Structures of a ZIP zinc transporter reveal an inward-open conformation with a binuclear metal center in the transport pathway.

Structural and functional characterization of the extracellular domain of the human CaSR with bound Mg2+ and a tryptophan derivative.

The ethylene-forming enzyme (EFE) from Pseudomonas syringae pv. phaseolicola PK2 is a member of the mononuclear non-heme Fe(II)- and 2-oxoglutarate (2OG)-dependent oxygenase superfamily. EFE converts 2OG into ethylene plus three CO2 molecules while also catalyzing the C5 hydroxylation of L-arginine (L-Arg) driven by the oxidative decarboxylation of 2OG to form succinate and CO2. Here we report eleven X-ray crystal structures of EFE that provide insight into the mechanisms of these two reactions. Binding of 2OG in the absence of L-Arg resulted in predominantly monodentate metal coordination, distinct from the typical bidentate metal-binding species observed in other family members. Subsequent addition of L-Arg resulted in compression of the active site, a conformational change of the carboxylate side chain metal ligand to allow for hydrogen bonding with the substrate, and creation of a twisted peptide bond involving this carboxylate and the following tyrosine residue. A reconfiguration of 2OG achieves bidentate metal coordination. The dioxygen binding site is located on the metal face opposite to that facing L-Arg, thus requiring reorientation of the generated ferryl species to catalyze L-Arg hydroxylation. Notably, a phenylalanyl side chain pointing towards the metal may hinder such a ferryl flip and promote ethylene formation. Extensive site-directed mutagenesis studies supported the importance of this phenylalanine and confirmed the essential residues used for substrate binding and catalysis. The structural and functional characterization described here suggests that conversion of 2OG to ethylene, atypical among Fe(II)/2OG oxygenases, is facilitated by the binding of L-Arg which leads to an altered positioning of the carboxylate metal ligand, a resulting twisted peptide bond, and the off-line geometry for dioxygen coordination.

The ZIP zinc transporter family is responsible for zinc uptake from the extracellular milieu or intracellular vesicles. The LIV-1 subfamily, containing nine out of the 14 human ZIP proteins, is featured with a large extracellular domain (ECD). The critical role of the ECD is manifested by disease-causing mutations on ZIP4, a representative LIV-1 protein. Here we report the first crystal structure of a mammalian ZIP4-ECD, which reveals two structurally independent subdomains and an unprecedented dimer centred at the signature PAL motif. Structure-guided mutagenesis, cell-based zinc uptake assays and mapping of the disease-causing mutations indicate that the two subdomains play pivotal but distinct roles and that the bridging region connecting them is particularly important for ZIP4 function. These findings lead to working hypotheses on how ZIP4-ECD exerts critical functions in zinc transport. The conserved dimeric architecture in ZIP4-ECD is also demonstrated to be a common structural feature among the LIV-1 proteins.

The ethylene-forming enzyme (EFE) is a non-heme Fe(II), 2-oxoglutarate (2OG), and L-arginine (L-Arg)-dependent oxygenase that catalyzes dual reactions: the generation of ethylene from 2OG and the C5 hydroxylation of L-Arg. Using an integrated molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) approach that references previous experimental studies, we tested the hypothesis that synergy between the conformation of L-Arg and the coordination mode of 2OG directs the reaction toward ethylene formation or L-Arg hydroxylation. The dynamics of EFE•Fe(III)•OO •− •2OG•L-Arg show that L-Arg can exist in conformation A (productive for hydroxylation) and conformation B (unproductive for hydroxylation). QM/MM calculations show that when 2OG is bound in an off-line mode and L-Arg is present in conformation A, the Fe(III)-OO •− intermediate undergoes the standard O 2 activation mechanism involving ferryldependent hydroxylation. With the same off-line 2OG coordination, but with conformation B of L-Arg, a unique pathway produces a half-bond ferric-bicarbonate intermediate that decomposes to ethylene, two CO 2 , and a ferrous-bicarbonate species. The results demonstrate that when 2OG is coordinated in off-line mode to the Fe center, the L-Arg conformation acts as a switch that directs the reaction toward ethylene formation or hydroxylation. Analysis of the electronic structure shows that the L-Arg conformation defines the precise location of an unpaired β electron in the Fe(III)-OO − complex, either in a π* ∥ orbital that triggers ethylene formation or a π* ⊥ orbital that cascades to L-Arg hydroxylation. A change in 2OG coordination from off-line to in-line reduces stabilization of the 2OG C1 carboxylate such that neither conformation of L-Arg produces the ethylene-forming half-bond ferric-bicarbonate intermediate. Thus, L-Arg conformation-dependent changes in the electronic structure of the Fe(III)-OO •− orbitals, together with the 2OG binding mode-associated stabilization of the C1-carboxylate, distinguish whether the EFE reaction proceeds via the ethylene-forming pathway or catalyzes a hydroxylation mechanism.