residues play key roles in catalysis. The structure of galactokinase described here serves as a model for understanding the functional consequences of point mutations known to result in Type II galactosemia in humans.

Crystal structure of 4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol kinase, an enzyme in the non-mevalonate pathway of isoprenoid synthesis

The crystal structure of the enzyme 4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol (CDP-ME) kinase from the thermophilic bacterium Thermus thermophilus HB8 has been determined at 1.7-A resolution. This enzyme catalyzes phosphorylation of the 2-hydroxyl group of CDP-ME, the fourth step of the non-mevalonate pathway, which is essential for isoprenoid biosynthesis in several pathogenic microorganisms. Since this pathway is absent in humans, it is an important target for the development of novel antimicrobial compounds. The structure of the enzyme is similar to the structures of mevalonate kinase and homoserine kinase, members of the GHMP superfamily. Lys8 and Asp125 are active site residues in mevalonate kinase that also appear to play a catalytic role in CDP-ME kinase. Both the mevalonate and the non-mevalonate pathways therefore involve closely related kinases with similar mechanisms. Assaying the enzyme showed that CDP-ME kinase will phosphorylate CDP-ME but not 4-(uridine 5'-diphospho)-2-C-methyl-D-erythritol, indicating the substrate pyrimidine moiety is involved in important interactions with the enzyme.

Biosynthesis of isoprenoids: crystal structure of 4-diphosphocytidyl-2C-methyl-D-erythritol kinase

4-Diphosphocytidyl-2C-methyl-d-erythritol kinase, an essential enzyme in the nonmevalonate pathway of isopentenyl diphosphate and dimethylallyl diphosphate biosynthesis, catalyzes the single ATP-dependent phosphorylation stage affording 4-diphosphocytidyl-2C-methyl-d-erythritol-2-phosphate. The 2-A resolution crystal structure of the Escherichia coli enzyme in a ternary complex with substrate and a nonhydrolyzable ATP analogue reveals the molecular determinants of specificity and catalysis. The enzyme subunit displays the alpha/beta fold characteristic of the galactose kinase/homoserine kinase/mevalonate kinase/phosphomevalonate kinase superfamily, arranged into cofactor and substrate-binding domains with the catalytic center positioned in a deep cleft between domains. Comparisons with related members of this superfamily indicate that the core regions of each domain are conserved, whereas there are significant differences in the substrate-binding pockets. The nonmevalonate pathway is essential in many microbial pathogens and distinct from the mevalonate pathway used by mammals. The high degree of sequence conservation of the enzyme across bacterial species suggests similarities in structure, specificity, and mechanism. Our model therefore provides an accurate template to facilitate the structure-based design of broad-spectrum antimicrobial agents.

Expression and purification of Arg196 and Lys272 mutants of mevalonate kinase from Methanococcus jannaschii

In microorganisms and plants, mevalonate kinase is involved in the biosynthesis of isoprenoid derivatives, one of the largest groups of natural products. We subcloned the gene of mevalonate kinase from Methanococcus jannaschii into a bacterial expression vector pLM1 with six continuous histidine codons attached to the 5' end of the gene. A variety of mutant expression plasmids including pMMK(R196K), pMMK(R196Q), pMMK(R196V), pMMK(K272R), and pMMK(K272A) have been constructed using site-directed mutagenesis. The wild-type protein and mutants were overexpressed and purified with a nickel HiTrap chelating metal affinity column to homogeneity. CD spectroscopy of wild-type protein and mutants indicates that none of the above mutations induces significant secondary structural changes. The results from kinetic studies showed that Arg196 is an essential residue for the function of the enzyme. Kinetic studies of Lys272 mutants indicate that salt bridge Lys272-Glu14 plays an important role in maintaining the active site microenvironment that is essential for catalytic activity of the enzyme.

XOL-1, primary determinant of sexual fate in C. elegans, is a GHMP kinase family member and a structural prototype for a class of developmental regulators

In Caenorhabditis elegans, an X chromosome-counting mechanism specifies sexual fate. Specific genes termed X-signal elements, which are present on the X chromosome, act in a concerted dose-dependent fashion to regulate levels of the developmental switch gene xol-1. In turn, xol-1 levels determine sexual fate and the activation state of the dosage compensation mechanism. The crystal structure of the XOL-1 protein at 1.55 A resolution unexpectedly reveals that xol-1 encodes a GHMP kinase family member, despite sequence identity of 10% or less. Because GHMP kinases, thus far, have only been characterized as small molecule kinases involved in metabolic pathways, for example, amino acid and cholesterol synthesis, XOL-1 is the first member that controls nonmetabolic processes. Biochemical investigations demonstrated that XOL-1 does not bind ATP under standard conditions, suggesting that XOL-1 acts by a mechanism distinct from that of other GHMP kinases. In addition, we have cloned a XOL-1 ortholog from Caenorhabditis briggsae, a related nematode that diverged from C. elegans approximately 50-100 million years ago. These findings demonstrate an unanticipated role for GHMP kinase family members as mediators of sexual differentiation and dosage compensation and, possibly, other aspects of differentiation and development.

Functional analysis of disease-causing mutations in human galactokinase

Galactokinase (EC 2.7.1.6) catalyzes the first committed step in the catabolism of galactose. The sugar is phosphorylated at position 1 at the expense of ATP. Lack of fully functional galactokinase is one cause of the inherited disease galactosemia, the main clinical manifestation of which is early onset cataracts. Human galactokinase (GALK1) was expressed in and purified from Escherichia coli. The recombinant enzyme was both soluble and active. Product inhibition studies showed that the most likely kinetic mechanism of the enzyme was an ordered ternary complex one in which ATP is the first substrate to bind. The lack of a solvent kinetic isotope effect suggests that proton transfer is unlikely to be involved in the rate determining step of catalysis. Ten mutations that are known to cause galactosemia were constructed and expressed in E. coli. Of these, five (P28T, V32M, G36R, T288M and A384P) were insoluble following induction and could not be studied further. Four of the remainder (H44Y, R68C, G346S and G349S) were all less active than the wild-type enzyme. One mutant (A198V) had kinetic properties that were essentially wild-type. These results are discussed both in terms of galactokinase structure-function relationships and how these functional changes may relate to the causes of galactosemia.

The kinetic mechanism of phosphomevalonate kinase

Phosphomevalonate kinase catalyzes an essential step in the so-called mevalonate pathway, which appears to be the sole pathway for the biosynthesis of sterols and other isoprenoids in mammals and archea. Despite the well documented importance of this pathway in the cause and prevention of human disease and that it is the biosynthetic root of an enormous diverse class of metabolites, the mechanism of phosphomevalonate kinase from any organism is not yet well characterized. The first structure of a phosphomevalonate kinase from Streptococcus pneumoniae was solved recently. The enzyme exhibits an atypical P-loop that is a conserved defining feature of the GHMP kinase superfamily. In this study, the kinetic mechanism of the S. pneumoniae enzyme is characterized in the forward and reverse directions using a combination of classical initial-rate methods including alternate substrate inhibition using ADPbetaS. The inhibition patterns strongly support that in either direction the substrates bind randomly to the enzyme prior to chemistry, a random sequential bi-bi mechanism. The kinetic constants are as follows: k(cat(forward)) = 3.4 s(-1), K(i(ATP)) = 137 microm, K(m(ATP)) = 74 microm, K(i(pmev)) = 7.7 microm, K(m(pmev)) = 4.2 microm; k(cat(reverse)) = 3.9 s(-1), K(i(ADP)) = 410 microm, K(m(ADP)) = 350 microm, K(i(ppmev)) = 14 microm, K(m(ppmev)) = 12 microm, where pmev and ppmev represent phosphomevalonate and diphosphomevalonate, respectively.

Cloning, expression, and purification of His-tagged rat mevalonate kinase

Mevalonate kinase catalyzes the phosphorylation of mevalonic acid to form mevalonate 5-phosphate, which plays a key role in regulating cholesterol biosynthesis in animal cells. Deficiency of mevalonate kinase activity in the human body has been linked to mevalonic aciduria and hyperimmunoglobulinemia D/periodic fever syndrome (HIDS). We cloned the gene of rat mevalonate kinase into a bacterial expression vector pLM1 with six continuous histidine codons attached to the 5(') of the gene. The cloned gene was overexpressed in Escherichia coli and the soluble protein was purified with a nickel HiTrap chelating metal affinity column in 90% yield to apparent homogeneity. The purified rat mevalonate kinase had a dimeric structure composed of identical subunits. Based on SDS-PAGE, the subunit was 42 kDa. The specific activity of the purified His-tagged rat mevalonate kinase was 32.7 micromol/min/mg and the optimal pH was found to be 7.0-8.0 in phosphate buffer. The Michaelis constant K(M) was 35 microM for (RS)-mevalonate and 953 microM for ATP, respectively. The V(max) was determined to be 38.7 micromol/min/mg. The overexpression of rat mevalonate kinase in E. coli and one-step purification of the highly active rat mevalonate kinase will facilitate further our investigation of this enzyme through site-directed mutagenesis and enzyme-catalyzed reactions with substrate analogs.

Sequence and structure classification of kinases

Kinases are a ubiquitous group of enzymes that catalyze the phosphoryl transfer reaction from a phosphate donor (usually ATP) to a receptor substrate. Although all kinases catalyze essentially the same phosphoryl transfer reaction, they display remarkable diversity in their substrate specificity, structure, and the pathways in which they participate. In order to learn the relationship between structural fold and functional specificities in kinases, we have done a comprehensive survey of all available kinase sequences (>17,000) and classified them into 30 distinct families based on sequence similarities. Of these families, 19, covering nearly 98% of all sequences, fall into seven general structural folds for which three-dimensional structures are known. These fold groups include some of the most widespread protein folds, such as Rossmann fold, ferredoxin fold, ribonuclease H fold, and TIM beta/alpha-barrel. On the basis of this classification system, we examined the shared substrate binding and catalytic mechanisms as well as variations of these mechanisms in the same fold groups. Cases of convergent evolution of identical kinase activities occurring in different folds are discussed.

Structural genomics of proteins from conserved biochemical pathways and processes

During the past year, X-ray crystallographers and solution NMR spectroscopists have made significant progress towards the complete structural characterization of conserved biochemical pathways and processes. Some of these advances were made in the context of nascent structural genomics programs, which promise to accelerate structural studies of biologically and medically important proteins. The results of high-throughput protein production, crystallization, structure determination, homology modeling and functional annotation published by two such programs have provided insight into the evolution and function of enzymes in the isoprenoid biosynthesis and ribulose monophosphate pathways.

The structure of a binary complex between a mammalian mevalonate kinase and ATP: insights into the reaction mechanism and human inherited disease

Mevalonate kinase catalyzes the ATP-dependent phosphorylation of mevalonic acid to form mevalonate 5-phosphate, a key intermediate in the pathways of isoprenoids and sterols. Deficiency in mevalonate kinase activity has been linked to mevalonic aciduria and hyperimmunoglobulinemia D/periodic fever syndrome (HIDS). The crystal structure of rat mevalonate kinase in complex with MgATP has been determined at 2.4-A resolution. Each monomer of this dimeric protein is composed of two domains with its active site located at the domain interface. The enzyme-bound ATP adopts an anti conformation, in contrast to the syn conformation reported for Methanococcus jannaschii homoserine kinase. The Mg(2+) ion is coordinated to both beta- and gamma-phosphates of ATP and side chains of Glu(193) and Ser(146). Asp(204) is making a salt bridge with Lys(13), which in turn interacts with the gamma-phosphate. A model of mevalonic acid can be placed near the gamma-phosphoryl group of ATP; thus, the C5 hydroxyl is located within 4 A from Asp(204), Lys(13), and the gamma-phosphoryl of ATP. This arrangement of residues strongly suggests: 1) Asp(204) abstracts the proton from C5 hydroxyl of mevalonate; 2) the penta-coordinated gamma-phosphoryl group may be stabilized by Mg(2+), Lys(13), and Glu(193); and 3) Lys(13) is likely to influence the pK(a) of the C5 hydroxyl of the substrate. V377I and I268T are the most common mutations found in patients with HIDS. Val(377) is located over 18 A away from the active site and a conservative replacement with Ile is unlikely to yield an inactive or unstable protein. Ile-268 is located at the dimer interface, and its Thr substitution may disrupt dimer formation.