Crystal structures of starch binding domain from Rhizopus oryzae glucoamylase in complex with isomaltooligosaccharide: insights into polysaccharide binding mechanism of CBM21 family.

Glucoamylases are responsible for hydrolysis of starch and polysaccharides to yield ß-D-glucose. Rhizopus oryzae glucoamylase (RoGA) is composed of an N-terminal starch binding domain (SBD) and a C-terminal catalytic domain connected by an O-glycosylated linker. Two carbohydrate binding sites in RoSBD have been identified, site I is created by three highly conserved aromatic residues, Trp47, Tyr83, and Tyr94, and site II is built up by Tyr32 and Phe58. Here, the two crystal structures of RoSBD in complex with only α-(1,6)-linked isomaltotriose (RoSBD-isoG3) and isomaltotetraose (RoSBD-isoG4) have been determined at 1.2 and 1.3 Å, respectively. Interestingly, site II binding is observed in both complexes, while site I binding is only found in the RoSBD-isoG4 complex. Hence, site II acts as the recognition binding site for carbohydrate and site I accommodates site II to bind isoG4. Site I participates in sugar binding only when the number of glucosyl units of oligosaccharides is more than three. Taken together, two carbohydrate binding sites in RoSBD cooperate to reinforce binding mode of glucoamylase with polysaccharides as well as the starch.

Integrative Model to Coordinate the Oligomerization and Aggregation Mechanisms of CCL5.

CC-type chemokine ligand 5 (CCL5) is involved in the pathogenesis of many inflammatory conditions. Under physiological conditions, CCL5 oligomerization and aggregation are considered to be responsible for its inflammatory properties. The structural basis of CCL5 oligomerization remains controversial because the current oligomer models contain no consensus interactions. In this study, NMR and biophysical analyses proposed evidence that the CC-type CCL5 dimer acts as the basic unit to constitute the oligomer and that CCL5 oligomerizes alternatively through E66-K25 and E66-R44/K45 interactions. In addition, a newly determined trimer structure, constituted by CCL5 and the E66S mutant, reported an interfacial interaction through the N-terminal 12FAY14 sequence. The interaction contributes to CCL5 aggregation and precipitation but not to oligomerization. In accordance with the observations, an integrative model explains the CCL5 oligomerization and aggregation mechanism in which CCL5 assembly consists of two types of dimer-dimer interactions and one aggregation mechanism. For full-length CCL5, the molecular accumulation triggers oligomerization through the E66-K25 and E66-R44/K45 interactions, and the 12FAY14 interaction acts as a secondary effect to derive aggregation and precipitation. In contrast, the E66-R44/K45 interaction might dominate in CCL5 N-terminal truncations, and the interaction would lead to the filament-like formation in solution.

Roles of the Hydrophobic Gate and Exit Channel in Vigna radiata Pyrophosphatase Ion Translocation.

Membrane-embedded pyrophosphatase (M-PPase) hydrolyzes pyrophosphate to drive ion (H+ and/or Na+) translocation. We determined crystal structures and functions of Vigna radiata M-PPase (VrH+-PPase), the VrH+-PPase-2Pi complex and mutants at hydrophobic gate (residue L555) and exit channel (residues T228 and E225). Ion pore diameters along the translocation pathway of three VrH+-PPases complexes (Pi-, 2Pi- and imidodiphosphate-bound states) present a unique wave-like profile, with different pore diameters at the hydrophobic gate and exit channel, indicating that the ligands induced pore size alterations. The 2Pi-bound state with the largest pore diameter might mimic the hydrophobic gate open. In mutant structures, ordered waters detected at the hydrophobic gate among VrH+-PPase imply the possibility of solvation, and numerous waters at the exit channel might signify an open channel. A salt-bridge, E225-R562 is at the way out of the exit channel of VrH+-PPase; E225A mutant makes the interaction eliminated and reveals a decreased pumping ability. E225-R562 might act as a latch to regulate proton release. A water wire from the ion gate (R-D-K-E) through the hydrophobic gate and into the exit channel may reflect the path of proton transfer.

Human CCL5 trimer: expression, purification and initial crystallographic studies.

The chemokine CCL5 is considered to be a potential therapeutic target because of its ability to recruit immune cells to inflammatory sites. CCL5 aggregates under physiological conditions, and high-order oligomer formation is considered to be significant for cell migration, immune-cell activation and HIV cell entry. The structure of the high-order oligomer is unknown and the mechanism by which the oligomer is derived has yet to be established. Here, a CCL5 mutant (CCL5-E66S) which is deficient in oligomer formation was mixed with native CCL5 to prepare a protein trimer. At an optimized ratio the trimeric CCL5 crystallized, and the crystal belonged to the tetragonal space group P41212, with unit-cell parameters a = 56.6, b = 56.6, c = 154.1 Å. The Matthews coefficient (VM) of the crystal is 2.58 Å3 Da-1 (three molecules in the asymmetric unit), with a solvent content of 52.32%. Diffraction data were collected to a resolution of 1.87 Šand the statistics indicated satisfactory data quality. The new structure will reveal the interfaces in the CCL5 oligomer, therefore assisting in understanding the mechanism of CCL5 oligomerization.

Insights into the mechanism of membrane pyrophosphatases by combining experiment and computer simulation.

Membrane-integral pyrophosphatases (mPPases) couple the hydrolysis of pyrophosphate (PPi) to the pumping of Na+, H+, or both these ions across a membrane. Recently solved structures of the Na+-pumping Thermotoga maritima mPPase (TmPPase) and H+-pumping Vigna radiata mPPase revealed the basis of ion selectivity between these enzymes and provided evidence for the mechanisms of substrate hydrolysis and ion-pumping. Our atomistic molecular dynamics (MD) simulations of TmPPase demonstrate that loop 5-6 is mobile in the absence of the substrate or substrate-analogue bound to the active site, explaining the lack of electron density for this loop in resting state structures. Furthermore, creating an apo model of TmPPase by removing ligands from the TmPPase:IDP:Na structure in MD simulations resulted in increased dynamics in loop 5-6, which results in this loop moving to uncover the active site, suggesting that interactions between loop 5-6 and the imidodiphosphate and its associated Mg2+ are important for holding a loop-closed conformation. We also provide further evidence for the transport-before-hydrolysis mechanism by showing that the non-hydrolyzable substrate analogue, methylene diphosphonate, induces low levels of proton pumping by VrPPase.

Membrane pyrophosphatases from Thermotoga maritima and Vigna radiata suggest a conserved coupling mechanism.

Membrane-bound pyrophosphatases (M-PPases), which couple proton/sodium ion transport to pyrophosphate synthesis/hydrolysis, are important in abiotic stress resistance and in the infectivity of protozoan parasites. Here, three M-PPase structures in different catalytic states show that closure of the substrate-binding pocket by helices 5-6 affects helix 13 in the dimer interface and causes helix 12 to move down. This springs a 'molecular mousetrap', repositioning a conserved aspartate and activating the nucleophilic water. Corkscrew motion at helices 6 and 16 rearranges the key ionic gate residues and leads to ion pumping. The pumped ion is above the ion gate in one of the ion-bound structures, but below it in the other. Electrometric measurements show a single-turnover event with a non-hydrolysable inhibitor, supporting our model that ion pumping precedes hydrolysis. We propose a complete catalytic cycle for both proton and sodium-pumping M-PPases, and one that also explains the basis for ion specificity.

Two unique ligand-binding clamps of Rhizopus oryzae starch binding domain for helical structure disruption of amylose.

The N-terminal starch binding domain of Rhizopus oryzae glucoamylase (RoSBD) has a high binding affinity for raw starch. RoSBD has two ligand-binding sites, each containing a ligand-binding clamp: a polyN clamp residing near binding site I is unique in that it is expressed in only three members of carbohydrate binding module family 21 (CBM21) members, and a Y32/F58 clamp located at binding site II is conserved in several CBMs. Here we characterized different roles of these sites in the binding of insoluble and soluble starches using an amylose-iodine complex assay, atomic force microscopy, isothermal titration calorimetry, site-directed mutagenesis, and structural bioinformatics. RoSBD induced the release of iodine from the amylose helical cavity and disrupted the helical structure of amylose type III, thereby significantly diminishing the thickness and length of the amylose type III fibrils. A point mutation in the critical ligand-binding residues of sites I and II, however, reduced both the binding affinity and amylose helix disruption. This is the first molecular model for structure disruption of the amylose helix by a non-hydrolytic CBM21 member. RoSBD apparently twists the helical amylose strands apart to expose more ligand surface for further SBD binding. Repeating the process triggers the relaxation and unwinding of amylose helices to generate thinner and shorter amylose fibrils, which are more susceptible to hydrolysis by glucoamylase. This model aids in understanding the natural roles of CBMs in protein-glycan interactions and contributes to potential molecular engineering of CBMs.