Structural and Dynamic Features of Acinetobacter baumannii OXA-66 ß-Lactamase Explain Its Stability and Evolution of Novel Variants.

OXA-66 is a member of the OXA-51 subfamily of class D ß-lactamases native to the Acinetobacter genus that includes Acinetobacter baumannii, one of the ESKAPE pathogens and a major cause of drug-resistant nosocomial infections. Although both wild type OXA-66 and OXA-51 have low catalytic activity, they are ubiquitous in the Acinetobacter genomes. OXA-51 is also remarkably thermostable. In addition, newly emerging, single and double amino acid variants show increased activity against carbapenems, indicating that the OXA-51 subfamily is growing and gaining clinical significance. In this study, we used molecular dynamics simulations, X-ray crystallography, and thermal denaturation data to examine and compare the dynamics of OXA-66 wt and its gain-of-function variants: I129L (OXA-83), L167V (OXA-82), P130Q (OXA-109), P130A, and W222L (OXA-234). Our data indicate that OXA-66 wt also has a high melting temperature, and its remarkable stability is due to an extensive and rigid hydrophobic bridge formed by a number of residues around the active site and harbored by the three loops, P, Ω, and ß5-ß6. Compared to the WT enzyme, the mutants exhibit higher flexibility only in the loop regions, and are more stable than other robust carbapenemases, such as OXA-23 and OXA-24/40. All the mutants show increased rotational flexibility of residues I129 and W222, which allows carbapenems to bind. Overall, our data support the hypothesis that structural features in OXA-51 and OXA-66 promote evolution of multiple highly stable variants with increased clinical relevance in A. baumannii.

CarboGrove: a resource of glycan-binding specificities through analyzed glycan-array datasets from all platforms.

Glycan arrays continue to be the primary resource for determining the glycan-binding specificity of proteins. The volume and diversity of glycan-array data are increasing, but no common method and resource exist to analyze, integrate, and use the available data. To meet this need, we developed a resource of analyzed glycan-array data called CarboGrove. Using the ability to process and interpret data from any type of glycan array, we populated the database with the results from 35 types of glycan arrays, 13 glycan families, 5 experimental methods, and 19 laboratories or companies. In meta-analyses of glycan-binding proteins, we observed glycan-binding specificities that were not uncovered from single sources. In addition, we confirmed the ability to efficiently optimize selections of glycan-binding proteins to be used in experiments for discriminating between closely related motifs. Through descriptive reports and a programmatically accessible Application Programming Interface, CarboGrove yields unprecedented access to the wealth of glycan-array data being produced and powerful capabilities for both experimentalists and bioinformaticians.

The structure of a doripenem-bound OXA-51 class D ß-lactamase variant with enhanced carbapenemase activity.

OXA-51 is a class D ß-lactamase that is thought to be the native carbapenemase of Acinetobacter baumannii. Many variants of OXA-51 containing active site substitutions have been identified from A. baumannii isolates, and some of these substitutions increase hydrolytic activity toward carbapenem antibiotics. We have determined the high-resolution structures of apo OXA-51 and OXA-51 with one such substitution (I129L) with the carbapenem doripenem trapped in the active site as an acyl-intermediate. The structure shows that acyl-doripenem adopts an orientation very similar to carbapenem ligands observed in the active site of OXA-24/40 (doripenem) and OXA-23 (meropenem). In the OXA-51 variant/doripenem complex, the indole ring of W222 is oriented away from the doripenem binding site, thereby eliminating a clash that is predicted to occur in wildtype OXA-51. Similarly, in the OXA-51 variant complex, L129 adopts a different rotamer compared to I129 in wildtype OXA-51. This alternative position moves its side chain away from the hydroxyethyl moiety of doripenem and relieves another potential clash between the enzyme and carbapenem substrates. Molecular dynamics simulations of OXA-51 and OXA-51 I129L demonstrate that compared to isoleucine, a leucine at this position greatly favors a rotamer that accommodates the ligand. These results provide a molecular justification for how this substitution generates enhanced binding affinity for carbapenems, and therefore helps explain the prevalence of this substitution in clinical OXA-51 variants.

Clinical Variants of the Native Class D ß-Lactamase of Acinetobacter baumannii Pose an Emerging Threat through Increased Hydrolytic Activity against Carbapenems.

The threat posed by the chromosomally encoded class D ß-lactamase of Acinetobacter baumannii (OXA-51/66) has been unclear, in part because of its relatively low affinity and turnover rate for carbapenems. Several hundred clinical variants of OXA-51/66 have been reported, many with substitutions of active-site residues. We determined the kinetic properties of OXA-66 and five clinical variants with respect to a wide variety of ß-lactam substrates. The five variants displayed enhanced activity against carbapenems and in some cases against penicillins, late-generation cephalosporins, and the monobactam aztreonam. Molecular dynamics simulations show that in OXA-66, P130 inhibits the side-chain rotation of I129 and thereby prevents doripenem binding because of steric clash. A single amino acid substitution at this position (P130Q) in the variant OXA-109 greatly enhances the mobility of both I129 and a key active-site tryptophan (W222), thereby facilitating carbapenem binding. This expansion of substrate specificity represents a very worrisome development for the efficacy of ß-lactams against this troublesome pathogen.