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I Schlichting

Publications and source records attributed to I Schlichting.

At least 37 records · Page 2Linked to original sources
PubMed1998

Structure of peptide deformylase and identification of the substrate binding site.

Peptide deformylase is an essential metalloenzyme required for the removal of the formyl group at the N terminus of nascent polypeptide chains in eubacteria. The Escherichia coli enzyme uses Fe2+ and nearly retains its activity on substitution of the metal ion by Ni2+. We have solved the structure of the Ni2+ enzyme at 1.9-A resolution by x-ray crystallography. Each of the three monomers in the asymmetric unit contains one Ni2+ ion and, in close proximity, one molecule of polyethylene glycol. Polyethylene glycol is shown to be a competitive inhibitor with a KI value of 6 mM with respect to formylmethionine under conditions similar to those used for crystallization. We have also solved the structure of the inhibitor-free enzyme at 2.5-A resolution. The two structures are identical within the estimated errors of the models. The hydrogen bond network stabilizing the active site involves nearly all conserved amino acid residues and well defined water molecules, one of which ligates to the tetrahedrally coordinated Ni2+ ion.

Amidohydrolases↗
PubMed1998

Loop closure and intersubunit communication in tryptophan synthase.

Crystal structures of wild-type tryptophan synthase alpha2beta2 complexes from Salmonella typhimurium were determined to investigate the mechanism of allosteric activation of the alpha-reaction by the aminoacrylate intermediate formed at the beta-active site. Using a flow cell, the aminoacrylate (A-A) intermediate of the beta-reaction () was generated in the crystal under steady state conditions in the presence of serine and the alpha-site inhibitor 5-fluoroindole propanol phosphate (F-IPP). A model for the conformation of the Schiff base between the aminoacrylate and the beta-subunit cofactor pyridoxal phosphate (PLP) is presented. The structure is compared with structures of the enzyme determined in the absence (TRPS) and presence (TRPSF-IPP) of F-IPP. A detailed model for binding of F-IPP to the alpha-subunit is presented. In contrast to findings by Hyde et al. [(1988) J. Biol. Chem. 263,17857-17871] and Rhee et al. [(1997) Biochemistry 36, 7664-7680], we find that the presence of an alpha-site alone ligand is sufficient for loop alphaL6 closure atop the alpha-active site. Part of this loop, alphaThr183, is important not only for positioning the catalytic alphaAsp60 but also for coordinating the concomitant ordering of loop alphaL2 upon F-IPP binding. On the basis of the three structures, a pathway for communication between the alpha- and beta-active sites has been established. The central element of this pathway is a newly defined rigid, but movable, domain that on one side interacts with the alpha-subunit via loop alphaL2 and on the other side with the beta-active site. These findings provide a structural basis for understanding the allosteric properties of tryptophan synthase.

Alanine↗
PubMed1998

Crystal structure of yeast thymidylate kinase complexed with the bisubstrate inhibitor P1-(5'-adenosyl) P5-(5'-thymidyl) pentaphosphate (TP5A) at 2.0 A resolution: implications for catalysis and AZT activation.

The crystal structure of yeast thymidylate kinase (TmpK) complexed with the bisubstrate inhibitor P1-(5'-adenosyl) P5-(5'-thymidyl) pentaphosphate (TP5A) was determined at 2.0 A resolution. In this complex, TmpK adopts a closed conformation with a region (LID) of the protein closing upon the substrate and forming a helix. The interactions of TmpK and TP5A strongly suggest that arginine 15, which is located in the phosphate binding loop (P-loop) sequence, plays a catalytic role by interacting with an oxygen atom of the transferred phosphoryl group. Unlike other nucleoside monophosphate kinases where basic residues from the LID region participate in stabilizing the transition state, TmpK lacks such residues in the LID region. We attribute this function to Arg 15 of the P-loop. TmpK plays an important role in the phosphorylation of the AIDS prodrug AZT. The structures of TmpK with dTMP and with AZT-MP [Lavie, A., et al. (1997) Nat. Struct. Biol. 4, 601-604] implicate the movement of Arg15 in response to AZT-MP binding as an important factor in the 200-fold reduced catalytic rate with AZT-MP. TmpK from Escherichia coli lacks this arginine in its P-loop while having basic residues in the LID region. This suggested that, if such a P-loop movement were to occur in the E. coli TmpK upon AZT-MP binding, it should not have such a detrimental effect on catalysis. This hypothesis was tested, and as postulated, E. coli TmpK phosphorylates AZT-MP only 2.5 times slower than dTMP.

Biotransformation↗
PubMed1998

Iron center, substrate recognition and mechanism of peptide deformylase.

Eubacterial proteins are synthesized with a formyl group at the N-terminus which is hydrolytically removed from the nascent chain by the mononuclear iron enzyme peptide deformylase. Catalytic efficiency strongly depends on the identity of the bound metal. We have determined by X-ray crystallography the Fe2+, Ni2+ and Zn2+ forms of the Escherichia coli enzyme and a structure in complex with the reaction product Met-Ala-Ser. The structure of the complex, with the tripeptide bound at the active site, suggests detailed models for the mechanism of substrate recognition and catalysis. Differences of the protein structures due to the identity of the bound metal are extremely small and account only for the observation that Zn2+ binds more tightly than Fe2+ or Ni2+. The striking loss of catalytic activity of the Zn2+ form could be caused by its reluctance to change between tetrahedral and five-fold metal coordination believed to occur during catalysis. N-terminal formylation and subsequent deformylation

Amidohydrolases↗
PubMed1997

Crystal structure of cytochrome P-450cam complexed with the (1S)-camphor enantiomer.

The crystal structure of cytochrome P-450cam complexed with the enantiomer (1S)-camphor has been solved to 1.8 angstroms resolution and compared with the structure of the (1R)-camphor P-450cam complex. The overall protein structure is the same for both enantiomer complexes. However, the orientation of the substrates in the heme pocket differs. In contrast to (1R)-camphor, the (1S)-enantiomer binds in at least two orientations. The major binding mode of (1S)-camphor resembles the one of the (1R)-enantiomer in that there is a hydrogen bond between Tyr-96 and the quinone group of camphor, and the 10-methyl group points towards the I-helix. The binding differs in that C-5 is not at a position suitable for hydroxylation. In the other orientation (1S)-camphor is not hydrogen bonded, but C-5 is located suitably for hydroxylation.

Camphor↗
PubMed1997

Structures of active conformations of UMP kinase from Dictyostelium discoideum suggest phosphoryl transfer is associative.

UMP/CMP kinase from Dictyostelium discoideum (UmpKdicty) catalyzes the specific transfer of the terminal phosphate of ATP to UMP or CMP. Crystal structures of UmpKdicty with substrates and the transition state analogs AlF3 or BeF2 that lock UmpKdicty in active conformations were solved. The positions of the catalytic Mg2+ and the highly conserved lysine of the P loop are virtually invariant in the different structures. In contrast, catalytic arginines move to stabilize charges that develop during this reaction. The location of the arginines indicates formation of negative charges during the reaction at the transferred phosphoryl group, but not at the phosphate bridging oxygen atoms. This is consistent with an associative phosphoryl transfer mechanism but not with a dissociative one.

Animals↗
PubMed1997

Out of the blue: the photocycle of the photoactive yellow protein.

A first real glance at the structural, spectral and temporal interplay that constitutes the photocycle of the photoactive yellow protein (PYP) has been obtained from a combination of time-resolved crystallography with mutational analysis and spectroscopic studies.

Bacterial Proteins↗
PubMed1997

The bottleneck in AZT activation.

Nucleoside-based inhibitors of reverse transcriptase were the first drugs to be used in the chemotherapy of AIDS. After entering the cell, these substances are activated to their triphosphate form by cellular kinases, after which they are potent chain terminators for the growing viral DNA. The two main factors limiting their efficacy are probably interrelated. These are the insufficient degree of reduction of viral load at the commencement of treatment and the emergence of resistant variants of the virus. The reason for the relatively poor suppression of viral replication appears to be inefficient metabolic activation. Thus, for the most extensively used drug, 3'-azido-3'-deoxythymidine (AZT), whereas phosphorylation to the monophosphate is facile, the product is a very poor substrate for the next kinase in the cascade, thymidylate kinase. Because of this, although high concentrations of the monophosphate can be reached in the cell, the achievable concentration of the active triphosphate is several orders of magnitude lower. Determination of the structure of thymidylate kinase as a complex with AZT monophosphate (AZTMP) together with studies on the kinetics of its phosphorylation have now led to a detailed understanding of the reasons for and consequences of the poor substrate properties.

Anti-HIV Agents↗
PubMed1996

Crystal structure of the Trypanosoma cruzi trypanothione reductase.mepacrine complex.

The three-dimensional structure of the complex between Trypanosoma cruzi trypanothione reductase (TR) (EC 1.6.4.8) and the antiparasitic drug mepacrine (quinacrine) has been solved at 2.9 angstoms resolution. Mepacrine is a competitive inhibitor of TR but does not affect human glutathione reductase (GR), a closely related host enzyme. Of particular importance for inhibitor binding are four amino acid residues in the disulfide substrate-binding site of TR that are not conserved in human GR, namely, Glu-18 (Ala-34 in GR), Trp-21 (Arg-37), Ser-109 (Ile-113), and Met-113 (Asn-117). The acridine ring of mepacrine is fixed at the active site close to the hydrophobic wall formed by Trp-21 and Met-113. Specific pairwise interactions between functional groups of the drug and amino acid side chains include the ring nitrogen and Met-113, the chlorine atom and Trp-21, and the oxymethyl group and Ser-109. The alkylamino chain of mepacrine points into the inner region of the active site and is held in position by a solvent-mediated hydrogen bond to Glu-18. The structure of the complex shows for the first time the atomic interactions between TR and an inhibitory ligand. This is a crucial step towards the rational design of inhibitors that might be suited as drugs against Chagas' disease.

Acridines↗
PubMed1995

Substrate and product structural requirements for binding of nucleotides to H-ras p21: the mechanism of discrimination between guanosine and adenosine nucleotides.

The interaction of the protein product of the H-ras oncogene with a series of nucleoside di- and triphosphates has been examined to investigate the tolerance of the active site to departures from the GTP or GDP structures. Nucleotides which bind relatively strongly could be used as competitors of GDP in a simple filter binding assay to give semiquantitave estimates of their affinities. For more weakly binding nucleotides or to obtain quantitative data, a transient kinetic method was used which was based on determination of the association and dissociation rate constants. The results obtained indicate that substantial modification of the sugar or phosphate structure is tolerated with little or moderate loss of affinity, but that large losses in affinity occur on modification of the base structure. In particular, replacing the guanine by an adenine residue leads to a dramatic loss of affinity. Thus, discrimination against ATP and ADP is very high (relative affinities of ATP and GTP 1:10(7)). This is due not only to loss of positive (stabilizing) interactions, but especially to the introduction of negative ones.

Acid Anhydride Hydrolases↗
PubMed1994

Structural and kinetic analysis of a channel-impaired mutant of tryptophan synthase.

Substrate channeling is a process by which two sequential enzymes interact to transfer a metabolite (or intermediate) from one enzyme active site to the next without allowing free diffusion of the metabolite. Channeling is thought to play an important role in metabolic regulation and cellular modulation of enzymatic activities. Although there are numerous examples of sequential enzyme pairs in glycolysis and other biosynthetic pathways that are thought to exhibit channeling, this is as yet controversial. Tryptophan synthase is considered the best example for an enzyme displaying channeling behavior between subunits. Tryptophan synthase is an alpha 2 beta 2 tetrameric enzyme complex, which catalyzes the last two steps in the biosynthesis of L-tryptophan. The alpha subunit catalyzes the cleavage of indole-3-glycerol phosphate to indole and glyceraldehyde-3-phosphate; the beta subunit catalyzes the condensation of indole with serine to form tryptophan, in a reaction mediated by pyridoxal phosphate. The inability to trap free indole in the steady-state reaction and analysis of the kinetics of single turnover reactions have led to the postulate that indole may pass directly from the alpha to the beta site without diffusion through solution (Demoss, J. A. (1962) Biochim. Biophys. Acta 62, 279-293; Matchett, W. M. (1974) J. Biol. Chem. 249, 4041-4049). The crystal structure of tryptophan synthase from Salmonella typhimurium has provided additional support for substrate channeling by elucidating a 25-A hydrophobic tunnel connecting the two catalytic sites (Hyde, C. C., Ahmed, S. A., Padlan, E. A., Miles, E. W., and Davies, D. R. (1988) J. Biol. Chem. 263, 17857-17871). The structure suggests that mutation of a residue lining the tunnel to a more bulky residue might impede or block the passage of indole during catalysis thus enabling detection of indole during a single enzyme turnover. A mutant of tryptophan synthase has been prepared in which one of the residues lining the tunnel, beta Cys-170, has been replaced with a bulkier tryptophan residue. Kinetic and structural analyses of the beta C170W mutant by rapid chemical quench methods and x-ray crystallographic analysis show both the transient formation of indole and the obstruction of the tunnel, thus providing direct evidence for the substrate channeling mechanism.

Catalysis↗
PubMed1994

Crystal structure of photolysed carbonmonoxy-myoglobin.

Myoglobin is a globular haem protein that reversibly binds ligands such as O2 and CO. Single photons of visible light can break the covalent bond between CO and the haem iron in carbon-monoxy-myoglobin (MbCO) and thus form an unstable intermediate, Mb*CO, with the CO inside the protein. The ensuing rebinding process has been extensively studied as a model for the interplay of dynamics, structure and function in protein reactions. We have used X-ray crystallography at liquid-helium temperatures to determine the structure of Mb*CO to a resolution of 1.5 A. The photodissociated CO lies on top of the haem pyrrole ring C. Comparison with the CO-bound and unligated myoglobin structures reveals that on photodissociation of the CO, the haem 'domes', the iron moves partially out of the haem plane, the iron-proximal histidine bonds is compressed, the F helix is strained and the distal histidine swings towards the outside of the ligand-binding pocket.

Animals↗
PubMed1994

Crystallization and preliminary X-ray structure analysis of thermally unstable p21(H-ras) guanosine complexes.

p21 is a small guanine nucleotide binding protein that is involved in intracellular signal transduction. Biochemical data suggest that the presence of the beta-phosphate is essential for strong binding of guanine nucleotides to the protein. Guanosine or GMP bind six orders of magnitude more weakly to p21 than GDP or GTP. Moreover, the thermal stability of the protein is dramatically reduced when bound to GMP or guanosine. We have crystallized C-terminally truncated forms of p21(H-ras), with guanosine or GMP bound, in the space groups P4(3)2(1)2, P2(1)2(1)2 and P2(1). The crystals diffract in the range 2.8-2.2 A. Details of the crystallization procedures, the characterization of the crystals and preliminary results of structure determination are described. An unexpected electron-density peak was found close to the position of the beta-phosphate in the phosphate-binding loop.

Journal Article↗
PubMed1994

Structure of the regulatory domain of scallop myosin at 2.8 A resolution.

The regulatory domain of scallop myosin is a three-chain protein complex that switches on this motor in response to Ca2+ binding. This domain has been crystallized and the structure solved to 2.8 A resolution. Side-chain interactions link the two light chains in tandem to adjacent segments of the heavy chain bearing the IQ-sequence motif. The Ca(2+)-binding site is a novel EF-hand motif on the essential light chain and is stabilized by linkages involving the heavy chain and both light chains, accounting for the requirement of all three chains for Ca2+ binding and regulation in the intact myosin molecule.

Amino Acid Sequence↗
PubMed1994

The structure of Trypanosoma cruzi trypanothione reductase in the oxidized and NADPH reduced state.

The three-dimensional structure of trypanothione reductase (TR) (EC 1.6.4.8) from Trypanosoma cruzi has been solved at 0.33 nm resolution by molecular replacement using the structure of C. fasciculata TR as a starting model. Elucidation of the T. cruzi TR structure represents the first step in the rational design of a drug against Chagas' disease. The structure of T. cruzi TR is compared with those of C. fasciculata TR as well as human and E. coli glutathione reductase (GR). In the FAD-binding domain, TR has two insertions, each about 10 residues long, which do not occur in GR. The first one is a rigid loop stabilizing the position of helix 91-117 which is responsible for the wider active site of TR as compared to GR. The second insertion does not occur where it is predicted by sequence alignment; rather the residues extend three strands of the 4-stranded beta-sheet by one or two residues each. This increases the number of hydrogen bonds within the sheet structure. The structure of the NADPH.TR complex has been solved at 0.33 nm resolution. The nicotinamide ring is sandwiched between the flavin ring and the side chain of Phe-198 which undergoes the same conformational change upon coenzyme binding as Tyr-197 in GR. In addition to Arg-222 and Arg-228, which are conserved in TR and GR, Tyr-221--the last residue of the second beta-sheet strand of the beta alpha beta dinucleotide binding fold--is in hydrogen bonding distance to the 2' phosphate group of NADPH.

Amino Acid Sequence↗
PubMed1993

Kinetic and structural analysis of the Mg(2+)-binding site of the guanine nucleotide-binding protein p21H-ras.

The coordination and binding of the Mg2+ ion in the nucleotide-binding site of p21 have been investigated using site-directed mutagenesis, kinetic methods, and phosphorous NMR. Mg2+ in the p21.nucleotide.Mg2+ complex appears to be in fast equilibrium with the solvent. The dissociation constant between Mg2+ and the p21.GDP complex was determined to be 2.8 microM. It decreases 30- or 16-fold on substituting Ser-17 or Asp-57 with alanine, respectively, whereas the T35A mutation has no effect. All three mutations influence the dissociation constants and the association and dissociation rate constants of the interaction between guanine nucleotides and p21, but to a different degree. We conclude that Thr-35 is only complexed to Mg2+ in the GTP conformation and both Asp-57 and Ser-17 appear to be critical for both GDP and GTP binding. 31P NMR spectra of the GDP and Gpp(NH)p (guanosine-5'-(beta,gamma-imido)triphosphate) complexes of mutated p21 show a remarkable perturbation of the guanine nucleotide-binding site compared to wild-type protein. The mutant proteins show reduced GTPase rates, which are not stimulated by the GTPase-activating protein GAP. p21(S17A) has been reported to function just as p21(S17N) as a dominant negative inhibitor of normal p21. We find that it inhibits oncogenic p21-induced survival of primary neurons.

Amino Acid Sequence↗