Biomedical subjects

Simon E V Phillips

Publications and source records attributed to Simon E V Phillips.

22 records · Page 2Linked to original sources

PubMed2002

Metal ions bound at the active site of the junction-resolving enzyme T7 endonuclease I.

T7 endonuclease I is a nuclease that is selective for the structure of the four-way DNA junction. The active site is similar to those of a number of restriction enzymes. We have solved the crystal structure of endonuclease I with a wild-type active site. Diffusion of manganese ions into the crystal revealed two peaks of electron density per active site, defining two metal ion-binding sites. Site 1 is fully occupied, and the manganese ion is coordinated by the carboxylate groups of Asp55 and Glu65, and the main chain carbonyl of Thr66. Site 2 is partially occupied, and the metal ion has a single protein ligand, the remaining carboxylate oxygen atom of Asp55. Isothermal titration calorimetry showed the sequential exothermic binding of two manganese ions in solution, with dissociation constants of 0.58 +/- 0.019 and 14 +/- 1.5 mM. These results are consistent with a two metal ion mechanism for the cleavage reaction, in which the hydrolytic water molecule is contained in the first coordination sphere of the site 1-bound metal ion.

Amino Acid Substitution↗

PubMed2002

Structural and functional studies of an intermediate on the pathway to operator binding by Escherichia coli MetJ.

We present the results of in vitro DNA-binding assays for a mutant protein (Q44K) of the E. coli methionine repressor, MetJ, as well as the crystal structure at 2.2 A resolution of the apo-mutant bound to a 10-mer oligonucleotide encompassing an 8 bp met-box sequence. The wild-type protein binds natural operators co-operatively with respect to protein concentration forming at least a dimer of repressor dimers along operator DNAs. The minimum operator length is thus 16 bp, each MetJ dimer interacting with a single met-box site. In contrast, the Q44K mutant protein can also bind stably as a single dimer to 8 bp target sites, in part due to additional contacts made to the phosphodiester backbone outside the 8 bp target via the K44 side-chains. Protein-protein co-operativity in the mutant is reduced relative to the wild-type allowing the properties of an intermediate on the pathway to operator site saturation to be examined for the first time. The crystal structure of the decamer complex shows a unique conformation for the protein bound to the single met-box site, possibly explaining the reduced protein-protein co-operativity. In both the extended and minimal DNA complexes formed, the mutant protein makes slightly different contacts to the edges of DNA base-pairs than the wild-type, even though the site of amino acid substitution is distal from the DNA-binding motif. Quantitative binding assays suggest that this is not due to artefacts caused by the crystallisation conditions but is most likely due to the relatively small contribution of such direct contacts to the overall binding energy of DNA-protein complex formation, which is dominated by sequence-dependent distortions of the DNA duplex and by the protein-protein contact between dimers.

Bacterial Proteins↗

PubMed2002

The structure of AhrC, the arginine repressor/activator protein from Bacillus subtilis.

In the Gram-positive bacterium Bacillus subtilis the concentration of the amino acid L-arginine is controlled by the transcriptional regulator AhrC. The hexameric AhrC protein binds in an L-arginine-dependent manner to pseudo-palindromic operators within the promoter regions of arginine biosynthetic and catabolic gene clusters. AhrC binding results in the repression of transcription of biosynthetic genes and in the activation of transcription of catabolic genes. The crystal structure of AhrC has been determined at 2.7 A resolution. Each subunit of the protein has two domains. The C-terminal domains are arranged with 32 point-group symmetry and mediate the major intersubunit interactions. The N-terminal domains are located around this core, where they lie in weakly associated pairs but do not obey strict symmetry. A structural comparison of AhrC with the arginine repressor from the thermophile B. stearothermophilus reveals close similarity in regions implicated in L-arginine binding and DNA recognition, but also reveals some striking sequence differences, especially within the C-terminal oligomerization domain, which may contribute to the different thermostabilities of the proteins. Comparison of the crystal structure of AhrC with a 30 A resolution model obtained by combining X-ray structure-factor amplitudes with phases derived from electron-microscopic analyses of AhrC crystals confirms the essential accuracy of the earlier model and suggests that such an approach may be more widely useful for obtaining low-resolution phase information.

Amino Acid Sequence↗

PubMed2002

Crystallization and preliminary X-ray structure determination of jack bean urease with a bound antibody fragment.

Urease allows organisms to use exogenous and internally generated urea as a nitrogen source, by catalyzing the hydrolysis of urea to form ammonia and carbon dioxide. Urease may also participate in the systemic nitrogen-transport pathways and possibly acts as a toxic defence protein. Jack bean urease (JBU) was the first nickel-metalloenzyme identified and was crystallized as early as 1926. Despite this, the structure has not yet been determined. An antibody fragment, Fv, that has a high affinity for JBU has been used to aid crystallization. The complex, which retains full enzyme activity, forms very small crystals that diffract weakly to 3.3 A. The crystals belong to the rhombohedral space group R32, with unit-cell parameters a = b = 228.6, c = 130.9 A. The structure of the urease molecule has been solved by molecular replacement using the structure of homogenous enzyme from Klebsiella aerogenes as a search model.

Crystallization↗