RESUMEN
Searching for functional square lattices in layered superconductor systems offers an explicit clue to modify the electron behavior and find exotic properties. The trigonal SnAs3 structural units in SnAs-based systems are relatively conformable to distortion, which provides the possibility to achieve structurally topological transformation and higher superconducting transition temperatures. In the present work, the functional As square lattice was realized and activated in Li0.6 Sn2 As2 and NaSnAs through a topotactic structural transformation of trigonal SnAs3 to square SnAs4 under pressure, resulting in a record-high Tc among all synthesized SnAs-based compounds. Meanwhile, the conductive channel transfers from the out-of-plane pz orbital to the in-plane px +py orbitals, facilitating electron hopping within the square 2D lattice and boosting the superconductivity. The reorientation of p-orbital following a directed local structure transformation provides an effective strategy to modify layered superconducting systems.
RESUMEN
Esterase EstB from Burkholderia gladioli belongs to a novel class of esterases homologous to penicillin binding proteins, notably DD-peptidase and class C ß-lactamases. It can cleave the side chain acetyl ester group from cephalosporins leaving the ß-lactam ring intact, which is a feature of relevance to industrial biocatalytic applications in the production of semisynthetic cephalosporin derivatives. Due to its important role as a potential biocatalyst in industry, the significance of EstB has been greatly appreciated. However, the molecular basis for those residues involving catalysis of EstB remains elusive. By analyzing the crystal structure of EstB, we identified a conserved water molecule in active-site cavity which might mediate an intramolecular proton transfer (PT) from Lys78 to Asp186 via Tyr133. Then a combined computational approach including molecular dynamics (MD) simulations and quantum mechanics/molecular mechanics (QM/MM) calculations was employed to explore this presumable PT mode in the native enzyme. A 30 ns MD simulation of the enzyme highlights the conserved H-bond network involving Lys78, Tyr133, Asp186, and the conserved water molecule in the active site. In particular, the water molecule did not exchange with bulk solvent, indicating its structural and functional relevance. The energy profile calculated by QM/MM approach displayed a notably low PT barrier (2.2 kcal/mol) and a dramatic energy difference (14.1 kcal/mol) in reactants versus immediate products, which implies that the proposed proton shuttle is concerted and energetically favorable. Our studies offer a reasonable pathway to yield a free base by assisting Lys78 deprotonation, thereby paving the way for future studies on Ser75 activation that is a critical step in catalysis by EstB, as well as biocatalyst development by rational attempts. This PT mode would also afford clues for the forthcoming investigation on acyltransferase LovD that is homologous to EstB.
