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Malcolm D Walkinshaw

Publications and source records attributed to Malcolm D Walkinshaw.

At least 19 recordsLinked to original sources
PubMed2007

Configurations of nickel-cyclam antiviral complexes and protein recognition.

Nickel(II)-xylylbicyclam is a potent anti-HIV agent and binds strongly to the CXCR4 co-receptor. We have investigated configurational equilibria of Ni(II)-cyclam derivatives, since these are important for receptor recognition. Crystallographic studies show that both trans and cis configurations are readily formed: [Ni(cyclam)(OAc)(2)] x H(2)O adopts the trans-III configuration with axial monodentate acetates, as does [Ni(benzylcyclam)(NO(3))(2)] with axial nitrate ligands, whereas [Ni(benzylcyclam)(OAc)](OAc)2 x H(2)O has an unusual folded cis-V configuration with Ni(II) coordination to bidentate acetate. UV/Vis and NMR studies show that the octahedral trans-III configuration slowly converts to square-planar trans-I in aqueous solution. For Ni(II)-xylylbicyclam, a mixture of cis-V and trans-I configurations was detected in solution. X-ray diffraction studies showed that crystals of lysozyme soaked in Ni(II)-cyclam or Ni(II) (2)-xylylbicyclam contain two major binding sites, one involving Ni(II) coordination to Asp101 and hydrophobic interactions between the cyclam ring and Trp62 and Trp63, and the second hydrophobic interactions with Trp123. For Ni(II)-cyclam bound to Asp101, the cis-V configuration predominates.

Antiviral Agents↗
PubMed2006

The first crystal structure of phosphofructokinase from a eukaryote: Trypanosoma brucei.

The crystal structure of the ATP-dependent phosphofructokinase (PFK) from Trypanosoma brucei provides the first detailed description of a eukaryotic PFK, and enables comparisons to be made with the crystal structures of bacterial ATP-dependent and PPi-dependent PFKs. The structure reveals that two insertions (the 17-20 and 329-348 loops) that are characteristic of trypanosomatid PFKs, but absent from bacterial and mammalian ATP-dependent PFKs, are located within and adjacent to the active site, and are in positions to play important roles in the enzyme's mechanism. The 90 residue N-terminal extension forms a novel domain that includes an "embracing arm" across the subunit boundary to the symmetry-related subunit in the tetrameric enzyme. Comparisons with the PPi-dependent PFK from Borrelia burgdorferi show that several features thought to be characteristic of PPi-dependent PFKs are present in the trypanosome ATP-dependent PFK. These two enzymes are generally more similar to each other than to the bacterial or mammalian ATP-dependent PFKs. However, there are critical differences at the active site of PPi-dependent PFKs that are sufficient to prevent the binding of ATP. This crystal structure of a eukaryotic PFK has enabled us to propose a detailed model of human muscle PFK that shows active site and other differences that offer opportunities for structure-based drug discovery for the treatment of sleeping sickness and other diseases caused by the trypanosomatid family of protozoan parasites.

Adenosine Triphosphate↗
PubMed2006

Crystallization and X-ray data analysis of the 10 kDa C-terminal lid subdomain from Caenorhabditis elegans Hsp70.

Hsp70 is an important molecular chaperone involved in the regulation of protein folding. Crystals of the C-terminal 10 kDa helical lid domain (residues 542-640) from a Caenorhabditis elegans Hsp70 homologue have been produced that diffract X-rays to approximately 3.4 A. Crystals belong to space group I2(1)2(1)2(1), with unit-cell parameters a = b = 197, c = 200 A. The Matthews coefficient, self-rotation function and Patterson map indicate 24 monomers in the asymmetric unit, showing non-crystallographic 432 symmetry. Molecular-replacement studies using the corresponding domain from rat, the only eukaryotic homologue with a known structure, failed and a mercury derivative was obtained. Preliminary MAD phasing using SHELXD and SHARP for location and refinement of the heavy-atom substructure and SOLOMON for density modification produced interpretable maps with a clear protein-solvent boundary. Further density-modification, model-building and refinement are currently under way.

Animals↗
PubMed2006

A kinetically trapped intermediate of FK506 binding protein forms in vitro: chaperone machinery dominates protein folding in vivo.

We have characterised the stability, binding and enzymatic properties of three human FK506 binding proteins (FKBP-12) differing only by the length and sequence of their N-terminus. One construct has a short hexa-his tag (6H-FKBP12); the second longer fusion protein (6HL-FKBP12) contains an additional thrombin protease cleavage site; the third has the long fusion tag removed and is essentially native FKBP-12 (cFKBP12). The proteins were purified both under native conditions and also using a refolding protocol. All three natively purified proteins have, within experimental error, the same peptidyl-prolyl isomerase (PPIase) activity (k(cat)/K(m) approximately 1 x 10(6)M(-1)s(-1)), and bind a natural inhibitor, rapamycin, with the same high affinity (K(d) approximately 6 nM). However, refolding of the protein containing the longer tag in vitro results in reduced PPIase activity (the k(cat)/K(m) was reduced from 1 x 10(6)M(-1)s(-1) to 0.81 x 10(6)M(-1)s(-1)) and a 6-fold affinity loss for rapamycin. Addition of both the long and short N-terminal his-tags slows the refolding kinetics of FKBP-12. However, the shorter his-tagged fusion protein regains fully native activity (> or =95%) while the longer regains only approximately 80-85% of native activity. Equilibrium urea denaturation titrations, isothermal titration calorimetry (ITC), analytical gel-filtration, and fluorescence binding data show that this loss of activity is not due to gross misfolding events, but is rather caused by the formation of a stable but subtly misfolded protein that has reduced peptidyl-prolyl isomerase (PPIase) activity and reduced affinity for rapamycin. The difference in behaviour between the in vitro refolded and native forms is due to the dominant role of the cellular chaperone/folding machinery.

Histidine↗
PubMed2006

A proton delivery pathway in the soluble fumarate reductase from Shewanella frigidimarina.

The mechanism for fumarate reduction by the soluble fumarate reductase from Shewanella frigidimarina involves hydride transfer from FAD and proton transfer from the active-site acid, Arg-402. It has been proposed that Arg-402 forms part of a proton transfer pathway that also involves Glu-378 and Arg-381 but, unusually, does not involve any bound water molecules. To gain further insight into the importance of this proton pathway we have perturbed it by substituting Arg-381 by lysine and methionine and Glu-378 by aspartate. Although all the mutant enzymes retain measurable activities, there are orders-of-magnitude decreases in their k(cat) values compared with the wild-type enzyme. Solvent kinetic isotope effects show that proton transfer is rate-limiting in the wild-type and mutant enzymes. Proton inventories indicate that the proton pathway involves multiple exchangeable groups. Fast scan protein-film voltammetric studies on wild-type and R381K enzymes show that the proton transfer pathway delivers one proton per catalytic cycle and is not required for transporting the other proton, which transfers as a hydride from the reduced, protonated FAD. The crystal structures of E378D and R381M mutant enzymes have been determined to 1.7 and 2.1 A resolution, respectively. They allow an examination of the structural changes that disturb proton transport. Taken together, the results indicate that Arg-381, Glu-378, and Arg-402 form a proton pathway that is completely conserved throughout the fumarate reductase/succinate dehydrogenase family of enzymes.

Amino Acids↗
PubMed2006

The chaperone function of cyclophilin 40 maps to a cleft between the prolyl isomerase and tetratricopeptide repeat domains.

Cyclophilin 40 (CyP40), an immunophilin cochaperone present in steroid receptor-Hsp90 complexes, contains an N-terminal peptidylprolyl isomerase (PPIase) domain separated from a C-terminal Hsp90-binding tetratricopeptide repeat (TPR) domain by a 30-residue linker. To map CyP40 chaperone function, CyP40 deletion mutants were prepared and analysed for chaperone activity. CyP40 fragments containing the PPIase domain plus linker or the linker region and the adjoining TPR domain retained chaperone activity, whilst individually, the catalytic and TPR domains were devoid of chaperoning ability. CyP40 chaperone function then, is localized within the linker that forms a binding cleft with potential to accommodate non-native substrates.

Amino Acid Sequence↗
PubMed2006

Mechanism of Mos1 transposition: insights from structural analysis.

We present the crystal structure of the catalytic domain of Mos1 transposase, a member of the Tc1/mariner family of transposases. The structure comprises an RNase H-like core, bringing together an aspartic acid triad to form the active site, capped by N- and C-terminal alpha-helices. We have solved structures with either one Mg2+ or two Mn2+ ions in the active site, consistent with a two-metal mechanism for catalysis. The lack of hairpin-stabilizing structural motifs is consistent with the absence of a hairpin intermediate in Mos1 excision. We have built a model for the DNA-binding domain of Mos1 transposase, based on the structure of the bipartite DNA-binding domain of Tc3 transposase. Combining this with the crystal structure of the catalytic domain provides a model for the paired-end complex formed between a dimer of Mos1 transposase and inverted repeat DNA. The implications for the mechanisms of first and second strand cleavage are discussed.

Amino Acid Motifs↗
PubMed2006

Fumarate reductase: structural and mechanistic insights from the catalytic reduction of 2-methylfumarate.

The soluble fumarate reductase (FR) from Shewanella frigidimarina can catalyse the reduction of 2-methylfumarate with a k(cat) of 9.0 s(-1) and a K(M) of 32 microM. This produces the chiral molecule 2-methylsuccinate. Here, we present the structure of FR to a resolution of 1.5 A with 2-methylfumarate bound at the active site. The mode of binding of 2-methylfumarate allows us to predict the stereochemistry of the product as (S)-2-methylsuccinate. To test this prediction we have analysed the product stereochemistry by circular dichroism spectroscopy and confirmed the production of (S)-2-methylsuccinate.

Catalysis↗
PubMed2006

The role of Thr268 and Phe393 in cytochrome P450 BM3.

In flavocytochrome P450 BM3 there are several active site residues that are highly conserved throughout the P450 superfamily. Of these, a phenylalanine (Phe393) has been shown to modulate heme reduction potential through interactions with the implicitly conserved heme-ligand cysteine. In addition, a distal threonine (Thr268) has been implicated in a variety of roles including proton donation, oxygen activation and substrate recognition. Substrate binding in P450 BM3 causes a shift in the spin state from low- to high-spin. This change in spin-state is accompanied by a positive shift in the reduction potential (DeltaE(m) [WT+arachidonate (120 microM)]=+138 mV). Substitution of Thr268 by an alanine or asparagine residue causes a significant decrease in the ability of the enzyme to generate the high-spin complex via substrate binding and consequently leads to a decrease in the substrate-induced potential shift (DeltaE(m) [T268A+arachidonate (120 microM)]=+73 mV, DeltaE(m) [T268N+arachidonate (120 microM)]=+9 mV). Rate constants for the first electron transfer and for oxy-ferrous decay were measured by pre-steady-state stopped-flow kinetics and found to be almost entirely dependant on the heme reduction potential. More positive reduction potentials lead to enhanced rate constants for heme reduction and more stable oxy-ferrous species. In addition, substitutions of the threonine lead to an increase in the production of hydrogen peroxide in preference to hydroxylated product. These results suggest an important role for this active site threonine in substrate recognition and in maintaining an efficiently functioning enzyme. However, the dependence of the rate constants for oxy-ferrous decay on reduction potential raises some questions as to the importance of Thr268 in iron-oxo stabilisation.

Base Sequence↗
PubMed2006

Differential binding of inhibitors to active and inactive CDK2 provides insights for drug design.

The cyclin-dependent kinases (CDKs) have been characterized in complex with a variety of inhibitors, but the majority of structures solved are in the inactive form. We have solved the structures of six inhibitors in both the monomeric CDK2 and binary CDK2/cyclinA complexes and demonstrate that significant differences in ligand binding occur depending on the activation state. The binding mode of two ligands in particular varies substantially in active and inactive CDK2. Furthermore, energetic analysis of CDK2/cyclin/inhibitors demonstrates that a good correlation exists between the in vitro potency and the calculated energies of interaction, but no such relationship exists for CDK2/inhibitor structures. These results confirm that monomeric CDK2 ligand complexes do not fully reflect active conformations, revealing significant implications for inhibitor design while also suggesting that the monomeric CDK2 conformation can be selectively inhibited.

Crystallography, X-Ray↗
PubMed2005

A surface plasmon resonance-based assay for small molecule inhibitors of human cyclophilin A.

A simple protocol for generating a highly stable and active surface plasmon resonance (SPR) sensor surface of recombinant human hexahistidine cyclophilin A (His-CypA) is described. The sensor surface was sensitive and stable enough to allow, for the first time, the screening and ranking of several novel small-molecule (Mr approximately 250-500 Da) ligands in a competition binding assay with cyclosporin A (CsA). It also allowed us to accurately determine the kinetic rate constants for the interaction between His-CypA and CsA. His-CypA was first captured on a Ni2+-nitrilotriacetic acid (NTA) sensor chip and was then briefly covalently stabilized, coupling via primary amines. The significant baseline drift observed due to dissociation of weakly bound His-CypA from the Ni2+-NTA moiety was eliminated, resulting in a surface that was stable for at least 36 h. In addition, immobilized protein activity levels were high, typically between 85 and 95%, assayed by the interaction between His-CypA and CsA. The mean equilibrium dissociation constant for CsA (K(dCsA)) binding to the immobilized His-CypA was 23+/-6 nM, with on and off rates of 0.53+/-0.1 microM(-1) s(-1) and 1.2+/-0.1 (x 10(-2)) s(-1), respectively. These values agree well with the values for the corresponding binding constants determined from steady-state and kinetic fluorescence titrations in solution.

Binding, Competitive↗
PubMed2005

Crystal structure of dihydrofolate reductase from Plasmodium vivax: pyrimethamine displacement linked with mutation-induced resistance.

Pyrimethamine (Pyr) targets dihydrofolate reductase of Plasmodium vivax (PvDHFR) as well as other malarial parasites, but its use as antimalarial is hampered by the widespread high resistance. Comparison of the crystal structures of PvDHFR from wild-type and the Pyr-resistant (SP21, Ser-58 --> Arg + Ser-117 --> Asn) strain as complexes with NADPH and Pyr or its analog lacking p-Cl (Pyr20) clearly shows that the steric conflict arising from the side chain of Asn-117 in the mutant enzyme, accompanied by the loss of binding to Ser-120, is mainly responsible for the reduction in binding of Pyr. Pyr20 still effectively inhibits both the wild-type and SP21 proteins, and the x-ray structures of these complexes show how Pyr20 fits into both active sites without steric strain. These structural insights suggest a general approach for developing new generations of antimalarial DHFR inhibitors that, by only occupying substrate space of the active site, would retain binding affinity with the mutant enzymes.

Amino Acid Substitution↗
PubMed2005

Protein recognition of macrocycles: binding of anti-HIV metallocyclams to lysozyme.

The macrocyclic antiviral drug xylyl-bicyclam blocks entry of HIV into cells by targeting the CXCR4 coreceptor, a seven-helix transmembrane G-protein-coupled receptor. Its affinity for CXCR4 is enhanced by binding to Cu2+, Ni2+, or Zn2+. Metallocyclams have a rich configurational chemistry and proteins may bind selectively to specific metallocyclam configurations. Our studies of lysozyme reveal structural details of protein-metallocyclam interactions that are important for receptor recognition. Solution NMR studies show that Cu-cyclam interacts with specific tryptophan residues of lysozyme (Trp-62, Trp-63, and Trp-123). Two major binding sites for both Cu-cyclam and Cu2-xylyl-bicyclam were detected by x-ray crystallography. In the first site, Cu2+ in one cyclam ring of Cu2-xylyl-bicyclam adopts a trans configuration and is coordinated to a carboxylate oxygen of Asp-101, whereas for Cu-cyclam two ring NH groups form H bonds to the carboxylate oxygens of Asp-101, stabilizing an unusual cis (folded) cyclam configuration. For both complexes in this site, a cyclam ring is sandwiched between the indole side chains of two tryptophan residues (Trp-62 and Trp-63). In the second site, a trans cyclam ring is stacked on Trp-123 and H bonded to the backbone carbonyl of Gly-117. We show that there is a pocket in a model of the human CXCR4 coreceptor in which trans and cis configurations of metallobicyclam can bind by direct metal coordination to carboxylate side chains, cyclam-NH...carboxylate H bonding, together with hydrophobic interactions with tryptophan residues. These studies provide a structural basis for the design of macrocycles that bind stereospecifically to G-coupled and other protein receptors.

Animals↗
PubMed2005

Structural and biochemical studies of human proliferating cell nuclear antigen complexes provide a rationale for cyclin association and inhibitor design.

The interactions between the tumor suppressor protein p21WAF1 and the cyclin-dependent kinase (CDK) complexes and with proliferating cell nuclear antigen (PCNA) regulate and coordinate the processes of cell-cycle progression and DNA replication. We present the x-ray crystal structure of PCNA complexed with a 16-mer peptide related to p21 that binds with a Kd of 100 nM. Two additional crystal structures of native PCNA provide previously undescribed structures of uncomplexed human PCNA and show that significant changes on ligand binding include rigidification of a number of flexible regions on the surface of PCNA. In the competitive binding experiments described here, we show that a 20-mer sequence from p21 can be associated simultaneously with PCNA and CDK/cyclin complexes. A structural model for this quaternary complex is presented in which the C-terminal sequence of p21 acts like double-sided tape and docks to both the PCNA and cyclin molecules. The quaternary complex shows little direct interaction between PCNA and cyclin, giving p21 the role of an adaptor molecule. Taken together, the biochemical and structural results delineate a druggable inhibitor site on the surface of PCNA that may be exploited in the design of peptidomimetics, which will act independently of cyclin-groove inhibitors.

Amino Acid Sequence↗
PubMed2004

Octaheme tetrathionate reductase is a respiratory enzyme with novel heme ligation.

We have isolated a soluble cytochrome from Shewanella oneidensis that contains eight covalently attached heme groups and determined its crystal structure. One of these hemes exhibits novel ligation of the iron atom by the epsilon-amino group of a lysine residue, despite its attachment via a typical CXXCH motif. This heme is most likely the active site for tetrathionate reduction, a reaction catalyzed efficiently by this enzyme.

Binding Sites↗
PubMed2004

Altered substrate specificity in flavocytochrome b2: structural insights into the mechanism of L-lactate dehydrogenation.

Flavocytochrome b(2) from Saccharomyces cerevisiae is a l-lactate/cytochrome c oxidoreductase belonging to a large family of 2-hydroxyacid-dependent flavoenzymes. The crystal structure of the enzyme, with pyruvate bound at the active site, has been determined [Xia, Z.-X., and Mathews, F. S. (1990) J. Mol. Biol. 212, 837-863]. The authors indicate that the methyl group of pyruvate is in close contact with Ala198 and Leu230. These two residues are not well-conserved throughout the family of (S)-2-hydroxy acid oxidases/dehydrogenases. Thus, to probe substrate specificity in flavocytochrome b(2), these residues have been substituted by glycine and alanine, respectively. Kinetic studies on the L230A mutant enzyme and the A198G/L230A double mutant enzyme indicate a change in substrate selectivity for the enzyme toward larger (S)-2-hydroxy acids. In particular, the L230A enzyme is more efficient at utilizing (S)-2-hydroxyoctanoate by a factor of 40 as compared to the wild-type enzyme [Daff, S., Manson, F. D. C., Reid, G. A., and Chapman, S. K. (1994) Biochem. J. 301, 829-834], and the A198G/L230A double mutant enzyme is 6-fold more efficient with the aromatic substrate l-mandelate than it is with l-lactate [Sinclair, R., Reid, G. A., and Chapman, S. K. (1998) Biochem. J. 333, 117-120]. To complement these solution studies, we have solved the structure of the A198G/L230A enzyme in complex with pyruvate and as the FMN-sulfite adduct (both to 2.7 A resolution). We have also obtained the structure of the L230A mutant enzyme in complex with phenylglyoxylate (the product of mandelate oxidation) to 3.0 A resolution. These structures reveal the increased active-site volume available for binding larger substrates, while also confirming that the integrity of the interactions important for catalysis is maintained. In addition to this, the mode of binding of the bulky phenylglyoxylate at the active site is in accordance with the operation of a hydride transfer mechanism for substrate oxidation/flavin reduction in flavocytochrome b(2), whereas a mechanism involving the formation of a carbanion intermediate would appear to be sterically prohibited.

Amino Acid Sequence↗
PubMed2004

Half-sandwich arene ruthenium(II)-enzyme complex.

The 1.6 [Angstrom] X-ray crystal structure of [(eta(6)-p-cymene)Ru(lysozyme)Cl(2)], the first of a half-sandwich complex of a protein, shows selective ruthenation of Nepsilon of the imidazole ring of His15.

Arginine↗