Evidence of essential arginyl residues in chicken liver mevalonate-5-pyrophosphate decarboxylase

Enzymes of the mevalonate pathway of isoprenoid biosynthesis

The mevalonate pathway accounts for conversion of acetyl-CoA to isopentenyl 5-diphosphate, the versatile precursor of polyisoprenoid metabolites and natural products. The pathway functions in most eukaryotes, archaea, and some eubacteria. Only recently has much of the functional and structural basis for this metabolism been reported. The biosynthetic acetoacetyl-CoA thiolase and HMG-CoA synthase reactions rely on key amino acids that are different but are situated in active sites that are similar throughout the family of initial condensation enzymes. Both bacterial and animal HMG-CoA reductases have been extensively studied and the contrasts between these proteins and their interactions with statin inhibitors defined. The conversion of mevalonic acid to isopentenyl 5-diphosphate involves three ATP-dependent phosphorylation reactions. While bacterial enzymes responsible for these three reactions share a common protein fold, animal enzymes differ in this respect as the recently reported structure of human phosphomevalonate kinase demonstrates. There are significant contrasts between observations on metabolite inhibition of mevalonate phosphorylation in bacteria and animals. The structural basis for these contrasts has also recently been reported. Alternatives to the phosphomevalonate kinase and mevalonate diphosphate decarboxylase reactions may exist in archaea. Thus, new details regarding isopentenyl diphosphate synthesis from acetyl-CoA continue to emerge.

Crystal structures of Trypanosoma brucei and Staphylococcus aureus mevalonate diphosphate decarboxylase inform on the determinants of specificity and reactivity

Mevalonate diphosphate decarboxylase (MDD) catalyzes the ATP-dependent decarboxylation of mevalonate 5-diphosphate (MDP) to form isopentenyl pyrophosphate, a ubiquitous precursor for isoprenoid biosynthesis. MDD is a poorly understood component of this important metabolic pathway. Complementation of a temperature-sensitive yeast mutant by the putative mdd genes of Trypanosoma brucei and Staphylococcus aureus provides proof-of-function. Crystal structures of MDD from T. brucei (TbMDD, at 1.8 A resolution) and S. aureus (SaMDD, in two distinct crystal forms, each diffracting to 2.3 A resolution) have been determined. Gel-filtration chromatography and analytical ultracentrifugation experiments indicate that TbMDD is predominantly monomeric in solution while SaMDD is dimeric. The new crystal structures and comparison with that of the yeast Saccharomyces cerevisiae enzyme (ScMDD) reveal the structural basis for this variance in quaternary structure. The presence of an ordered sulfate in the structure of TbMDD reveals for the first time details of a ligand binding in the MDD active site and, in conjunction with well-ordered water molecules, comparisons with the related enzyme mevalonate kinase, structural and biochemical data derived on ScMDD and SaMDD, allows us to model a ternary complex with MDP and ATP. This model facilitates discussion of the molecular determinants of substrate recognition and contributions made by specific residues to the enzyme mechanism.

Identification of cysteine and lysine residues present at the active site of beef liver glutamate dehydrogenase by o-phthalaldehyde

Beef liver glutamate dehydrogenase (GDH) is inactivated by the bifunctional reagent, o-phthalaldehyde. The initial rate of inactivation follows pseudo first-order kinetics. The reaction of the enzyme with o-phthalaldehyde results in isoindole derivative formation which is characterized by typical fluorescence emission and excitation maximum at 410 nm and 337 nm, respectively. The inactivation of GDH by o-phthalaldehyde is partially prevented by alpha-ketoglutaric acid, whereas NADH does not provide any protection. This clearly indicates that cysteine and lysine residues are located near the alpha-ketoglutaric acid binding center. The dissociation constant of 2.2 mM was obtained for enzyme-alpha-ketoglutaric acid complex. Stoichiometry of o-phthalaldehyde binding with glutamate dehydrogenase showed that the formation of approximately one isoindole derivative per subunit of glutamate dehydrogenase is accompanied by complete loss of activity.

Study of the interaction of cadmium with membrane-bound succinate dehydrogenase

Cadmium ions inhibit membrane-bound succinate dehydrogenase with a second-order rate constant of 10.42 mM-1 s-1 at pH 7.35 and 25 degrees C. Succinate and malonate protect the enzyme against cadmium ion inhibition. The protection pattern exerted by succinate and malonate suggests that the group modified by cadmium is located at the active site. The pH curve of inactivation by Cd2+ indicates the involvement of an amino acid residue with pKa of 7.23.

Substrate binding order in mevalonate 5-diphosphate decarboxylase from chicken liver

The substrate binding order of chicken liver mevalonate 5-diphosphate decarboxylase was investigated by using competitive inhibitors of the substrates. Mevalonate and mevalonate 5-phosphate showed mixed inhibition when ATP was the varied substrate. Both analogues of ATP, adenosine 5'-O-(3-thiotriphosphate) and beta-tau methylene adenosine 5'-triphosphate showed uncompetitive inhibition against mevalonate 5-diphosphate. These results are in accordance with an ordered sequential mechanism with mevalonate 5-diphosphate as the first substrate to bind to the enzyme.

Inactivation of chicken liver mevalonate 5-diphosphate decarboxylase by sulfhydryl-directed reagents: evidence of a functional dithiol

Chicken liver mevalonate 5-diphosphate decarboxylase (ATP:(R)-5-diphosphomevalonate carboxy-lyase (dehydrating), EC 4.1.1.33) is inactivated by methylmethanethiosulfonate and 5,5'-dithiobis(2-nitrobenzoate). The presence of the substrates ATP or mevalonate 5-diphosphate protect very effectively against inactivation. The inactivation is second order with respect to methylmethanethiosulfonate, with an inactivation rate constant of (7.6 +/- 0.1).10(-5) microM-2.s-1, implying that the modifier may be reacting with more than one thiol in the enzyme. The enzyme is also inactivated by a number of dithiol-specific reagents, suggesting the presence of a functional dithiol. The determined pKapp values for enzyme modification by methyl methanethiosulfonate and phenylarsine oxide are 7.3 +/- 0.1 and 7.6 +/- 0.3, respectively. From the data presented, it is concluded that the enzyme possesses a functional dithiol that is important for substrate binding.

Physiological aspects and mechanism of action of mevalonate 5-diphosphate decarboxylase

1. This work reviews the present knowledge of the physiological role and mechanism of action of mevalonate 5-diphosphate decarboxylase, the third enzyme involved in the biosynthesis of cholesterol from mevalonic acid. 2. Published evidence indicates that this and other enzymes of the cholesterol biosynthetic pathway present coordinate fluctuations in activity in rat liver. A possible regulatory role for the brain decarboxylases from chicken and rat has been proposed. 3. From kinetic and stereochemical studies with the chicken liver enzyme it has been proposed that the reaction is initiated by the abstraction of a proton from the 3-hydroxyl group of mevalonate 5-diphosphate by a basic group in the enzyme, followed by the nucleophilic attack of the C-3 oxygen on P gamma of the lambda isomer of the beta, gamma bidentate MgATP2- in a SN2(P) reaction that goes with inversion of configuration at P.

Kinetic effects of ATP, divalent metal ions and pH on chicken liver mevalonate 5-diphosphate decarboxylase

The activity of chicken liver mevalonate 5-diphosphate decarboxylase was measured over a wide range of Mg2+ and ATP concentrations. It was found that free ATP activated the enzyme, whereas free Mg2+ had no effect on the enzyme activity. Computed analyses of free species concentrations and pH studies indicated that MgATP2- is the true substrate. The relative efficiencies of Mg2+, Mn2+, Cd2+, and Zn2+ as activating metal ions were evaluated in terms of V/Km for the corresponding (metal-ATP)2- complexes, and the relative ratios were: Mn2+ 100, Cd2+ 37, Mg2+ 14, Zn2+ 1.7. Inhibitory effects were demonstrated for all free divalent cations tested, except for Mg2+, and were in the order Zn2+ greater than Cd2+ greater than Mn2+.

The presence of functional arginine residues in phosphoenolpyruvate carboxykinase from Saccharomyces cerevisiae

Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase (ATP:oxaloacetate carboxy-lyase (transphosphorylating), EC 4.1.1.49) is completely inactivated by phenylglyoxal and 2,3-butanedione in borate buffer at pH 8.4, with pseudo-first-order kinetics and a second-order rate constant of 144 min-1 X M-1 and 21.6 min-1 X M-1, respectively. Phosphoenolpyruvate, ADP and Mn2+ (alone or in combination) protect the enzyme against inactivation, suggesting that the modification occurs at or near to the substrate-binding site. Almost complete restoration of activity was obtained when a sample of 2,3-butanedione-inactivated enzyme was freed of excess modifier and borate ions, suggesting that only arginyl groups are modified. The changes in the rate of inactivation in the presence of substrates and Mn2+ were used to determine the dissociation constants for enzyme-ligand complexes, and values of 23 +/- 3 microM, 168 +/- 44 microM and 244 +/- 54 microM were found for the dissociation constants for the enzyme-Mn2+, enzyme-ADP and enzyme-phosphoenolpyruvate complexes, respectively. Based on kinetic data, it is shown that 1 mol of reagent must combine per enzyme active unit in order to inactivate the enzyme. Complete inactivation of the carboxykinase can be correlated with the incorporation of 3-4 mol [7-14C]phenylglyoxal per mol of enzyme subunit. Assuming a stoichiometry of 1:1 between phenylglyoxal incorporation and arginine modification, our results suggest that the modification of only two of the three to four reactive arginine residues per phosphoenolpyruvate carboxykinase subunit is responsible for inactivation.

Mevalonate-5-diphosphate decarboxylase: stereochemical course of ATP-dependent phosphorylation of mevalonate 5-diphosphate

Chicken liver mevalonate-5-diphosphate decarboxylase catalyzes the reaction of mevalonate 5-diphosphate (MVADP) with ATP to produce isopentenyl diphosphate, ADP, CO2, and inorganic phosphate. The overall reaction involves an anti elimination of the tertiary hydroxyl and carboxyl groups. To investigate the mechanism for transfer of the terminal phosphoryl group of ATP to the C-3 oxygen of MVADP, we have carried out the reaction using stereospecifically labeled (Sp)-adenosine 5'-O-(3-thio[3-17O2,18O]triphosphate) [( gamma-17O2,18O]ATP gamma S) in place of ATP. The configuration of the [17O,18O]thiophosphate produced was found to be Rp, corresponding to overall inversion of configuration at phosphorus in the thiophosphoryl group transfer step. This result is consistent with the direct transfer of the thiophosphoryl group from (Sp)-[gamma-17O2,18O]ATP gamma S to MVADP at the active site. Our result does not indicate the involvement of a covalent thiophosphoryl-enzyme on the reaction pathway.

On the relationship between reaction order and stoichiometry in irreversible inhibition of enzymes

The relationship between reaction order and stoichiometry in inhibition of enzymes by potentially irreversible inhibitors is discussed. It is pointed out that in an overwhelming fraction of cases reported or imagined, the order in inhibitor concentration approximates one, regardless of actual stoichiometry. It follows that reaction order should not be employed as a reliable measure of the number of amino acid residues involved in the inactivation process.