Crystal structure of guanidinoacetate methyltransferase from rat liver: a model structure of protein arginine methyltransferase

Guanidinoacetate methyltransferase (GAMT) is the enzyme that catalyzes the last step of creatine biosynthesis. The enzyme is found in abundance in the livers of all vertebrates. Recombinant rat liver GAMT has been crystallized with S-adenosylhomocysteine (SAH), and the crystal structure has been determined at 2.5 A resolution. The 36 amino acid residues at the N terminus were cleaved during the purification and the truncated enzyme was crystallized. The truncated enzyme forms a dimer, and each subunit contains one SAH molecule in the active site. Arg220 of the partner subunit forms a pair of hydrogen bonds with Asp134 at the guanidinoacetate-binding site. On the basis of the crystal structure, site-directed mutagenesis on Asp134, and chemical modification and limited proteolysis studies, we propose a catalytic mechanism of this enzyme. The truncated GAMT dimer structure can be seen as a ternary complex of protein arginine methyltransferase (one subunit) complexed with a protein substrate (the partner subunit) and the product SAH. Therefore, this structure provides insight into the structure and catalysis of protein arginine methyltransferases.     

The blood–brain barrier transport and cerebral distribution of guanidinoacetate in rats: involvement of creatine and taurine transporters

Although the cerebral accumulation of guanidinoacetate (GAA) contributes to neurological complications in S -adenosylmethionine:guanidinoacetate N -methyltransferase (GAMT) deficiency, how GAA is abnormally distributed in the brain remains unknown. The purpose of this study was to investigate the transport of GAA across the blood–brain barrier (BBB) and in brain parenchymal cells in rats. [¹⁴C]GAA microinjected into the rat cerebrum was not eliminated from the brain, implying the negligible contribution of GAA efflux transport across the BBB. In contrast, in vivo analysis and an uptake study by TR-BBB cells, a rat in vitro BBB model, revealed that GAA was transported from the circulating blood across the BBB most likely via a creatine transporter (CRT). Although CRT at the BBB is almost saturated by endogenous creatine under physiological conditions, the creatine level in the blood significantly decreases in GAMT deficiency. This might lead to the increase of CRT-mediated blood-to-brain transport of GAA at the BBB. Furthermore, [¹⁴C]GAA was taken up by brain parenchymal cells in a concentrative manner most likely via taurine transporter and CRT. These characteristics of GAA transport across the BBB and in the brain parenchymal cells could be the key factors that facilitate GAA accumulation in the brains of patients with GAMT deficiency.     

Automated high-performance liquid chromatographic method for the determination of guanidinoacetic acid in dried blood spots: a tool for early diagnosis of guanidinoacetate methyltransferase deficiency

A new automated method for the assay of guanidinoacetic acid (GAA) in dried blood spot (DBS) on filter paper is reported. The method, based on reversed-phase (RP)-HPLC, precolumn derivatisation with benzoin and fluorescence detection, has shown good precision and sensitivity and requires only minimal sample handling. The validity of the method was demonstrated by analysing the neonatal blood spot of a patient affected by guanidinoacetate methyltransferase (GAMT) deficiency. GAA concentration was found to be nearly 12-fold higher than the mean control value. We propose this method as an inexpensive and widely applicable tool for the diagnosis of GAMT deficiency.     

Evidence that the inhibitory effects of guanidinoacetate on the activities of the respiratory chain, Na+,K+-ATPase and creatine kinase can be differentially prevented by taurine and vitamins E and C administration in rat striatum in vivo

Guanidinoacetate methyltransferase (GAMT) deficiency is an inherited neurometabolic disorder biochemically characterized by tissue accumulation of guanidinoacetate (GAA) and depletion of creatine. Affected patients present epilepsy and mental retardation whose etiopathogeny is unclear. In a previous study we showed that instrastriatal administration of GAA caused a reduction of Na(+),K(+)-ATPase and creatine kinase (CK) activities, as well as an increase in TBARS (an index of lipid peroxidation). In the present study we investigated the in vitro and in vivo effects of GAA on glucose uptake from [U-(14)C] acetate (citric acid cycle activity) and on the activities of complexes II, II-III, III and IV of the respiratory chain in striatum of rats. Results showed that 50 and 100 microM GAA (in vitro studies) and GAA administration (in vivo studies) significantly inhibited complexes II and II-III, respectively, but did not alter complexes III and IV, as well as CO(2) production. We also studied the influence of taurine or vitamins E and C on the inhibitory effects caused by intrastriatal administration of GAA on complexes II and II-III, Na(+),K(+)-ATPase and CK activities, and on TBARS in rat striatum. Pre-treatment with taurine and vitamins E and C revealed that taurine prevents the effects of intrastriatal administration of GAA on the inhibition of complex II, complex II-III, and Na(+),K(+)-ATPase activities. Vitamins E and C prevent the effects of intrastriatal administration of GAA on the inhibition of CK and Na(+),K(+)-ATPase activities, and on the increase of TBARS. The data suggest that GAA in vivo and in vitro treatment disturbs important parameters of striatum energy metabolism and that oxidative damage may be mediating these effects. It is presumed that defects in striatum bioenergetics might be involved in the pathophysiology of striatum damage characteristic of patients with GAMT-deficiency.     

A gibberellin methyltransferase modulates the timing of floral transition at the Arabidopsis shoot meristem

The transition from vegetative to reproductive growth is a key event in the plant life cycle. Plants therefore use a variety of environmental and endogenous signals to determine the optimal time for flowering to ensure reproductive success. These signals are integrated at the shoot apical meristem (SAM), which subsequently undergoes a shift in identity and begins producing flowers rather than leaves, while still maintaining pluripotency and meristematic function. Gibberellic acid (GA), an important hormone associated with cell growth and differentiation, has been shown to promote flowering in many plant species including Arabidopsis thaliana, but the details of how spatial and temporal regulation of GAs in the SAM contribute to floral transition are poorly understood. In this study, we show that the gene GIBBERELLIC ACID METHYLTRANSFERASE 2 (GAMT2), which encodes a GA-inactivating enzyme, is significantly upregulated at the SAM during floral transition and contributes to the regulation of flowering time. Loss of GAMT2 function leads to early flowering, whereas transgenic misexpression of GAMT2 in specific regions around the SAM delays flowering. We also found that GAMT2 expression is independent of the key floral regulator LEAFY but is strongly increased by the application of exogenous GA. Our results indicate that GAMT2 is a repressor of flowering that may act as a buffer of GA levels at the SAM to help prevent premature flowering.     

High-resolution structure of the soluble,respiratory-type Rieske protein from Thermus thermophilus: analysis and comparison

The structure of the soluble Rieske protein from Thermus thermophilus has been determined at a resolution of 1.3 A at pH 8.5 using multiwavelength anomalous dispersion (MAD) techniques. This is the first report of a Rieske protein from a menaquinone-utilizing organism. The structure shows an overall fold similar to previously reported Rieske proteins. A novel feature of this crystal form appears to be a shared hydrogen between the His-134 imidazole ring ligated to Fe2 of the [2Fe-2S] cluster and its symmetry partner, His-134', one being formally an imidazolate anion, Fe2-(His-134)N(epsilon)(-)...H-N(epsilon')(His-134')-Fe2', in which crystallographic C(2) axes pass equidistant between N(epsilon)...N(epsilon') and normal to the line defined by N(epsilon)...N(epsilon'). This provides evidence for a stable, oxidized cluster with a His(-) ligand and lends support to a previously proposed mechanism of coupled proton and electron transfer. A detailed comparison of the Thermus Rieske protein with six other Rieske and Rieske-type proteins indicates: (a) The cluster binding domain is tightly conserved. (b) The 3-D structure of the 10 beta-strand fold is conserved, even among the most divergent proteins. (c) There is an approximately linear relation between acid-pH redox potential and number of H-bonds to the cluster. (d) These proteins have two faces, one points into the larger complex (bc(1), b(6)f, or other), is involved in the proton coupled electron transfer function, and is highly conserved. The second is oriented toward the solvent and shows wide variation in charge, sequence, length, hydrophobicity, and secondary elements in the loops that connect the beta-sheets.     

Small molecule inhibitors that selectively block dengue virus methyltransferase

Crystal structure analysis of Flavivirus methyltransferases uncovered a flavivirus-conserved cavity located next to the binding site for its cofactor, S-adenosyl-methionine (SAM). Chemical derivatization of S-adenosyl-homocysteine (SAH), the product inhibitor of the methylation reaction, with substituents that extend into the identified cavity, generated inhibitors that showed improved and selective activity against dengue virus methyltransferase (MTase), but not related human enzymes. Crystal structure of dengue virus MTase with a bound SAH derivative revealed that its N6-substituent bound in this cavity and induced conformation changes in residues lining the pocket. These findings demonstrate that one of the major hurdles for the development of methyltransferase-based therapeutics, namely selectivity for disease-related methyltransferases, can be overcome.     

Sustaining S-adenosyl-l-methionine-dependent methyltransferase activity in plant cells

Many biochemical reactions in plants involve the transfer of a methyl group from S -adenosyl- l -methionine (SAM). The transfer of the methyl group from SAM generates S -adenosyl- l -homocysteine (SAH), a potent inhibitor of SAM-dependent methyltransferases (MTs). To mitigate the toxic effects of SAH on MT activity, SAH is removed by SAH hydrolase (SAHH, EC 3.3.1.1) in a reaction generating homocysteine and adenosine (Ado). However, SAHH catalyzes a reversible reaction that is favored to move in the direction of SAH hydrolysis only by removal of these products. Removal of Ado is reported to exert a greater influence on promoting SAH hydrolysis. Whereas animals appear to rely upon Ado deaminase (EC 3.5.4.4) to catabolize Ado, plants appear to use adenosine kinase (EC 2.7.1.20) for this important role. Compounds undergoing methylation represent a broad spectrum of chemically diverse substrates ranging from nucleic acids, lipids and cell wall components to comparatively simpler amines, alcohols and metal halides. Given the diverse nature of methyl acceptor compounds, it is very likely that the demand for SAM synthesis and SAH removal changes both temporally and spatially during the course of plant growth and development. Plants also use SAM as a precursor for the synthesis of ethylene, polyamines, biotin and nicotianamine. These uses are also expected to undergo changes reflective of the metabolic activities of different plants, plant organs, or cells. This review examines the various uses of SAM in plants and addresses how they allocate this resource to satisfy potentially competing needs.     

Catalytic mechanism of glycine N-methyltransferase

Methyltransfer reactions are some of the most important reactions in biological systems. Glycine N-methyltransferase (GNMT) catalyzes the S-adenosyl-l-methionine- (SAM-) dependent methylation of glycine to form sarcosine. Unlike most SAM-dependent methyltransferases, GNMT has a relatively high value and is weakly inhibited by the product S-adenosyl-l-homocysteine (SAH). The major role of GNMT is believed to be the regulation of the cellular SAM/SAH ratio, which is thought to play a key role in SAM-dependent methyltransfer reactions. Crystal structures of GNMT complexed with SAM and acetate (a potent competitive inhibitor of Gly) and the R175K mutated enzyme complexed with SAM were determined at 2.8 and 3.0 A resolutions, respectively. With these crystal structures and the previously determined structures of substrate-free enzyme, a catalytic mechanism has been proposed. Structural changes occur in the transitions from the substrate-free to the binary complex and from the binary to the ternary complex. In the ternary complex stage, an alpha-helix in the N-terminus undergoes a major conformational change. As a result, the bound SAM is firmly connected to protein and a "Gly pocket" is created near the bound SAM. The second substrate Gly binds to Arg175 and is brought into the Gly pocket. Five hydrogen bonds connect the Gly in the proximity of the bound SAM and orient the lone pair orbital on the amino nitrogen (N) of Gly toward the donor methyl group (C(E)) of SAM. Thermal motion of the enzyme leads to a collision of the N and C(E) so that a S(N)2 methyltransfer reaction occurs. The proposed mechanism is supported by mutagenesis studies.     

Essential catalytic role of Glu134 in endo-beta-1,3-1,4-D-glucan 4-glucanohydrolase from B. licheniformis as determined by site-directed mutagenesis.

Site-directed mutagenesis experiments designed to identify the active site of Bacillus licheniformis endo-beta-1,3-1,4-D-glucan 4-glucanohydrolase (beta-glucanase) have been performed. Putative catalytic residues were chosen on the basis of sequence similarity analysis to viral and eukaryotic lysozymes. Four mutant enzymes were expressed and purified from recombinant E. coli and their kinetics analysed with barley beta-glucan. Replacement of Glu134 by Gln produced a mutant (E134Q) that retains less than 0.3% of the wild-type activity. The other mutants, D133N, E160Q and D179N, are active but show different kinetic parameters relative to wild-type indicative of their participation in substrate binding and transition-state complex stabilization. Glu134 is essential for activity; it is comprised in a region of high sequence similarity to the active site of T4 lysozyme and matches the position of the general acid catalyst. These results strongly support a lysozyme-like mechanism for this family of Bacillus beta-glucan hydrolases with Glu134 being the essential acid catalyst.     

Cell-Type-Specific Spatiotemporal Expression of Creatine Biosynthetic Enzyme S-adenosylmethionine:guanidinoacetate N-methyltransferase in Developing Mouse Brain

Creatine is synthesized by S-adenosylmethionine:guanidinoacetate N-methyltransferase (GAMT), and the creatine/phosphocreatine shuttle system mediated by creatine kinase (CK) is essential for storage and regeneration of high-energy phosphates in cells. Although the importance of this system in brain development is evidenced by the hereditary nature of creatine deficiency syndrome, the spatiotemporal cellular expression patterns of GAMT in developing brain remain unknown. Here we show that two waves of high GAMT expression occur in developing mouse brain. The first involves high expression in mitotic cells in the ventricular zone of the brain wall and the external granular layer of the cerebellum at the embryonic and neonatal stages. The second was initiated by striking up-regulation of GAMT in oligodendrocytes during the second and third postnatal weeks (i.e., the active myelination stage), which continued to adulthood. Distinct temporal patterns were also evident in other cell types. GAMT was highly expressed in perivascular pericytes and smooth muscle cells after birth, but not in adults. In neurons, GAMT levels were low to moderate in neuroblasts residing in the ventricular zone, increased during the second postnatal week when active dendritogenesis and synaptogenesis occur, and decreased to very low levels thereafter. Moderate levels were observed in astrocytes throughout development. The highly regulated, cell type-dependent expression of GAMT suggests that local creatine biosynthesis plays critical roles in certain phases of neural development. In accordance with this idea, we observed increased CK expression in differentiating neurons; this would increase creatine/phosphocreatine shuttle system activity, which might reflect increased energy demand.

     

Guanidinoacetate Decreases Antioxidant Defenses and Total Protein Sulfhydryl Content in Striatum of Rats

Guanidinoacetate methyltransferase (GAMT) deficiency is an inherited neurometabolic disorder biochemically characterized by tissue accumulation of guanidinoacetate (GAA) and depletion of creatine. Affected patients present epilepsy and mental retardation whose pathogeny is unclear. In the present study we investigated the in vitro and in vivo (intrastriatal administration) effects of GAA on some oxidative stress parameters in rat striatum. Sixty-day-old rats were used for intrastriatal infusion of GAA. For the in vitro studies, 60-day-old Wistar rats were killed by decapitation and the striatum was pre-incubated for 1 h at 37°C in the presence of GAA at final concentrations ranging from 10 to 100 μM. Parameters of oxidative stress such as total radical-trapping antioxidant potential (TRAP), antioxidant enzymes (SOD, GPx, and CAT), protein carbonyl and thiol contents were measured. DNA damage was also evaluated. Results showed that GAA administration (in vivo studies) or the addition of 100 μM GAA to assays (in vitro studies) significantly decreased TRAP, SOD activity, and total thiol levels in rat striatum. In contrast, this guanidino compound did not alter protein carbonyl content and the activities of CAT and GPx. DNA damage was not found after intrastriatal administration of GAA. The data indicate that the metabolite accumulating in GAMT deficiency decreases antioxidant capacity and total thiol content in the striatum. It is therefore presumed that this pathomechanism may contribute at least in part to the pathophysiology of the brain injury observed in patients affected by GAMT deficiency.     

Identification of a tyrosine residue in rat guanidinoacetate methyltransferase that is photolabeled with S-adenosyl-L-methionine.

Exposure of rat guanidinoacetate methyltransferase to ultraviolet light in the presence of S-adenosyl-L-[methyl-3H]methionine ([methyl-3H]AdoMet) results in covalent linking of radioactivity to the enzyme protein. The incorporation of radioactivity shows no lag and is linear with respect to time up to 1 h. The photolabeling is saturable with [methyl-3H]AdoMet, and the binding constant of the enzyme for AdoMet determined in this experiment is similar to that obtained by equilibrium dialysis. Low concentrations of competitive inhibitors S-adenosyl-L-homocysteine and sinefungin effectively prevent the photoinduced labeling by AdoMet. Although guanidinoacetate methyltransferase is irreversibly inactivated upon ultraviolet irradiation in the absence of AdoMet, the enzyme inactivated by 1-h exposure to ultraviolet irradiation has been shown to bind AdoMet with an affinity identical to that of the native enzyme. These results indicate that photolabeling occurs at the active site. Following proteolysis of the [methyl-3H]-AdoMet-labeled enzyme with chymotrypsin, a radioactive peptide is isolated having a sequence Asp-Thr-X-Pro-Leu-Ser-Glu-Glu-Thr-Trp. The peptide corresponds to residues 134-143, with X being modified Tyr-136. The same peptide is photolabeled when [carboxy-14C]AdoMet is used. High-performance liquid chromatography of this peptide after acid hydrolysis and phenyl isothiocyanate derivatization suggests that the entire molecule of AdoMet is attached to Tyr-136.     

Formulating a fluorogenic assay to evaluate S-adenosyl-L-methionine analogues as protein methyltransferase cofactors

Protein methyltransferases (PMTs) catalyze arginine and lysine methylation of diverse histone and nonhistone targets. These posttranslational modifications play essential roles in regulating multiple cellular events in an epigenetic manner. In the recent process of defining PMT targets, S-adenosyl-L-methionine (SAM) analogues have emerged as powerful small molecule probes to label and profile PMT targets. To examine efficiently the reactivity of PMTs and their variants on SAM analogues, we transformed a fluorogenic PMT assay into a ready high throughput screening (HTS) format. The reformulated fluorogenic assay is featured by its uncoupled but more robust character with the first step of accumulation of the commonly-shared reaction byproduct S-adenosyl-L-homocysteine (SAH), followed by SAH-hydrolase-mediated fluorogenic quantification. The HTS readiness and robustness of the assay were demonstrated by its excellent Z' values of 0.83-0.95 for the so-far-examined 8 human PMTs with SAM as a cofactor (PRMT1, PRMT3, CARM1, SUV39H2, SET7/9, SET8, G9a and GLP1). The fluorogenic assay was further implemented to screen the PMTs against five SAM analogues (allyl-SAM, propargyl-SAM, (E)-pent-2-en-4-ynyl-SAM (EnYn-SAM), (E)-hex-2-en-5-ynyl-SAM (Hey-SAM) and 4-propargyloxy-but-2-enyl-SAM (Pob-SAM)). Among the examined 8 × 5 pairs of PMTs and SAM analogues, native SUV39H2, G9a and GLP1 showed promiscuous activity on allyl-SAM. In contrast, the bulky SAM analogues, such as EnYn-SAM, Hey-SAM and Pob-SAM, are inert toward the panel of human PMTs. These findings therefore provide the useful structure-activity guidance to further evolve PMTs and SAM analogues for substrate labeling. The current assay format is ready to screen methyltransferase variants on structurally-diverse SAM analogues.     

Detecting S-adenosyl-L-methionine-induced conformational change of a histone methyltransferase using a homogeneous time-resolved fluorescence-based binding assay

A homogeneous time-resolved fluorescence (HTRF)-based binding assay has been established to measure the binding of the histone methyltransferase (HMT) G9a to its inhibitor CJP702 (a biotin analog of the known peptide-pocket inhibitor, BIX-01294). This assay was used to characterize G9a inhibitors. As expected, the peptide-pocket inhibitors decreased the G9a-CJP702 binding signal in a concentration-dependent manner. In contrast, the S-adenosyl-L-methionine (SAM)-pocket compounds, SAM and sinefungin, significantly increased the G9a-CJP702 binding signal, whereas S-adenosyl-L-homocysteine (SAH) showed minimal effect. Enzyme kinetic studies showed that CJP702 is an uncompetitive inhibitor (vs. SAM) that has a strong preference for the E:SAM form of the enzyme. Other data presented suggest that the SAM/sinefungin-induced increase in the HTRF signal is secondary to an increased E:SAM or E:sinefungin concentration. Thus, the G9a-CJP702 binding assay not only can be used to characterize the peptide-pocket inhibitors but also can detect the subtle conformational differences induced by the binding of different SAM-pocket compounds. To our knowledge, this is the first demonstration of using an uncompetitive inhibitor as a probe to monitor the conformational change induced by compound binding with an HTRF assay.     

Mechanistic studies on transcriptional coactivator protein arginine methyltransferase 1

Protein arginine methyltransferases (PRMTs) catalyze the transfer of methyl groups from S-adenosylmethionine (SAM) to the guanidinium group of arginine residues in a number of important cell signaling proteins. PRMT1 is the founding member of this family, and its activity appears to be dysregulated in heart disease and cancer. To begin to characterize the catalytic mechanism of this isozyme, we assessed the effects of mutating a number of highly conserved active site residues (i.e., Y39, R54, E100, E144, E153, M155, and H293), which are believed to play key roles in SAM recognition, substrate binding, and catalysis. The results of these studies, as well as pH-rate studies, and the determination of solvent isotope effects (SIEs) indicate that M155 plays a critical role in both SAM binding and the processivity of the reaction but is not responsible for the regiospecific formation of asymmetrically dimethylated arginine (ADMA). Additionally, mutagenesis studies on H293, combined with pH studies and the lack of a normal SIE, do not support a role for this residue as a general base. Furthermore, the lack of a normal SIE with either the wild type or catalytically impaired mutants suggests that general acid/base catalysis is not important for promoting methyl transfer. This result, combined with the fact that the E144A/E153A double mutant retains considerably more activity then the single mutants alone, suggests that the PRMT1-catalyzed reaction is primarily driven by bringing the substrate guanidinium into the proximity of the S-methyl group of SAM and that the prior deprotonation of the substrate guanidinium is not required for methyl transfer.     

Direct detection of N-H[...]O=C hydrogen bonds in biomolecules by NMR spectroscopy

A nuclear magnetic resonance (NMR) experiment is described for the direct detection of N-H[...]O=C hydrogen bonds (H-bonds) in 15N and 13C isotope-labeled biomolecules. This quantitative 'long-range' HNCO-COSY (correlation spectroscopy) experiment detects and quantifies electron-mediated scalar couplings across the H-bond (H-bond scalar couplings), which connect the magnetically active (15)N and (13)C nuclei on both sides of the H-bond. Detectable H-bonds comprise the canonical backbone H-bonds in proteins as well as other H-bonds in proteins and nucleic acids with N-H donors and O=C (carbonylic or carboxylic) acceptors. Unlike other NMR observables, which provide only indirect evidence of the presence of H-bonds, the H-bond scalar couplings identify all partners of the H-bond, the donor, the donor proton and the acceptor, in a single experiment. The size of the scalar couplings can be related to H-bond geometries. The time required to detect the N-H[...]O=C H-bonds in small proteins (< or = approximately 10 kDa) is typically on the order of 1 d at millimolar concentrations, whereas H-bond detection for larger proteins (< or = approximately 30 kDa) may be possible within several days depending on concentration, isotope composition, magnetic field strength and molecular weight. The proteins ubiquitin (8.6 kDa), dimeric RANTES (2 x 8.5 kDa) and MAP30 (30 kDa) are used as examples to illustrate this procedure.     

The stimulating effect of cyclic AMP, glucagon and insulin on guanidine acetate-N-methyltransferase activity in rat liver and pancreas]

N6,O2'-dibutyrylcyclo-3',5'-AMP injected to intact rats alone or in combination with theophylline increases the activity of guanidine acetate methyltransferase (GAMT) in liver and pancreas. Cyclic 3',5'-AMP and its dibutyryl analog administered immediately or two hours after the suturing of common bile duct (SCBD) stimulate the increase of pancreatic GAMT activity 2-3 fold. Glucagon, injected intraabdominally simultaneously with SCBD and administration of theophylline, dramatically increases the theophylline effect on the GAMT activity. The freezing of rat pancreas pretreated witn secretin, a hormone structurally similar to glucagon, results in a 1.5-2-fold increase of creatine synthesis from S-adenosylmethionine and guanidinacetic acid. An hour after glucagon administration to intact rats the GAMT activity of liver increases 9 times. The effect of glucagon is enhanced by insulin. Cycloheximide inhibits the increase of GAMT activity, induced by glucagon or a combination of glucagon and insulin. Experiments on tissue homogenates demonstrate that 3',5'-AMP in concentrations of 10(-8) --10(-2) M does not affect the GAMT activity or to some extent inhibits the enzyme. The homogenate incubation in a medium containing 10(-5) M epinephrine or 10(-7) M caffeine and 5 mM Mg2+ leads to an increase in the GAMT activity. Oligomycin removes the stimulating effects of caffeine and Mg2+ on the enzyme activation. This is probably due to the presence of 3',5'-AMP-dependent protein kinase in the mechanism of GAMT activation by cyclic AMP.     

Crystal structures of BchU, a methyltransferase involved in bacteriochlorophyll c biosynthesis, and its complex with S-adenosylhomocysteine: implications for reaction mechanism

BchU plays a role in bacteriochlorophyll c biosynthesis by catalyzing methylation at the C-20 position of cyclic tetrapyrrole chlorin using S-adenosylmethionine (SAM) as a methyl source. This methylation causes red-shifts of the electronic absorption spectrum of the light-harvesting pigment, allowing green photosynthetic bacteria to adapt to low-light environments. We have determined the crystal structures of BchU and its complex with S-adenosylhomocysteine (SAH). BchU forms a dimer and each subunit consists of two domains, an N-terminal domain and a C-terminal domain. Dimerization occurs through interactions between the N-terminal domains and the residues responsible for the catalytic reaction are in the C-terminal domain. The binding site of SAH is located in a large cavity between the two domains, where SAH is specifically recognized by many hydrogen bonds and a salt-bridge. The electron density map of BchU in complex with an analog of bacteriochlorophyll c located its central metal near the SAH-binding site, but the tetrapyrrole ring was invisible, suggesting that binding of the ring to BchU is loose and/or occupancy of the ring is low. It is likely that His290 acts as a ligand for the central metal of the substrate. The orientation of the substrate was predicted by simulation, and allows us to propose a mechanism for the BchU directed methylation: the strictly conserved Tyr246 residue acts catalytically in the direct transfer of the methyl group from SAM to the substrate through an S(N)2-like mechanism.     

Dispensability of glutamic acid 48 and aspartic acid 134 for Mn2+-dependent activity of Escherichia coli ribonuclease HI

The activities of the eight mutant proteins of Escherichia coli RNase HI, in which the four carboxylic amino acids (Asp(10), Glu(48), Asp(70), and Asp(134)) involved in catalysis are changed to Asn (Gln) or Ala, were examined in the presence of Mn(2+). Of these proteins, the E48A, E48Q, D134A, and D134N proteins exhibited the activity, indicating that Glu(48) and Asp(134) are dispensable for Mn(2+)-dependent activity. The maximal activities of the E48A and D134A proteins were comparable to that of the wild-type protein. However, unlike the wild-type protein, these mutant proteins exhibited the maximal activities in the presence of >100 microM MnCl(2), and their activities were not inhibited at higher Mn(2+) concentrations (up to 10 mM). The wild-type protein contains two Mn(2+) binding sites and is activated upon binding of one Mn(2+) ion at site 1 at low ( approximately 1 microM) Mn(2+) concentrations. This activity is attenuated upon binding of a second Mn(2+) ion at site 2 at high (>10 microM) Mn(2+) concentrations. The cleavage specificities of the mutant proteins, which were examined using oligomeric substrates at high Mn(2+) concentrations, were identical to that of the wild-type protein at low Mn(2+) concentrations but were different from that of the wild-type protein at high Mn(2+) concentrations. These results suggest that one Mn(2+) ion binds to the E48A, E48Q, D134A, and D134N proteins at site 1 or a nearby site with weaker affinities. The binding analyses of the Mn(2+) ion to these proteins in the absence of the substrate support this hypothesis. When Mn(2+) ion is used as a metal cofactor, the Mn(2+) ion itself, instead of Glu(48) and Asp(134), probably holds water molecules required for activity.