Purification, crystallization and preliminary X-ray diffraction studies of the three components of the toluene 2,3-dioxygenase enzyme system. Corrigendum

A corrigendum to the article by Lee et al. (2005), Acta Cryst. F61, 669–672

Inhibitor binding mode and allosteric regulation of Na+-glucose symporters.

Sodium-dependent glucose transporters (SGLTs) exploit sodium gradients to transport sugars across the plasma membrane. Due to their role in renal sugar reabsorption, SGLTs are targets for the treatment of type 2 diabetes. Current therapeutics are phlorizin derivatives that contain a sugar moiety bound to an aromatic aglycon tail. Here, we develop structural models of human SGLT1/2 in complex with inhibitors by combining computational and functional studies. Inhibitors bind with the sugar moiety in the sugar pocket and the aglycon tail in the extracellular vestibule. The binding poses corroborate mutagenesis studies and suggest a partial closure of the outer gate upon binding. The models also reveal a putative Na+ binding site in hSGLT1 whose disruption reduces the transport stoichiometry to the value observed in hSGLT2 and increases inhibition by aglycon tails. Our work demonstrates that subtype selectivity arises from Na+-regulated outer gate closure and a variable region in extracellular loop EL5

The Sodium Sialic Acid Symporter From Staphylococcus aureus Has Altered Substrate Specificity

Mammalian cell surfaces are decorated with complex glycoconjugates that terminate with negatively charged sialic acids. Commensal and pathogenic bacteria can use host-derived sialic acids for a competitive advantage, but require a functional sialic acid transporter to import the sugar into the cell. This work investigates the sodium sialic acid symporter (SiaT) from Staphylococcus aureus (SaSiaT). We demonstrate that SaSiaT rescues an Escherichia coli strain lacking its endogenous sialic acid transporter when grown on the sialic acids N-acetylneuraminic acid (Neu5Ac) or N-glycolylneuraminic acid (Neu5Gc). We then develop an expression, purification and detergent solubilization system for SaSiaT and demonstrate that the protein is largely monodisperse in solution with a stable monomeric oligomeric state. Binding studies reveal that SaSiaT has a higher affinity for Neu5Gc over Neu5Ac, which was unexpected and is not seen in another SiaT homolog. We develop a homology model and use comparative sequence analyses to identify substitutions in the substrate-binding site of SaSiaT that may explain the altered specificity. SaSiaT is shown to be electrogenic, and transport is dependent upon more than one Na+ ion for every sialic acid molecule. A functional sialic acid transporter is essential for the uptake and utilization of sialic acid in a range of pathogenic bacteria, and developing new inhibitors that target these transporters is a valid mechanism for inhibiting bacterial growth. By demonstrating a route to functional recombinant SaSiaT, and developing the in vivo and in vitro assay systems, our work underpins the design of inhibitors to this transporter

The basis for non-canonical ROK family function in the N-acetylmannosamine kinase from the pathogen Staphylococcus aureus

In environments where glucose is limited, some pathogenic bacteria metabolize host-derived sialic acid as a nutrient source. N-Acetylmannosamine kinase (NanK) is the second enzyme of the bacterial sialic acid import and degradation pathway and adds phosphate to N-acetylmannosamine using ATP to prime the molecule for future pathway reactions. Sequence alignments reveal that Gram-positive NanK enzymes belong to the Repressor, ORF, Kinase (ROK) family, but many lack the canonical Zn-binding motif expected for this function, and the sugar-binding EXGH motif is altered to EXGY. As a result, it is unclear how they perform this important reaction. Here, we study the Staphylococcus aureus NanK (SaNanK), which is the first characterization of a Gram-positive NanK. We report the kinetic activity of SaNanK along with the ligand-free, N-acetylmannosamine-bound and substrate analog GlcNAc-bound crystal structures (2.33, 2.20, and 2.20 Å resolution, respectively). These demonstrate, in combination with small-angle X-ray scattering, that SaNanK is a dimer that adopts a closed conformation upon substrate binding. Analysis of the EXGY motif reveals that the tyrosine binds to the N-acetyl group to select for the "boat" conformation of N-acetylmannosamine. Moreover, SaNanK has a stacked arginine pair coordinated by negative residues critical for thermal stability and catalysis. These combined elements serve to constrain the active site and orient the substrate in lieu of Zn binding, representing a significant departure from canonical NanK binding. This characterization provides insight into differences in the ROK family and highlights a novel area for antimicrobial discovery to fight Gram-positive and S. aureus infections

Structure-function studies of iron-sulfur enzyme systems

Iron-sulfur clusters are among the most ancient of metallocofactors and serve a variety of biological functions in proteins, including electron transport, catalytic, and structural roles. Two kinds of multicomponent enzyme systems have been investigated by X-ray crystallography, the ferredoxin/thioredoxin system and bacterial Rieske non-heme iron dioxygenase (RDO) systems. The ferredoxin/thioredoxin system is a light sensitive system controlling the activities of key enzymes involved in the assimilatory (photosynthetic) and dissimilatory pathways in chloroplasts and photosynthetic bacteria. The system consists of a ferredoxin, ferredoxin:thioredoxin reductase (FTR), and two thioredoxins, Trx-m and Trx-f. In light, photosystem I reduces ferredoxin that reduces Trx-m and Trx-f. This two-electron reduction is catalyzed by FTR that contains a [4Fe-4S] center and a proximal disulfide bridge. When the first electron is delivered by the ferredoxin, an intermediate is formed where one thiol of the proximal disulfide attacks the disulfide bridge of thioredoxin. This results in a transient protein-protein complex held together by a mixed disulfide between FTR and Trx-m. This complex is stabilized by using a C40S mutant Trx-m and its structure have been determined. RDOs consists of a flavoprotein reductase and often a ferredoxin that transfer electrons from NAD(P)H to the terminal dioxygenase. The terminal dioxygenase catalyze the enantioselective addition of dioxygen in the initial degradation of aromatic compounds, producing cis-dihydrodiols. The structures of three dioxygenases, nitrobenzene dioxygenase (NBDO), 2-nitrotoluene (2NTDO) and toluene dioxygenase (TDO), as well as the two electron transfer proteins of the TDO system, toluene dioxygenase reductase (TDOR) and toluene dioxygenase ferredoxin (TDOF), have been determined. The dioxygenase structures are all alpha3beta3 heterohexamers similar to other RDOs. The catalytic a subunit contains a Rieske iron-sulfur cluster and a mononuclear iron at the active site. 2NTDO and NBDO are both able to degrade nitroaromatic compounds. Their structures and structures of NBDO in complex with two nitroarene substrates reveal the structural basis for the dihydroxylation of nitroarene compounds. The electron transfer pathway from NADH via TDOR and the TDOF to the TDO is described in relation to the obtained structures of the TDO system

Crystal structure of the HIV-2 neutralizing Fab fragment 7C8 with high specificity to the V3 region of gp125.

7C8 is a mouse monoclonal antibody specific for the third hypervariable region (V3) of the human immunodeficiency virus type 2 (HIV-2)-associated protein gp125. The three-dimensional crystal structure of the Fab fragment of 7C8, determined to 2.7 Å resolution, reveals a deep and narrow antigen-binding cleft with architecture appropriate for an elongated epitope. The highly hydrophobic cleft is bordered on one side by the negatively charged second complementarity determining region (CDR2) and the unusually long positively charged CDR3 of the heavy chain and, on the other side, by the CDR1 of the light chain. Analysis of 7C8 in complex with molecular models of monomeric and trimeric gp125 highlights the importance of a conserved stretch of residues FHSQ that is localized centrally on the V3 region of gp125. Furthermore, modeling also indicates that the Fab fragment neutralizes the virus by sterically impairing subsequent engagement of the gp125 trimer with the co-receptor on the target cell