The active site of hamster 3-hydroxy-3-methylglutaryl-CoA reductase resides at the subunit interface and incorporates catalytically essential acidic residues from separate polypeptides

We employed site-directed mutagenesis based on sequence comparisons and characterization of purified mutant enzymes to identify Glu558 and Asp766 of Syrian hamster 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (EC 1.1.1.34) as essential for catalysis. Mutant enzymes E558D, E558Q, and D766N had wild-type Km values for (S)-HMG-CoA and NADPH, but exhibited less than 0.5% of the wild-type catalytic activity. The inactive mutant polypeptides E558Q and D766N nevertheless can associate to generate an active enzyme. In vitro, 6% of the wild-type activity was observed when mutant polypeptides E558D and D766N were mixed in the absence of chaotropic agents. When mutant polypeptides E558Q and D766N were co-expressed in Escherichia coli, the resulting purified enzyme had 25% of wild-type activity. Hamster HMG-CoA reductase thus is a two-site, dimeric enzyme whose subunits associate to form an active site in which each monomer contributes at least one residue (e.g. Glu558 from one monomer and Asp766 from the other). The wild-type enzyme behaves as a dimer during size exclusion chromatography and has one HMG-CoA binding site per monomer. Syrian hamster HMG-CoA reductase thus appears to be a homodimer with two active sites which are located at the subunit interface.     

Role of arginine 439 in substrate binding of 5-aminolevulinate synthase

5-Aminolevulinate synthase (EC 2.3.1.37) catalyzes the first reaction in the heme biosynthetic pathway in nonplant eukaryotes and some prokaryotes. Homology sequence modeling between 5-aminolevulinate synthase and some other alpha-family pyridoxal 5'-phosphate-dependent enzymes indicated that the residue corresponding to the Arg-439 of murine erythroid 5-aminolevulinate synthase is a conserved residue in this family of pyridoxal 5'-phosphate-dependent enzymes. Further, this conserved arginine residue in several enzymes, e.g., aspartate aminotransferase, for which the three-dimensional structure is known, has been shown to interact with the substrate carboxyl group. To test whether Arg-439 is involved in substrate binding in murine erythroid 5-aminolevulinate synthase, Arg-439 and Arg-433 of murine erythroid 5-aminolevulinate synthase were each replaced by Lys and Leu using site-directed mutagenesis. The R439K mutant retained 77% of the wild-type activity; its K(m) values for both substrates increased 9-13-fold, while the activity of R433K increased 2-fold and the K(m) values for both substrates remained unchanged. R439L had no measurable activity as determined using a standard 5-aminolevulinate synthase enzyme-coupled activity assay. In contrast, the kinetic parameters for R433L were comparable to those of the wild-type. Dissociation constants (Kd) for glycine increased 5-fold for R439K and at least 30-fold for R439L, while Kd values for glycine for both R433K and R433L mutants were similar to those of the wild-type. However, there was not much difference in methylamine binding among the mutants and the wild-type, excepting of a 10-fold increase in K(d)methylamine for R439L. R439K proved much less thermostable than the wild-type enzyme, with the thermotransition temperature, T1/2, determined to be 8.3 degrees C lower than that of the wild-type enzyme. In addition, in vivo complementation analysis demonstrated that in the active site of murine erythroid 5-aminolevulinate synthase, R439 is contributed from the same subunit as K313 (which is involved in the Schiff base linkage of the pyridoxal 5'-phosphate cofactor) and D279 (which interacts electrostatically with the ring nitrogen of the cofactor), while another subunit provides R149. Taken together, these findings suggest that Arg-439 plays an important role in substrate binding of murine erythroid 5-aminolevulinate synthase.     

Directed mutagenesis studies of the metal binding site at the subunit interface of Escherichia coli inorganic pyrophosphatase

Recent crystallographic studies on Escherichia coli inorganic pyrophosphatase (E-PPase) have identified three Mg2+ ions/enzyme hexamer in water-filled cavities formed by Asn24, Ala25, and Asp26 at the trimer-trimer interface (Kankare, J., Salminen, T., Lahti, R., Cooperman, B., Baykov, A. A., and Goldman, A. (1996) Biochemistry 35, 4670-4677). Here we show that D26S and D26N substitutions decrease the stoichiometry of tight Mg2+ binding to E-PPase by approximately 0.5 mol/mol monomer and increase hexamer stability in acidic medium. Mg2+ markedly decelerates the dissociation of enzyme hexamer into trimers at pH 5.0 and accelerates hexamer formation from trimers at pH 7.2 with wild type E-PPase and the N24D variant, in contrast to the D26S and D26N variants, when little or no effect is seen. The catalytic parameters describing the dependences of enzyme activity on substrate and Mg2+ concentrations are of the same magnitude for wild type E-PPase and the three variants. The affinity of the intertrimer site for Mg2+ at pH 7.2 is intermediate between those of two Mg2+ binding sites found in the E-PPase active site. It is concluded that the metal ion binding site found at the trimer-trimer interface of E-PPase is a high affinity site whose occupancy by Mg2+ greatly stabilizes the enzyme hexamer but has little effect on catalysis.     

Interaction of pyridoxal 5'-phosphate with tryptophan-139 at the subunit interface of dimeric D-amino acid transaminase

The crystal structure of dimeric bacterial D-amino acid transaminase shows that the indole rings of the two Trp-139 side chains face each other in the subunit interface about 10 angstroms from the coenzyme, pyridoxal 5'-phosphate. To determine whether it has a role in the catalytic efficiency of the enzyme or interacts with the coenzyme, Trp-139 has been substituted by several different types of amino acids, and the properties of these recombinant mutant enzymes have been compared to the wild-type enzyme. In the native wild-type holoenzyme, the fluorescence of one of the three Trp residues per monomer is almost completely quenched, probably due to its interaction with PLP since in the native wild-type apoenzyme devoid of PLP, tryptophan fluorescence is not quenched. Upon reconstitution of this apoenzyme with PLP, the tryptophan fluorescence is quenched to about the same extent as it is in the native wild-type enzyme. The site of fluorescence quenching is Trp-139 since the W139F mutant in which Trp-139 is replaced by Phe has about the same amount of fluorescence as the wild-type enzyme. The circular dichroism spectra of the holo and the apo forms of both the wild-type and the W139F enzymes in the far-ultraviolet show about the same degree of ellipticity, consistent with the absence of extensive global changes in protein structure. Furthermore, comparison of the circular dichroism spectrum of the W139F enzyme at 280 nm with the corresponding spectral region of the wild-type enzyme suggests a restricted microenvironment for Trp-139 in the latter enzyme. The functional importance of Trp-139 is also demonstrated by the finding that its replacement by Phe, His, Pro, or Ala gives mutant enzymes that are optimally active at temperatures below that of the wild-type enzyme and undergo the E-PLP --> E-PMP transition as a function of D-Ala concentration with reduced efficiency. The results suggest that a fully functional dimeric interface with the two juxtaposed indole rings of Trp-139 is important for optimal catalytic function and maximum thermostability of the enzyme and, furthermore, that there might be energy transfer between Trp-139 and coenzyme PLP.     

Biphasic ordered induction of heme synthesis in differentiating murine erythroleukemia cells: role of erythroid 5-aminolevulinate synthase

During dimethyl sulfoxide (DMSO)-stimulated differentiation of murine erythroleukemia (MEL) cells, one of the early events is the induction of the heme biosynthetic pathway. While recent reports have clearly demonstrated that GATA-1 is involved in the induction of erythroid cell-specific forms of 5-aminolevulinate synthase (ALAS-2) and porphobilinogen (PBG) deaminase and that cellular iron status plays a regulatory role for ALAS-2, little is known about regulation of the remainder of the pathway. In the current study, we have made use of a stable MEL cell mutant (MEAN-1) in which ALAS-2 enzyme activity is not induced by DMSO, hexamethylene bisacetamide (HMBA), or butyric acid. In this cell line, addition of 2% DMSO to growing cultures results in the normal induction of PBG deaminase and coproporphyrinogen oxidase but not in the induction of the terminal two enzymes, protoporphyrinogen oxidase and ferrochelatase. These DMSO-treated cells did not produce mRNA for beta-globin and do not terminally differentiate. In addition, the cellular level of ALAS activity declines rapidly after addition of DMSO, indicating that ALAS-1 must turn over rapidly at this time. Addition of 75 microM hemin alone to the cultures did not induce cells to terminally differentiate or induce any of the pathway enzymes. However, the simultaneous addition of 2% DMSO and 75 microM hemin caused the cells to carry out a normal program of terminal erythroid differentiation, including the induction of ferrochelatase and beta-globin. These data suggest that induction of the entire heme biosynthetic pathway is biphasic in nature and that induction of the terminal enzymes may be mediated by the end product of the pathway, heme. We have introduced mouse ALAS-2 cDNA into the ALAS-2 mutant cell line (MEAN-1) under the control of the mouse metallothionein promoter (MEAN-RA). When Cd and Zn are added to cultures of MEAN-RA in the absence of DMSO, ALAS-2 is induced but erythroid differentiation does not occur and cells continue to grow normally. In the presence of metallothionein inducers and DMSO, the MEAN-RA cells induce in a fashion similar to that found with the wild-type 270 MEL cells. Induction of the activities of ALAS, PBG deaminase, coproporphyrinogen oxidase, and ferrochelatase occurs. In cultures of MEAN-RA where ALAS-2 had been induced with Cd plus Zn 24 h prior to DMSO addition, onset of heme synthesis occurs more rapidly than when DMSO and Cd plus Zn are added simultaneously. This study reveals that induction of ALAS-2 alone is not sufficient to induce terminal differentiation of the MEAN-RA cells, and it does not appear that ALAS-2 alone is the rate-limiting enzyme of the heme biosynthetic pathway during MEL cell differentiation.     

Essential histidyl residues at the active site(s) of sucrose-phosphate synthase from Prosopis juliflora

Antimicrobial resistance in nosocomial isolates is of increasing concern to the clinician, particularly in intensive care units. With more expensive drugs and prolonged periods of hospitalization required, resistance can result in increased healthcare costs. For the patient, infection with multiply resistant strains of bacteria is associated with high mortality rates. This review focuses on the prevalence of nosocomial infections throughout Europe, with particular emphasis on the prevalence of resistance to common antimicrobial agents. The beta-lactams are the most frequently prescribed antimicrobials, and the growing importance of extended spectrum beta-lactamases and the hyperproduction of chromosomal beta-lactamase by stably derepressed mutants in the development of microbial resistance are discussed. Given that the most common reason for modification of an initial empiric antibiotic treatment is the isolation of microorganisms not susceptible to the initial choice of treatment, the results from two European multicenter trials comparing the efficacy of the carbapenems, meropenem, and imipenem/cilastatin, for the treatment of serious nosocomial infections, are appraised. In light of these results, it can be concluded that the carbapenems are effective as initial empiric monotherapy for nosocomial infections because of their broad spectrum of efficacy and stability to beta-lactamases.     

Role of a single amino acid at the amino terminus of the simian virus 5 F2 subunit in syncytium formation

The fusion (F) protein of simian virus 5 (strain W3A) induced extensive cell fusion in BHK cells when expressed alone, while that of strain WR did not. Mutational analysis demonstrated that the fusing activity can be transferred to the WR F protein by a proline residue at position 22 of subunit W3A F2.     

The formation or the reduction of a disulfide bridge on the gamma subunit of chloroplast ATP synthase affects the inhibitory effect of the epsilon subunit

We have studied the change of the catalytic activity of chimeric complexes that were formed by chloroplast coupling factor 1 (CF1) -gamma, alpha and beta subunits of thermophilic bacterial F1 after formation or reduction of the disulfide bridge of different gamma subunits modified by oligonucleotide-directed mutagenesis techniques. For this purpose, three mutant gamma subunits were produced: gamma Delta194-230, here 37 amino acids from Pro-194 to Ile-230 are deleted, gammaC199A, Cys-199 is changed to Ala, and gamma Delta200-204, amino acids from Asp-200 to Lys-204 are deleted. All of the chimeric subunit complexes produced from each of these mutant CF1-gamma subunits and alpha and beta subunits from thermophilic bacterial F1 lost the sensitivity against thiol reagents when compared with the complex containing wild-type CF1-gamma. The pH optimum (pH 8.5-9.0) and the concentration of methanol to stimulate ATPase activities were not affected by these mutations. These indicate that the introduction of the mutations did not change the main features of ATPase activity of the chimeric complex. However, the interaction between gamma subunit and epsilon subunit was strongly influenced by the type of gamma subunit itself. Although the ATPase activity of the chimeric complex that contained gamma Delta200-204 or gammaC199A was inhibited by the addition of recombinant epsilon subunit from CF1 similarly to complexes containing the reduced wild-type gamma subunit, the recombinant epsilon subunit did not inhibit the ATPase of the complex, which contained the oxidized form of gamma subunit. Therefore the affinity of the epsilon subunit to the gamma subunit may be dependent on the state of the gamma subunit or the epsilon subunit may bind to the oxidized form of gamma subunit in a mode that does not inhibit the activity. The ATPase activity of the complex that contains gamma Delta194-230 was not efficiently inhibited by epsilon subunit. These results show that the formation or reduction of the disulfide bond on the gamma subunit may induce a conformational change in the region that directly affects the interaction of this subunit with the adjacent epsilon subunit.     

Evidence for two tissue-specific pathways for in vivo thyroxine 5'-deiodination in the rat

Propylthiouracil (PTU) is a well known inhibitor of thyroxine (T(4)) to triiodothyronine (T(3)) conversion as evidenced by its effect in several in vitro systems and by the decrease in serum T(3) caused by this drug in either rats or man receiving T(4) replacement. However, the failure of PTU to decrease the intrapituitary T(3) concentration and to completely blunt the serum T(3) concentration in T(4)-replaced athyreotic rats suggest that there may be a PTU-insensitive pathway of T(4) to T(3) conversion in some tissues. To address this question, we have studied the in vivo effect of PTU treatment on the generation of [(125)I]T(3) from [(125)I]T(4) in the serum and cerebral cortex (Cx), cerebellum (Cm), liver (L), and anterior pituitary (P) of euthyroid rats. Whereas PTU decreased the concentration of [(125)I]T(3) in the serum, L homogenates, and L nuclei after [(125)I]T(4), it did not affect the concentration of [(125)I]T(3) in homogenates or nuclei of Cx, Cm, or P. Iopanoic acid pretreatment significantly reduced the [(125)I]T(3) concentration in serum, homogenates, and cell nuclei of all these organs. Neither agent affected the metabolism or tissue distribution of simultaneously injected [(131)I]T(3). The presence of PTU in these tissues was evaluated by in vitro assessment of iodothyronine 5'-deiodinating activity using both [(125)I]rT(3) and [(125)I]T(4) as substrates. In agreement with the in vivo findings, generation of [(125)I]T(3) from T(4) in vitro was not affected by PTU in Cx, Cm, P but it was inhibited by 76% in L. However, rT(3) 5'-deiodination, known to be sensitive to PTU in these tissues, was inhibited in all four indicating that the PTU given in vivo was present in significant amounts. These results demonstrate that in rat Cx, Cm, and P unlike liver, PTU does not inhibit T(4) to T(3) conversion in vivo despite the presence of the drug in the tissues in amounts that significantly inhibit reverse T(3) 5'-deiodination. These results show that in vivo 5'-deiodination of T(4) proceeds via a PTU-insensitive pathway in the central nervous system and pituitary, while this pathway is not quantitatively important in the L. This mechanism accounts for the "locally generated" T(3) in central nervous system and pituitary and could also provide the approximately one-third of extrathyroidally produced T(3) not blocked by PTU administration in athyreotic T(4)-replaced rat.     

Mechanism of the synergistic induction of CYP2H by isopentanol plus ethanol: comparison to glutethimide and relation to induction of 5-aminolevulinate synthase

Mice homozygous for the osteopetrosis (op) mutation are characterized by defective differentiation of osteoclasts, monocytes, and tissue macrophages due to a lack of functional macrophage colony-stimulating factor (M-CSF/CSF-1) activity. In young (4-6 week-old) op/op mice, the bone marrow cavities were filled with spongious bone. In aged (50-72 week-old) op/op mice, the bone marrow cavities were markedly reconstructed and marrow hematopoiesis was expanded. Numbers of osteoclasts and bone marrow macrophages in aged op/op mice were increased but most of the osteoclasts were mononuclear cells and showed poorly developed ruffled borders. Lysosomes of bone marrow macrophages were laden with abundant crystalloid materials in aged op/op mice and aged littermate mice. However, such macrophages were not observed in young op/op mice nor in young littermates. In contrast to the marked increase in numbers of osteoclasts and macrophages in the bone marrow, the number of Kupffer cells in the liver did not increase in aged op/op mice. Kupffer cells in aged op/op mice did not show ultrastructural maturation with aging and contained a few crystalloid structures. M-CSF administration to aged op/op mice induced numerical increases in Kupffer cells and lysosomes in Kupffer cells, disappearance of crystalloid structures in lysosomes of Kupffer cells, and the development of ruffled border in osteoclasts. These findings indicate that M-CSF-independent mechanisms for macrophage and osteoclast development in aged op/op mice are restricted to bone marrow. M-CSF plays important roles in the differentiation of macrophage and osteoclast and the production and function of lysosomes.     

Isolation and characterization of the mitochondrial ATP synthase from Chlamydomonas reinhardtii. cDNA sequence and deduced protein sequence of the alpha subunit

We have isolated the F0F1-ATP synthase complex from oligomycin-sensitive mitochondria of the green alga Chlamydomonas reinhardtii. A pure and active ATP synthase was obtained by means of sonication, extraction with dodecyl maltoside and ion exchange and gel permeation chromatography in the presence of glycerol, DTT, ATP and PMSF [corrected]. The enzyme consists of 14 subunits as judged by SDS-PAGE. A cDNA clone encoding the ATP synthase alpha subunit has been sequenced. The deduced protein sequence contains a presequence of 45 amino acids which is not present in the mature protein. The mature protein is 58-70% identical to corresponding mitochondrial proteins from other organisms. In contrast to the ATP synthase beta subunit from C. reinhardtii (Franzen and Falk, Plant Mol Biol 19 (1992) 771-780), the protein does not have a C-terminal extension. However, the N-terminal domain of the mature protein is 15-18 residues longer than in ATP synthase alpha subunits from other organisms. Southern blot analysis indicates that the protein is encoded by a single-copy gene.     

Reaction of neuronal nitric-oxide synthase with oxygen at low temperature. Evidence for reductive activation of the oxy-ferrous complex by tetrahydrobiopterin

The reaction of reduced NO synthase (NOS) with molecular oxygen was studied at -30 degreesC. In the absence of substrate, the complex formed between ferrous NOS and O2 was sufficiently long lived for a precise spectroscopic characterization. This complex displayed similar spectral characteristics as the oxyferrous complex of cytochrome P450 (lambda max = 416.5 nm). It then decomposed to the ferric state. The oxidation of the flavin components was much slower and could be observed only at temperatures higher than -20 degreesC. In the presence of substrate (L-arginine), another, 12-nm blue-shifted, intermediate spectrum was formed. The breakdown of the latter species resulted in the production of Nomega-hydroxy-L-arginine in a stoichiometry of maximally 52% per NOS heme. This product formation took place also in the absence of the reductase domain of NOS. Both formation of the blue-shifted intermediate and of Nomega-hydroxy-L-arginine required the presence of tetrahydrobiopterin (BH4). We propose that the blue-shifted intermediate is the result of reductive activation of the oxygenated complex, and the electron is provided by BH4. These observations suggest that the reduction of the oxyferroheme complex may be the main function of BH4 in NOS catalysis.     

Functional analysis of the catalytic subunit of Dictyostelium PKA in vivo

Long-term amygdala kindling in rats produces increases in emotionality (Kalynchuk et al., Biol. Psychiatry, 41 (1997) 438-451). The present experiment was conducted to investigate whether this hyperemotionality is specific to amygdala kindling or whether it can be produced by kindling other structures. Rats received 99 convulsive or sham stimulations of either the amygdala, the hippocampus, or the caudate nucleus. One day after the stimulation phase, each rat's open-field activity and resistance to capture were assessed; the following day, each rat was tested on an elevated plus maze. The site of stimulation had a significant effect on the results of each of these tests. The amygdala-kindled and hippocampal-kindled rats explored less in the open field, were more resistant to capture from the open field, and engaged in a greater percentage of open-arm activity in the elevated plus maze than did the caudate-kindled rats or the sham-stimulated controls. The caudate-kindled rats were more active in the open field than their sham-stimulated controls, but they did not significantly differ from them in terms of the other measures. These results suggest that kindling-induced emotionality is produced by limbic kindling but not nonlimbic kindling.