Active-site protonation states in an Acyl-Enzyme intermediate of a Class A β-Lactamase with a Monobactam Substrate

The monobactam antibiotic aztreonam is used to treat cystic fibrosis patients with chronic pulmonary infections colonized by Pseudomonas aeruginosa strains expressing CTX-M extended-spectrum β-lactamases. The protonation states of active-site residues that are responsible for hydrolysis have been determined previously for the apo form of a CTX-M β-lactamase but not for a monobactam acyl-enzyme intermediate. Here we used neutron and high-resolution X-ray crystallography to probe the mechanism by which CTX-M extended-spectrum β-lactamases hydrolyze monobactam antibiotics. In these first reported structures of a class A β-lactamase in an acyl-enzyme complex with aztreonam, we directly observed most of the hydrogen atoms (as deuterium) within the active site. Although Lys 234 is fully protonated in the acyl intermediate, we found that Lys 73 is neutral. These findings are consistent with Lys 73 being able to serve as a general base during the acylation part of the catalytic mechanism, as previously proposed

On the fraction of dark matter in charged massive particles (CHAMPs)

From various cosmological, astrophysical and terrestrial requirements, we derive conservative upper bounds on the present-day fraction of the mass of the Galactic dark matter (DM) halo in charged massive particles (CHAMPs). If dark matter particles are neutral but decay lately into CHAMPs, the lack of detection of heavy hydrogen in sea water and the vertical pressure equilibrium in the Galactic disc turn out to put the most stringent bounds. Adopting very conservative assumptions about the recoiling velocity of CHAMPs in the decay and on the decay energy deposited in baryonic gas, we find that the lifetime for decaying neutral DM must be > (0.9-3.4)x 10^3 Gyr. Even assuming the gyroradii of CHAMPs in the Galactic magnetic field are too small for halo CHAMPs to reach Earth, the present-day fraction of the mass of the Galactic halo in CHAMPs should be < (0.4-1.4)x 10^{-2}. We show that redistributing the DM through the coupling between CHAMPs and the ubiquitous magnetic fields cannot be a solution to the cuspy halo problem in dwarf galaxies.Comment: 21 pages, 2 figures. To appear in JCA

XAFS Determination of As(V) Associated with Fe(III) Oxyhydroxides in Weathered Mine Tailings and Contaminated Soil from California, U.S.A.

The speciation of Arsenic (As) in soils and natural waters is critical in determining its environmental fate. As(III) is the most mobile and toxic of the inorganic arsenic species, but is readily oxidized to As(V), which although still quite toxic is far less mobile. Adsorption to sediment particles may remove As(V) from contaminated water, or the precipitation of arsenic minerals such as scorodite (FeAsO4·2H2O) may control the equilibrium aqueous concentration. We have employed XAFS spectroscopy to examine the solid-phase speciation of arsenic in mine tailings samples and a contaminated soil from California. Quantitative speciation of As was determined using XANES fitting methods and EXAFS analysis ; in all samples, As(V) predominates. In an oxidized tailings (no residual sulfides), we find As(V) adsorbed/coprecipitated on Fe(III) oxyhydroxides. In less oxidized tailings containing residual sulfides, there is evidence for scorodite a a similar Fe(III) arsenate and a reduced arsenic phase (arsenopyrite or arseniferous pyrite). In a soil contaminated by smelter waste, we detect Mg3(AsO4)2·8 H2O (hoernesite), and suggest the presence of an additional weakly bound As(V) species. XAFS spectroscopy is one of the few techniques for direct determination of arsenic speciation in complex matrices such as soils and mine tailings