Biomedical subjects

G E Schulz

Publications and source records attributed to G E Schulz.

At least 37 records · Page 2Linked to original sources

PubMed1999

The structure of the membrane protein squalene-hopene cyclase at 2.0 A resolution.

Squalene cyclases catalyze a cationic cyclization cascade, which is homologous to a key step in cholesterol biosynthesis. The structure of the enzyme from Alicyclobacillus acidocaldarius has been determined in a new crystal form at 2.0 A resolution (1 A=0.1 nm) and refined to an R-factor of 15.3 % (Rfree=18.7 %). The structure indicates how the initial protonation and the final deprotonation of squalene occur and how the transient carbocations are stabilized. The pathways of the flexible educt squalene from the membrane interior to the active center cavity and of the rigid fused-ring product hopene in the reverse direction are discussed. The enzyme contains eight so-called QW-sequence repeats that fortify the alpha/alpha-barrels by an intricate interaction network. They are unique to the known triterpene cyclases and are presumed to shield these enzymes against the released enthalpy of the highly exergonic catalyzed reaction. The enzyme is a monotopic membrane protein, the membrane-binding interactions of which are described and compared with those of two prostaglandin-H2 synthase isoenzymes, the only other structurally characterized proteins of this type. In the crystals the membrane-binding regions face each other, suggesting a micelle-type detergent structure between them.

Amino Acid Sequence↗

PubMed1999

Strategy for membrane protein crystallization exemplified with OmpA and OmpX.

The bacterial outer membrane proteins OmpA and OmpX were modified in such a manner that they yielded bulky crystals diffracting X-rays isotropically beyond 2 A resolution and permitting detailed structural analyses. The procedure involved semi-directed mutagenesis, mass production into inclusion bodies, and (re)naturation therefrom; it should be applicable for a broader range of membrane proteins.

Bacterial Outer Membrane Proteins↗

PubMed1999

Chaperone-assisted expression of authentic bovine adrenodoxin reductase in Escherichia coli.

Adrenodoxin reductase is an essential component of the mitochondrial monooxygenase systems that are involved in the synthesis of steroid hormones and related compounds. After removing by mutagenesis a secondary ribosome binding site and an mRNA loop formed between the gene and the vector, large amounts of the enzyme could be produced in Escherichia coli by coexpression with the HSP60-chaperone system. The purified protein was homogeneous enough for reproducible crystallization. The crystals diffracted X-rays isotropically beyond 1.7 A resolution permitting a structure analysis.

Animals↗

PubMed1999

Locating proper non-crystallographic symmetry in low-resolution electron-density maps with the program GETAX.

Non-crystallographic symmetry averaging for improving and extending an initial set of phases can be crucial at an early stage of a protein structure analysis. A method is described which detects the position of a proper rotation axis in a surprisingly poor electron-density map and is fast enough to run through a large number of axis orientations. It uses a simple multimer mask to define the searching unit, which is then shifted through the whole unit cell looking for the position with the highest correlation coefficient between the interrelated parts. Appropriate weighting and averaging enhances the signal-to-noise ratio. Examples of the application of this algorithm are given. The use of the local rotation axis for phasing is commented on. A search of the Protein Data Bank showed that 27% of the unique crystal forms contain proper local n-fold axes, which could have been located with the presented method.

Adenylate Kinase↗

PubMed1999

Homogenization and crystallization of histidine ammonia-lyase by exchange of a surface cysteine residue.

Histidase (histidine ammonia-lyase, EC 4.3.1.3) from Pseudomonas putida was expressed in Escherichia coli and purified. In the absence of thiols the tetrameric enzyme gave rise to undefined aggregates and suitable crystals could not be obtained. The solvent accessibility along the chain was predicted from the amino acid sequence. Among the seven cysteines, only one was labeled as 'solvent-exposed'. The exchange of this cysteine to alanine abolished all undefined aggregations and yielded readily crystals diffracting to 1.8 A resolution.

Crystallography↗

PubMed1998

The structure of a trimeric archaeal adenylate kinase.

The adenylate kinase from the hyperthermophilic archaean species Sulfolobus acidocaldarius has been cloned, expressed in Escherichia coli, purified and crystallized. The crystal structure was elucidated by multiple isomorphous replacement and non-crystallographic density averaging. The structure was refined at 2.6 A (1 A=0.1 nm) resolution. The enzyme is trimeric, in contrast to previous solution measurements that suggested a dimeric structure, and in contrast to the vast majority of adenylate kinases, which are monomeric. In large parts of each subunit the chain fold resembles the known enzyme structure from eubacteria and eukaryotes although the sequence homology is negligible. Since the asymmetric unit contains two trimers with and without bound AMP at the AMP sites and with an ADP at one of the six ATP sites, the analysis shows the enzyme in several states. The conformational differences between these states resemble those of other adenylate kinases. Because of sequence homology, the structure presented provides a good model for the methanococcal adenylate kinases.

Adenosine Diphosphate↗

PubMed1998

Substrate binding to a cyclodextrin glycosyltransferase and mutations increasing the gamma-cyclodextrin production.

Bacterial cyclodextrin glycosyltransferases use starch to produce cyclic maltooligosaccharides (cyclodextrins) which are of interest in various applications. The cyclization reaction gives rise to a spectrum of ring sizes consisting of predominantly six to eight glucosyl units. Using the enzyme from Bacillus circulans strain no. 8, binding studies have been performed with several substrates and analogues. The observed binding modes differ in detail, but agree in general with data on homologous enzymes. Based on these binding studies, two mutations were designed that changed the production spectrum from the predominant product beta-cyclodextrin of the wild-type enzyme towards gamma-cyclodextrin, which is of practical interest because it is rare and can encapsulate larger nonpolar compounds.

Bacillus↗

PubMed1998

Structure of cyclodextrin glycosyltransferase complexed with a derivative of its main product beta-cyclodextrin.

Crystals of the inactive mutant Glu257-->Ala of cyclodextrin glycosyltransferase were soaked with the cyclodextrin (CD) derivative S-(alpha-D-glucopyranosyl)-6-thio-beta-CD. The structural analysis showed its beta-CD moiety with no density indication for the exocyclic glucosyl unit. For steric reasons, however, the position of this unit is restricted to be at only two of the seven glucosyl groups of beta-CD. The analysis indicated that the enzyme can cyclize branched alpha-glucans. The ligated beta-CD moiety revealed how the enzyme binds its predominant cyclic product. The conformation of the ligated beta-CD was intermediate between the more symmetrical conformation in beta-CD dodecahydrate crystals and the conformation of a bound linear alpha-glucan chain. Its scissile bond was displaced by 2.8 A from the position in linear alpha-glucans. Accordingly, the complex represents the situation after the cyclization reaction but before diffusion into the solvent, where a more symmetrical conformation is assumed, or the equivalent state in the reverse reaction. Furthermore, a unifying nomenclature for oligosaccharide-binding subsites in proteins is proposed.

Bacillus↗

PubMed1998

The mechanism of the elongation and branching reaction of poly(ADP-ribose) polymerase as derived from crystal structures and mutagenesis.

The binding site for the acceptor substrate poly(ADP-ribose) in the elongation reaction of the ADP-ribosyl transferase poly(ADP-ribose) polymerase (PARP) was detected by cocrystallizing the enzyme with an NAD+ analogue. The site was confirmed by mutagenesis studies. In conjunction with the binding site of the donor NAD+, the bound acceptor reveals the geometry of the elongation reaction. It shows in particular that the strictly conserved glutamate residue of all ADP-ribosylating enzymes (Glu988 of PARP) facilitates the reaction by polarizing both, donor and acceptor. Moreover, the binding properties of the acceptor site suggest a mechanism for the branching reaction, that also explains the dual specificity of this transferase for elongation and branching, which is unique among polymer-forming enzymes.

Adenosine Diphosphate Ribose↗

PubMed1998

Inhibitor and NAD+ binding to poly(ADP-ribose) polymerase as derived from crystal structures and homology modeling.

Inhibitors of poly(ADP-ribose) polymerase (PARP, EC 2.4.2.30) are of clinical interest because they have potential for improving radiation therapy and chemotherapy of cancer. The refined binding structures of four such inhibitors are reported together with the refined structure of the unligated catalytic fragment of the enzyme. Following their design, all inhibitors bind at the position of the nicotinamide moiety of the substrate NAD+. The observed binding mode suggests inhibitor improvements that avoid other NAD(+)-binding enzymes. Because the binding pocket of NAD+ has been strongly conserved during evolution, the homology with ADP-ribosylating bacterial toxins could be used to extend the bound nicotinamide, which is marked by the inhibitors, to the full NAD+ molecule.

Amino Acid Sequence↗

PubMed1998

Isoprenoid biosynthesis: manifold chemistry catalyzed by similar enzymes.

Isoprenoids comprise a family of more than 23,000 natural products, among them the precursors of cholesterol and taxol. The structures of three isoprenoid-cyclizing enzymes have recently been determined and here are placed in the greater context of isoprenoid biosynthesis. On the basis of reaction mechanisms, a subdivision into class I and class II enzymes is proposed. The chain folds of these enzymes also appear to fall into the same two distinct classes, suggesting that class I and class II enzymes have evolved separately.

Alkyl and Aryl Transferases↗

PubMed1998

Porin mutants with new channel properties.

The general diffusion porin from Rhodopseudomonas blastica was produced in large amounts in Escherichia coli inclusion bodies and (re)natured to the exact native structure. Here, we report on 13 mutants at the pore eyelet giving rise to new diffusion properties as measured in planar lipid bilayer experiments. The crystal structures of seven of these mutants were established. The effects of charge-modifying mutations at the pore eyelet are consistent with the known selectivity for cations. Deletions of 16 and 27 residues of the constriction loop L3 resulted in labile trimers and pores. The reduction of the eyelet cross section by introducing tryptophans gave rise to a closely correlated decrease of the conductivities. A mutant with six newly introduced tryptophans in the eyelet closed its pore in a defined manner within seconds under a voltage of 20 mV, suggesting the existence of two states. The results indicate that the pore can be engineered in a rational manner.

Amino Acid Substitution↗

PubMed1998

Structure of the outer membrane protein A transmembrane domain.

The outer membrane protein A of Escherichia coli (OmpA) is an intensely studied example in the field of membrane protein folding. We have determined the structure of the OmpA transmembrane domain consisting of residues 1-171, by X-ray diffraction analysis, to a resolution of 2.5 A. It consists of a regular, extended eight-stranded beta-barrel and appears to be constructed like an inverse micelle with large water-filled cavities, but does not form a pore. Surprisingly, the cavities seem to be highly conserved during evolution. The structure corroborates the concept that all outer membrane proteins consist of beta-barrels. The structure constitutes a beta-barrel membrane anchor that appears to be the outer membrane equivalent of the single-chain alpha-helix anchor of the inner membrane.

Amino Acid Sequence↗

PubMed1997

Structure, catalysis and supramolecular assembly of adenylate kinase from maize.

The crystal structure of adenylate kinase from maize ligated with an inhibitor has been determined by molecular replacement and refined to 3.5-A resolution. The enzyme keeps the ATP/ADP/AMP equilibrium in the cell. In the C4 plant maize, it has the special task to recycle the AMP produced in large amounts in primary CO2 assimilation. The established structure explains the side reaction with CMP. Moreover, it shows infinite rods that can be readily discerned in the crystal packing. In comparison with homologues, two structural differences that are crucial for this supramolecular assembly are evident. We propose that the rods represent a natural inactive storage form that assembles at night when maize stops CO2 assimilation and thus most of the AMP production in its C4 cycle. The enzyme is particularly abundant in mesophyll chloroplasts, where such an assembly would release appreciable amounts of water that can be used in other processes during the night.

Adenylate Kinase↗

PubMed1997

Structure and mechanism of L-fucose isomerase from Escherichia coli.

The three-dimensional structure of L-fucose isomerase from Escherichia coli has been determined by X-ray crystallography at 2.5 A resolution. This ketol isomerase converts the aldose L-fucose into the corresponding ketose L-fuculose using Mn2+ as a cofactor. Being a hexamer with 64,976 Da per subunit, L-fucose isomerase is the largest structurally known ketol isomerase. The enzyme shows neither sequence nor structural similarity with other ketol isomerases. The hexamer obeys D3 symmetry and forms the crystallographic asymmetric unit. The strict and favorably oriented local symmetry allowed for a computational phase extension from 7.3 A to 2.5 A resolution. The structure was solved with an L-fucitol molecule bound to the catalytic center such that the hydroxyl groups at positions 1 and 2 are ligands of the manganese ion. Most likely, L-fucitol mimics a bound L-fucose molecule in its open chain form. The protein environment suggests strongly that the reaction belongs to the ene-diol type.

Aldose-Ketose Isomerases↗

PubMed1997

Structure and function of a squalene cyclase.

The crystal structure of squalene-hopene cyclase from Alicyclobacillus acidocaldarius was determined at 2.9 angstrom resolution. The mechanism and sequence of this cyclase are closely related to those of 2,3-oxidosqualene cyclases that catalyze the cyclization step in cholesterol biosynthesis. The structure reveals a membrane protein with membrane-binding characteristics similar to those of prostaglandin-H2 synthase, the only other reported protein of this type. The active site of the enzyme is located in a large central cavity that is of suitable size to bind squalene in its required conformation and that is lined by aromatic residues. The structure supports a mechanism in which the acid starting the reaction by protonating a carbon-carbon double bond is an aspartate that is coupled to a histidine. Numerous surface alpha helices are connected by characteristic QW-motifs (Q is glutamine and W is tryptophan) that tighten the protein structure, possibly for absorbing the reaction energy without structural damage.

Amino Acid Sequence↗

PubMed1997

Crystal structures and mechanism of 6-phospho-beta-galactosidase from Lactococcus lactis.

The initial structural model of 6-phospho-beta-galactosidase from Lactococcus lactis was refined to an R-factor of 16.4% (R[free] = 23.6%) to 2.3 A resolution (1 A = 0.1 nm), and the structures of three other crystal forms were solved by molecular replacement. The four structural models are essentially identical. The catalytic center of the enzyme is approximately at the mass center of the molecule and can only be reached through a 20 A long channel, which is observed with an "open" or "closed" entrance. The closed entrance is probably too small for the educt lactose-6-phosphate to enter, but large enough for the first product glucose to leave. Among the presented structures is a complex between an almost inactive mutant and the second product galactose-6-phosphate, which is exclusively bound at side-chains. A superposition (onto the native enzyme) of galactose-6-phosphate as bound to the mutant suggests the geometry of a postulated covalent intermediate. The binding mode of the educt was modeled, starting from the bound galactose-6-phosphate. A tightly fixed tryptophan is used as a chopping-board for splitting the disaccharide, and several other aromatic residues in the active center cavity are likely to participate in substrate transport/binding.

Bacterial Proteins↗

PubMed1997

Infinite non-crystallographic symmetries in crystals of a globular protein.

The monomeric enzyme 6-phospho-beta-galactosidase crystallized in three forms, all of which can be considered as fiber associations when judged from the packing contacts. In one case the internal fiber symmetry is a row of parallel twofold axes that are displaced by a' along the fiber axis. In the crystal, these twofold axes are non-crystallographic, but they define the x axis by a = 2a'. Antiparallel and asymmetric fiber association along y and z, respectively, results in space group P2(1). The two other crystal forms contain fibers with internal 2(1) axes (translation c') defining the z axis by c = 2c'. Antiparallel fiber association results in sheets with internal twofold and 2(1) axes defining the yz plane. These sheets associate along x in two ways; in one case the internal sheet symmetry becomes crystallographic, while it remains non-crystallographic in the other. All non-crystallographic fiber symmetries run infinitely through the crystal. In one case they cause exact systematic absences that mimic space group P2(1)2(1)2(1) instead of the actual P2(1)2(1)2. The relationship between contact areas and local symmetries is discussed.

Journal Article↗