Biomedical subjects

R Huber

Publications and source records attributed to R Huber.

At least 253 records · Page 14Linked to original sources

PubMed1997

Crystal structure of herbicide-detoxifying maize glutathione S-transferase-I in complex with lactoylglutathione: evidence for an induced-fit mechanism.

Glutathione S-transferases (GSTs) -I and -III are involved in herbicide metabolism in maize and have been intensively studied. Starting with plant tissue from Zea mays var. mutin recombinant GST-I was prepared by heterologous expression in Escherichia coli. The enzyme was crystallized in the presence of lactoylglutathione, a ligand formerly never observed in a GST structure and known as an intermediate of the pharmacologically relevant glyoxalase system. The crystal structure of GST-I has been determined at 2.5 A resolution and exhibits the GST-typical dimer of two identical subunits, each consisting of 214 residues. Compared with other plant GSTs the three-dimensional structure of GST-I primarily shows structural differences in the hydrophobic substrate binding site, the linker segment and the C-terminal region. Furthermore, a comparison of the ligand-bound GST-I structure with the apo structure of GST-III indicates the movement of a ten-residue loop upon binding of the ligand to the active site. This is the first structure-based evidence for an induced fit mechanism of glutathione S-transferases, which has previously been postulated for class pi enzymes. Together with GST-III, GST-I may explain herbicide resistance and selectivity in maize as well as in other agronomic relevant crops.

Arabidopsis↗

PubMed1997

Voltage dependent binding of annexin V, annexin VI and annexin VII-core to acidic phospholipid membranes.

Annexin V, VI and VII-core (delta1-107) are members of the annexin protein family and bind to acidic phospholipid membranes in a calcium dependent manner. They also show ion channel activity under certain conditions. As annexins bind peripherally to lipid membranes, ion channel formation must consist of at least two steps: An adsorption reaction regulating the binding of annexin to the membrane surface and the opening and closing of the active species controlling the channel activity. By using the baseline current through the patch clamp seal as a probe for unoccupied binding sites at the membrane, we show that the adsorption of annexins to membranes is not only calcium dependent but also strongly voltage dependent. Whereas the free transfer energies at low calcium concentrations are similar for all three annexins, the binding of annexin V becomes much tighter with higher calcium levels, compared to annexin VI and VII-core. This correlates with the finding that annexin VI and VII-core display channel activity much more often than annexin V if one assumes that a high coverage of the membrane surface with annexins stabilizes the bilayer. At higher protein concentrations weaker binding is observed in agreement with the previously reported anti-cooperativity of membrane binding.

Adsorption↗

PubMed1997

Converting blood coagulation factor IXa into factor Xa: dramatic increase in amidolytic activity identifies important active site determinants.

The coagulation factors IXa (fIXa) and Xa (fXa) share extensive structural and functional homology; both cleave natural substrates effectively only with a cofactor at a phospholipid surface. However, the amidolytic activity of fIXa is 10(4)-fold lower than that of fXa. To identify determinants of this poor reactivity, we expressed variants of truncated fIXa (rf9a) and fXa (rf10a) in Escherichia coli. The crystal structures of fIXa and fXa revealed four characteristic active site components which were subsequently exchanged between rf9a and rf10a. Exchanging Glu219 by Gly or exchanging the 148 loop did not increase activity of rf9a, whereas corresponding mutations abolished reactivity of rf10a. Exchanging Ile213 by Val only moderately increased reactivity of rf9a. Exchanging the 99 loop, however, dramatically increased reactivity. Furthermore, combining all four mutations essentially introduced fXa properties into rf9a: the amidolytic activity was increased 130-fold with fXa substrate selectivity. The results suggest a 2-fold origin of fIXa's poor reactivity. A narrowed S3/S4 subsite disfavours interaction with substrate P3/P4 residues, while a distorted S1 subsite disfavours effective cleavage of the scissile bond. Both defects could be repaired by introducing fXa residues. Such engineered coagulation enzymes will be useful in diagnostics and in the development of therapeutics.

Amidohydrolases↗

PubMed1997

Closed structure of phosphoglycerate kinase from Thermotoga maritima reveals the catalytic mechanism and determinants of thermal stability.

BACKGROUND: Phosphoglycerate kinase (PGK) is essential in most living cells both for ATP generation in the glycolytic pathway of aerobes and for fermentation in anaerobes. In addition, in many plants the enzyme is involved in carbon fixation. Like other kinases, PGK folds into two distinct domains, which undergo a large hinge-bending motion upon catalysis. The monomeric 45 kDa enzyme catalyzes the transfer of the C1-phosphoryl group from 1, 3-bisphosphoglycerate to ADP to form 1,3-bisphosphoglycerate to ADP to form 3-phosphoglycerate and ATP. For decades, the conformation of the enzyme during catalysis has been enigmatic. The crystal structure of PGK from the hyperthermophilic organism Thermotoga maritima (TmPGK) represents the first structure of an extremely thermostable PGK. It adds to a series of four known crystal structures of PGKs from mesophilic via moderately thermophilic to a hyperthermophilic organism, allowing a detailed analysis of possible structural determinants of thermostability. RESULTS: The crystal structure of TmPGK was determined to 2.0 A resolution, as a ternary complex with the product 3-phosphoglycerate and the product analogue AMP-PNP (adenylyl-imido diphosphate). The complex crystallizes in a closed conformation with a drastically reduced inter-domain angle and a distance between the two bound ligands of 4.4 A, presumably representing the active conformation of the enzyme. The structure provides new details of the catalytic mechanism. An inter-domain salt bridge between residues Arg62 and Asp200 forms a strap to hold the two domains in the closed state. We identify Lys197 as a residue involved in stabilization of the transition state phosphoryl group, and so term it the 'phosphoryl gripper'. CONCLUSIONS: The hinge-bending motion of the two domains upon closure of the structure, as seen in the Trypanosoma PGK structure, is confirmed. This closed conformation obviously occurs after binding of both substrates and is locked by the Arg62-Asp200 salt bridge. Re-orientations in the conserved active-site loop region around Thr374 also bring both domains into direct contact in the core region of the former inter-domain cleft, to form the complete catalytic site. Comparison of extremely thermostable TmPGK with less thermostable homologues reveals that its increased rigidity is achieved by a raised number of intramolecular interactions, such as an increased number of ion pairs and additional stabilization of alpha helix and loop regions. The covalent fusion with triosephosphate isomerase might represent an additional stabilization strategy.

Amino Acid Sequence↗

PubMed1997

Catalytic domain structure of vampire bat plasminogen activator: a molecular paradigm for proteolysis without activation cleavage.

The saliva of the blood-eating vampire bat Desmodus rotundus contains plasminogen activators (PAs) that maintain the fluidity of the prey's blood by activating plasminogen and dissolving developing fibrin clots. D. rotundus salivary PAs (DSPAs) are composed of evolutionarily conserved domains reminiscent of human tissue-type PA (tPA), but their catalytic domain lacks a plasmin-sensitive "activation cleavage site". Despite this, all DSPAs are intrinsically active and enormously stimulated in the presence of fibrin. The recombinant catalytic domain of DSPAalpha1 has been crystallized in a covalent complex with Glu-Gly-Arg-chloromethyl ketone and its structure solved at 2.9 A resolution. The structure is similar to that of activated two-chain human tPA. Despite its single-chain status, the activation domain is observed in an enzymatically active conformation, with a functional substrate binding site and active site accommodating the peptidylmethylene inhibitor. The activation pocket, which normally receives the N-terminal Ile16, is occupied by the side chain of Lys156, whose distal ammonium group makes an internal salt bridge with the carboxylate group of Asp194. Lys156 is in a groove shielded from the bulk solvent by the intact "activation loop" (Gln10-Phe21), favoring Lys156-Asp194 salt bridge formation and stabilization of a functional substrate binding site. Together with the characteristic 186 insertion loop, the activation loop could act as a switch, effecting full single-chain enzymatic activity upon binding to fibrin.

Amino Acid Sequence↗

PubMed1997

Structure of the thrombin complex with triabin, a lipocalin-like exosite-binding inhibitor derived from a triatomine bug.

Triabin, a 142-residue protein from the saliva of the blood-sucking triatomine bug Triatoma pallidipennis, is a potent and selective thrombin inhibitor. Its stoichiometric complex with bovine alpha-thrombin was crystallized, and its crystal structure was solved by Patterson search methods and refined at 2.6-A resolution to an R value of 0.184. The analysis revealed that triabin is a compact one-domain molecule essentially consisting of an eight-stranded beta-barrel. The eight strands A to H are arranged in the order A-C-B-D-E-F-G-H, with the first four strands exhibiting a hitherto unobserved up-up-down-down topology. Except for the B-C inversion, the triabin fold exhibits the regular up-and-down topology of lipocalins. In contrast to the typical ligand-binding lipocalins, however, the triabin barrel encloses a hydrophobic core intersected by a unique salt-bridge cluster. Triabin interacts with thrombin exclusively via its fibrinogen-recognition exosite. Surprisingly, most of the interface interactions are hydrophobic. A prominent exception represents thrombin's Arg-77A side chain, which extends into a hydrophobic triabin pocket forming partially buried salt bridges with Glu-128 and Asp-135 of the inhibitor. The fully accessible active site of thrombin in this complex is in agreement with its retained hydrolytic activity toward small chromogenic substrates. Impairment of thrombin's fibrinogen converting activity or of its thrombomodulin-mediated protein C activation capacity upon triabin binding is explained by usage of overlapping interaction sites of fibrinogen, thrombomodulin, and triabin on thrombin. These data demonstrate that triabin inhibits thrombin via a novel and unique mechanism that might be of interest in the context of potential therapeutic applications.

Amino Acid Sequence↗

PubMed1997

Structures of class pi glutathione S-transferase from human placenta in complex with substrate, transition-state analogue and inhibitor.

BACKGROUND: Glutathione S-transferases (GSTs) are detoxification enzymes, found in all aerobic organisms, which catalyse the conjugation of glutathione with a wide range of hydrophobic electrophilic substrates, thereby protecting the cell from serious damage caused by electrophilic compounds. GSTs are classified into five distinct classes (alpha, mu, pi, sigma and theta) by their substrate specificity and primary structure. Human GSTs are of interest because tumour cells show increased levels of expression of single classes of GSTs, which leads to drug resistance. Structural differences between classes of GST can therefore be utilised to develop new anti-cancer drugs. Many mutational and structural studies have been carried out on the mu and alpha classes of GST to elucidate the reaction mechanism, whereas knowledge about the pi class is still limited. RESULTS: We have solved the structures of the pi class GST hP1-1 in complex with its substrate, glutathione, a transition-state complex, the Meisenheimer complex, and an inhibitor, S-(rho-bromobenzyl)-glutathione, and refined them to resolutions of 1.8 A, 2.0 A and 1.9 A, respectively. All ligand molecules are well-defined in the electron density. In all three structures, an additionally bound N-morpholino-ethansulfonic acid molecule from the buffer solution was found. CONCLUSIONS: In the structure of the GST-glutathione complex, two conserved water molecules are observed, one of which hydrogen bonds directly to the sulphur atom of glutathione and the other forms hydrogen bonds with residues around the glutathione-binding site. These water molecules are absent from the structure of the Meisenheimer complex bound to GST, implicating that deprotonation of the cysteine occurs during formation of the ternary complex which involves expulsion of the inner bound water molecule. The comparison of our structures with known mu class GST structures show differences in the location of the electrophile-binding site (H-site), explaining the different substrate specificities of the two classes. Fluorescence measurements are in agreement with the position of the N-morpholino-ethansulfonic acid, close to Trp28, identifying a possible ligandin-substrate binding site.

Alkanesulfonic Acids↗

PubMed1997

Slow-binding inhibition of Escherichia coli cystathionine beta-lyase by L-aminoethoxyvinylglycine: a kinetic and X-ray study.

The pyridoxal 5'-phosphate (PLP)-dependent cystathionine beta-lyase (CBL) was previously found to be inhibited by the natural toxins rhizobitoxine and l-aminoethoxyvinylglycine (AVG). The present study characterizes the interaction of Escherichia coli CBL with AVG and methoxyvinylglycine (MVG) by a combination of kinetic methods and X-ray crystallography. Upon AVG treatment, time-dependent, slow-binding inhibition [Morrison, J. F. (1982) Trends Biochem. Sci. 7, 102-105] was observed due to the generation of a long-lived, slowly dissociating enzyme-inhibitor complex. Kinetic analysis revealed a one-step inhibition mechanism (CBL + AVG --> CBLAVG, Ki = 1.1 +/- 0.3 microM) with an association rate constant (k1) of 336 +/- 40 M-1 s-1. This value is several orders of magnitude lower than typical bimolecular rate constants of ES formation, suggesting that additional steps occur before formation of the first detectable CBLAVG complex. Loss of activity is paralleled by the conversion of the pyridoxaldimine 426 nm chromophore to a 341 nm-absorbing species. On the basis of the recently solved structure of native CBL [Clausen, T., et al. (1996) J. Mol. Biol. 262, 202-224], it was possible to elucidate the X-ray structure of the CBLAVG complex and to refine it to an R-factor of 16.4% at 2.2 A resolution. The refined structure reveals the geometry of the bound inhibitor and its interactions with residues in the active site of CBL. Both the X-ray structure and the absorbance spectrum of the CBLAVG complex are compatible with a ketimine as the reaction product. Thus, the inhibitor seems to bind in a similar way to CBL as the substrate, but after alpha-proton abstraction, the reaction proceeds in a CBL nontypical manner, i.e. protonation of PLP-C4', resulting in the "dead-end" ketimine PLP derivative. The CBLAVG structure furthermore suggests a binding mode for rhizobitoxine and explains the failure of MVG to inhibit CBL.

Crystallography, X-Ray↗

PubMed1997

Analysis of exon/intron structure and 400 kb of genomic sequence surrounding the 5'-promoter and 3'-terminal ends of the human glypican 3 (GPC3) gene.

GPC3, the gene modified in the Simpson-Golabi-Behmel gigantism/overgrowth syndrome (SGBS), is shown to span more than 500 kb of genomic sequence, with the transcript beginning 197 bp 5' of the translational start site. The Xq26.1 region containing GPC3 as the only known gene has been extended to > 900 kb by sequence analysis of flanking BAC clones. Two GC isochores (40.6 and 42.6% GC) are observed at the 5' and 3' ends of the locus, with a large repertoire of repetitive sequences that includes an unusual cluster of four L1 elements > 92% identical over 2.8 kb. Eight exons, accounting for the full 2.4-kb GPC3 cDNA, have been sequenced along with neighboring intronic regions. PCR assays have been developed to amplify each exon and exon/intron junction sequence, to help discriminate instances of SGBS among individuals with overgrowth syndromes and to facilitate mutational analysis of lesions in the gene.

Amino Acid Sequence↗

PubMed1997

Cloning, purification, crystallization, and preliminary X-ray diffraction analysis of cystathionine gamma-synthase from E. coli.

The Escherichia coli metB gene has been PCR-extracted from genomic DNA and placed under the control of a tac and a T7 promoter in plasmids pCYB1 and pET22b(+), respectively, to produce overexpressing bacterial strains for the gene product, cystathionine gamma-synthase. Efficient purification procedures have been developed for a C-terminally intein-tagged version and the wild-type target protein, yielding the product in a quantity and homogeneity amenable to high-resolution single-crystal X-ray analysis. Crystals have been obtained in space group P1 with unit cell constants a=82.2 A, b=84.2 A, c=116.2 A, alpha=107.0 degrees, beta=96.3 degrees, gamma=108.0 degrees, suggesting eight monomers per asymmetric unit (V[M]=2.23 A3/Da). Crystals diffract to beyond 2.6 A resolution and a data set complete to 2.8 A resolution has been collected using a rotating anode X-ray source. A cryogenic buffer system has been developed to allow synchrotron data collection. Patterson self rotation searches reveal the presence of two independent tetramers with local 222 symmetry in an asymmetric unit. The crystallographic results corroborate and extend previous solution studies regarding the quaternary organization of the enzyme.

Carbon-Oxygen Lyases↗

PubMed1997

Mechanism of inhibition of the human matrix metalloproteinase stromelysin-1 by TIMP-1.

Matrix metalloproteinases (MMPs) are zinc endopeptidases that are required for the degradation of extracellular matrix components during normal embryo development, morphogenesis and tissue remodelling. Their proteolytic activities are precisely regulated by endogenous tissue inhibitors of metalloproteinases (TIMPs). Disruption of this balance results in diseases such as arthritis, atherosclerosis, tumour growth and metastasis. Here we report the crystal structure of an MMP-TIMP complex formed between the catalytic domain of human stromelysin-1 (MMP-3) and human TIMP-1. TIMP-1, a 184-residue protein, has the shape of an elongated, contiguous wedge. With its long edge, consisting of five different chain regions, it occupies the entire length of the active-site cleft of MMP-3. The central disulphide-linked segments Cys 1-Thr 2-Cys 3-Val 4 and Ser 68-Val 69 bind to either side of the catalytic zinc. Cys 1 bidentally coordinates this zinc, and the Thr-2 side chain extends into the large specificity pocket of MMP-3. This unusual architecture of the interface between MMP-3 and TIMP-1 suggests new possibilities for designing TIMP variants and synthetic MMP inhibitors with potential therapeutic applications.

Binding Sites↗

PubMed1997

Thermus thermophilus cytochrome-c552: A new highly thermostable cytochrome-c structure obtained by MAD phasing.

The three-dimensional structure of cytochrome-c552 from Thermus thermophilus has been determined by the multiple anomalous dispersion technique using synchrotron radiation and refined to a resolution of 1.28 A. Data collection at 90 K and the recording of three data sets (f'-minimum: 7125 eV, f"-maximum: 7138 eV and reference for scaling: 10,077 eV) resulted in an initial electron density of very high quality at 2.1 A, which was readily interpretable for model building. The model was refined to an R value of 19.1% (Rfree=22.4%) at 1.28 A resolution using a fourth data set collected at a photon energy of 11,810 eV. Comparison of this thermophilic cytochrome with its mesophilic mitochondrial or bacterial counterparts reveals significant structural differences which are discussed with respect to their importance for thermostability and binding between this cytochrome and its corresponding ba3-oxidase. Amino acid sequence similarities to other class I cytochromes are very weak and entirely limited to the region around the CXXCH motif close to the N terminus. The N-terminal two-thirds of cytochrome-c552 cover spatial regions around the heme prosthetic group that are similar to those observed for other cytochromes. The actual secondary structural elements that are responsible for that shielding do not, however, correlate well to other structures. Only the N-terminal helix (containing the heme binding cysteine residues) aligns reasonably well with other class I cytochromes. The most striking differences that distinguish the present structure from all other class I cytochromes is the C-terminal one-third of the molecule that wraps around the remainder of the structure as a stabilizing clamp, the existence of an extended beta-sheet covering one edge of the heme and the lack of any internal water molecule.

Bacterial Proteins↗

PubMed1997

Structural mapping of the active site specificity determinants of human tissue-type plasminogen activator. Implications for the design of low molecular weight substrates and inhibitors.

The recent structure determination of the catalytic domain of tissue-type plasminogen activator (tPA) suggested residue Arg174 could play a role in P3/P4 substrate specificity. Six synthetic chromogenic tPA substrates of the type R-Xaa-Gly-Arg-p-nitroanilide, in which R is an N-terminal protection group, were synthesized to test this property. Although changing the residue Xaa (in its L or D form) at position P3 from the hydrophobic Phe to an acidic residue, Asp or Glu, gave no improvement in catalytic efficiency, comparative analysis of the substrates indicated a preference for an acidic substituent occupying the S3 site when the S4 site contains a hydrophobic or basic moiety. The 2.9 A structure determination of the catalytic domain of human tPA in complex with the bis-benzamidine inhibitor 2, 7-bis-(4-amidinobenzylidene)-cycloheptan-1-one reveals a three-site interaction, salt bridge formation of the proximal amidino group of the inhibitor with Asp189 in the primary specificity pocket, extensive hydrophobic surface burial, and a weak electrostatic interaction between the distal amidino group of the inhibitor and two carbonyl oxygens of the protein. The latter position was previously occupied by the guanidino group of Arg174, which swings out to form the western edge of the S3 pocket. These data suggest that the side chain of Arg174 is flexible, and does not play a major role in the S4 specificity of tPA. On the other hand, this residue would modulate S3 specificity, and may be exploited to fine tune the specificity and selectivity of tPA substrates and inhibitors.

Arginine↗

PubMed1997

Cytochrome-c552 from Thermus thermophilus: a functional and crystallographic investigation.

The eubacterium Thermus thermophilus expresses terminal oxidases of the ba3- and caa3-type. The soluble cytochrome-c552 of this organism has been isolated by a new method and characterized. In contrast to previous studies, but in line with coexpression at low aeration, the cytochrome was unambiguously identified as the substrate of the ba3-oxidase. In the presence of TMPD and ascorbate, biphasic Eadie-Hofstee plots with kmax = 250 s-1 at 25 degrees C are observed upon addition of cytochrome-c552. Surprisingly, the caa3-oxidase with its single covalently bound cytochrome-c also exhibits a biphasic redox activity with kmax = 185 s-1 in the presence of TMPD and ascorbate only. Further addition of cytochrome-c552 does not lead to enhanced activity. Crystals of cytochrome-c552 were obtained by vapor diffusion using the sitting-drop method in the presence of ammonium sulfate as precipitant. They diffract to 1.28 A resolution using synchrotron radiation. The structure has been solved by MAD phasing.

Amino Acid Sequence↗

PubMed1997

Lysine 156 promotes the anomalous proenzyme activity of tPA: X-ray crystal structure of single-chain human tPA.

Tissue type plasminogen activator (tPA) is the physiological initiator of fibrinolysis, activating plasminogen via highly specific proteolysis; plasmin then degrades fibrin with relatively broad specificity. Unlike other chymotrypsin family serine proteinases, tPA is proteolytically active in a single-chain form. This form is also preferred for therapeutic administration of tPA in cases of acute myocardial infarction. The proteolytic cleavage which activates most other chymotrypsin family serine proteinases increases the catalytic efficiency of tPA only 5- to 10-fold. The X-ray crystal structure of the catalytic domain of recombinant human single-chain tPA shows that Lys156 forms a salt bridge with Asp194, promoting an active conformation in the single-chain form. Comparisons with the structures of other serine proteinases that also possess Lys156, such as trypsin, factor Xa and human urokinase plasminogen activator (uPA), identify a set of secondary interactions which are required for Lys156 to fulfil this activating role. These findings help explain the anomalous single-chain activity of tPA and may suggest strategies for design of new therapeutic plasminogen activators.

Amino Acid Chloromethyl Ketones↗

PubMed1997

The three-dimensional structure of recombinant leech-derived tryptase inhibitor in complex with trypsin. Implications for the structure of human mast cell tryptase and its inhibition.

The x-ray crystal structure of recombinant leech-derived tryptase inhibitor (rLDTI) has been solved to a resolution of 1.9 A in complex with porcine trypsin. The nonclassical Kazal-type inhibitor exhibits the same overall architecture as that observed in solution and in rhodniin. The complex reveals structural aspects of the mast cell proteinase tryptase. The conformation of the binding region of rLDTI suggests that tryptase has a restricted active site cleft. The basic amino terminus of rLDTI, apparently flexible from previous NMR measurements, approaches the 148-loop of trypsin. This loop has an acidic equivalent in tryptase, suggesting that the basic amino terminus could make favorable electrostatic interactions with the tryptase molecule. A series of rLDTI variants constructed to probe this hypothesis confirmed that the amino-terminal Lys-Lys sequence plays a role in inhibition of human lung tryptase but not of trypsin or chymotrypsin. The location of such an acidic surface patch is in accordance with the known low molecular weight inhibitors of tryptase.

Amino Acid Sequence↗

PubMed1997

Bioincorporation of telluromethionine into proteins: a promising new approach for X-ray structure analysis of proteins.

A simple and efficient method for the specific and quantitative replacement of the naturally occurring amino acid methionine by its isosteric analogue telluromethionine in the expression of recombinant proteins has been developed. The method requires a controlable and competitive expression system like the bacteriophage T7 polymerase/promoter in a methionine-auxotrophic host. Using methionine-auxotrophic Escherichia coli strains, incorporation of telluromethionine at high yields has been achieved for human recombinant annexin V, human mitochondrial transamidase, Arabidopsis glutathione-S-transferase and the N-terminal domain of Salmonella tailspike adhesion protein as confirmed by amino acid, mass-spectrometric and X-ray analyses. Expressed and purified telluromethionine-proteins and native proteins were found to crystallise isomorphously. In terms of efficient bio-expression, isomorphism of crystals and relative abundance of methionine residues, the production of telluromethionine-proteins as heavy-atom derivatives offers a valid and general approach in X-ray analysis by the method of multiple isomorphous replacement.

Amidinotransferases↗

PubMed1997

Substrate binding and catalysis by L-arginine:glycine amidinotransferase--a mutagenesis and crystallographic study.

L-Arginine:glycine amidinotransferase catalyzes the committed step in the biosynthesis of creatine. Eight active-site mutants, D170N, D254N, H303V, D305A, R322E, S355A, C407S, and C410A of recombinant human L-arginine:glycine amidinotransferase were prepared by site-directed mutagenesis and enzymatically characterized. The crystal structures of the three mutants D170N, D254N, and C407S have been determined at 0.28-nm, 0.29-nm and 0.236-nm resolution, respectively. The mutation of active-site residues which are involved in substrate-binding yielded inactive mutants. Substitution of Asp254, which is not directly involved in substrate binding but is thought to transfer protons in concert with the His303 imidazole group, results in a strongly (2000-fold) reduced activity. However, the substitution of Cys410, a residue near the active site but not involved in catalysis or substrate binding, by Ala does not change the kinetic properties with respect to the wild-type enzyme. The loss of enzymatic activity of the D170N, D254N, C407S and likely all other mutants is solely due to the inserted point mutations, affecting substrate binding or transition-state stabilization, and not due to major conformational rearrangements of the protein. These results show that a His-Asp pair on one side of the substrate and a Cys on the other side are key residues for activity and are part of a disjoint triad. The imidazole ring of the His is proposed to act as a general acid/general base during catalysis whereas the Cys acts as a nucleophile analogous to Cys25 of papain-like cysteine proteinases.

Amidinotransferases↗