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The crystallographic structure of the Escherichia coli OXA-1 ␤-lactamase has been established at 1.5-Å resolution and refined to R ‫ס‬ 0.18. The 28.2-kD oxacillinase is a class D serine ␤-lactamase that is especially active against the penicillin-type ␤-lactams oxacillin and cloxacillin. In contrast to the structures of OXA-2, OXA-10, and OXA-13 belonging to other subclasses, the OXA-1 molecule is monomeric rather than dimeric and represents the subclass characterized by an enlarged ⍀ loop near the ␤-lactam binding site. The 6-residue hydrophilic insertion in this loop cannot interact directly with substrates and, instead, projects into solvent. In this structure at pH 7.5, carboxylation of the conserved Lys 70 in the catalytic site is observed. One oxygen atom of the carboxylate group is hydrogen bonded to Ser 120 and Trp 160. The other oxygen atom is more exposed and hydrogen bonded to the O␥ of the reactive Ser 67. In the overlay of the class D and class A binding sites, the carboxylate group is displaced ca. 2.6 Å from the carboxylate group of Glu 166 of class A enzymes. However, each group is equidistant from the site of the water molecule expected to function in hydrolysis, and which could be activated by the carboxylate group of Lys 70. In this ligand-free OXA-1 structure, no water molecule is seen in this site, so the water molecule must enter after formation of the acyl-Ser 67 intermediate. Abbreviations: HEPES, N-(2-hydroxyethyl)piperazine-NЈ-(2-ethanesulfonic acid); MPD, 2-methyl-2,4-pentanediol; PEG, polyethylene glycol; rmsd, root of the mean of the squared deviations; rpm, revolutions per minute.Article and publication are at http://www.proteinscience.org/cgi

Structural differences at subsite 2 of the D-alanine-D-lactate ligase help explain a substrate specificity shift (D-alanine to D-lactate) leading to remodeled cell wall peptidoglycan and vancomycin resistance in Gram-positive pathogens.

All together: A concise strategy for the first diastereoselective total synthesis of (±)‐schindilactone A is reported. The synthesis features a ring‐closing metathesis, a thiourea/cobalt‐catalyzed Pauson–Khand reaction, and a thiourea/palladium‐catalyzed carbonylative annulation reaction. The chemistry can be applied to the synthesis of structures related to schindilactone A.

A bacterial response to the clinical use of class A beta-lactamase inhibitors such as tazobactam and clavulanic acid is the expression of variant beta-lactamases with weaker binding affinities for these mechanism-based inhibitors. Some of these inhibitor-resistant variants contain a glycine mutation at Ser130, a conserved active site residue known to be adventitiously involved in the inhibition mechanism. The crystallographic structure of a complex of tazobactam with the Ser130Gly variant of the class A SHV-1 beta-lactamase has been determined to 1.8 A resolution. Two reaction intermediates are observed. The primary intermediate is an acyclic species bound to the reactive Ser70. It is poorly primed for catalytic hydrolysis because its ester carbonyl group is completely displaced from the enzyme's oxyanion hole. A smaller fraction of the enzyme contains a Ser70-bound aldehyde resulting from hydrolytic loss of the triazoyl-sulfinyl amino acid moiety from the primary species. This first structure of a class A beta-lactamase lacking Ser130, the side chain of which functions in beta-lactam binding and possibly in catalysis, gives crystallographic evidence that the acylation step of beta-lactam turnover can occur without Ser130. Unexpectedly, the crystal structure of the uncomplexed Ser130Gly enzyme, also determined to 1.8 A resolution, shows that a critical Glu166-activated water molecule is missing from the catalytic site. Comparison of this uncomplexed variant with the wild-type structure reveals that Ser130 is required for orienting the side chain of Ser70 and ensuring the hydrogen bonding of Ser70 to both Lys73 and the catalytic water molecule.

No abstract

A cesium continuum that fills the channels and cavities of zeolite X has been prepared, and its structure has been determined by single-crystal x-ray crystallography. The three-dimensional continuum is cationic to balance the negative charge of the zeolite framework. Its valence electrons, only 0.3 per Cs(+) ion, are widely delocalized over 95 percent of the cesium ions in the crystal. The continuum has a unit cell formula of (Cs(122))(86+) and contains Cs(13) and Cs(14) clusters (one per supercage) arranged like the atoms in diamond, with one Cs(2) appendix (in the sodalite cavity) per cluster.

No abstract