The Lon AAA+ protease (LonA) plays important roles in protein homeostasis and regulation of diverse biological processes. LonA behaves as a homomeric hexamer in the presence of magnesium (Mg(2+)) and performs ATP-dependent proteolysis. However, it is also found that LonA can carry out Mg(2+)-dependent degradation of unfolded protein substrate in an ATP-independent manner. Here we show that in the presence of Mg(2+) LonA forms a non-secluded hexameric barrel with prominent openings, which explains why Mg(2+)-activated LonA can operate as a diffusion-based chambered protease to degrade unstructured protein and peptide substrates efficiently in the absence of ATP. A 1.85 Å crystal structure of Mg(2+)-activated protease domain reveals Mg(2+)-dependent remodeling of a substrate-binding loop and a potential metal-binding site near the Ser-Lys catalytic dyad, supported by biophysical binding assays and molecular dynamics simulations. Together, these findings reveal the specific roles of Mg(2+) in the molecular assembly and activation of LonA.

Pt II azido complexes [Pt(bpy) (N 3 ) 2 ] (1), [Pt(phen) (N 3 ) 2 ] (2) and trans-[Pt(N 3 ) 2 (py) 2 ] (3) incorporating the bidentate diimine ligands 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen) or the monodentate pyridine (py) respectively, have been synthesised from their chlorido precursors and characterised by x-ray crystallography; complex 3 shows significant deviation from squareplanar geometry (N 3 -Pt-N 3 angle 146.7°) as a result of steric congestion at the Pt centre. The novel Pt IV complexes trans, cis-[Pt(bpy)(OAc) 2 (N 3 ) 2 ] (4), trans, cis-[Pt(phen)(OAc) 2 (N 3 ) 2 ] (5), trans, trans, trans-[Pt(OAc) 2 (N 3 ) 2 (py) 2 ] (6), were obtained from 1-3 via oxidation with H 2 O 2 in acetic acid followed by reaction of the intermediate with acetic anhydride. Complexes 4-6 exhibit interesting structural and photochemical properties that were studied by x-ray, NMR and UV-vis spectroscopy and TDDFT. These Pt IV complexes exhibit greater absorption at longer wavelengths (ε = 9756 M −1 cm −1 at 315 nm for 4; ε = 796 M −1 cm −1 at 352 nm for 5; ε = 16900 M −1 cm −1 at 307 nm for 6, in aqueous solution) than previously reported Pt IV azide complexes, due to the presence of aromatic amines, and 4-6 undergo photoactivation with both UVA (365 nm) and visible green light (514 nm). The UV-vis spectra of complexes 4-6 were calculated using TD-DFT; the nature of the transitions contributing to the UV-vis bands provide insight into the mechanism of production of the observed photoproducts. The UV-vis spectra of 1-3 were also simulated by computational methods and comparison between Pt II and Pt IV electronic and structural properties allowed further elucidation of the photochemistry of 4-6.

Knowledge of the bonding and selectivity of organic mercury, [H3C-Hg]+ (MeHg+), and inorganic Hg2+ for protein and DNA functional groups is important for understanding the mechanism of heavy metal poisoning. Herein, we elucidate (1) the differences between inorganic Hg2+ and organic MeHg+ in their interactions with different ligands of biological interest, (2) the protein and DNA functional groups that Hg2+ and MeHg+ target in aqueous solution, and (3) the likelihood of "soft" Hg2+ displacing the "borderline" Zn2+ bound to "harder" nitrogen/oxygen-containing side chains such as His and Asp/Glu. The results reveal that, relative to Hg2+, the lower positive charge on MeHg+ results in a longer and weaker bond with a given ligand, in accord with the observed kinetic lability of MeHg+ complexes. They also indicate that negatively charged or polar amino acid side chains containing S-/O-/S/N donors could coordinate to both organic MeHg+ and inorganic Hg2+. In addition, Gua and Cyt could also coordinate to MeHg+ and disrupt Gua...Cyt base pairing. A key novel finding is that Hg2+ is a far better electron acceptor than Zn2+, and can thus accept more negative charge from the Zn ligands than the native Zn2+, thus enhancing Hg-ligand interactions and enabling Hg2+ to displace the native cofactor from zinc essential enzymes and "structural" Zn proteins. The results herein support several possible mechanisms for Hg poisoning. Ways that mercury poisoning could be prevented in cells are discussed.