I04-1-Macromolecular Crystallography (fixed wavelength)
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Abstract: Periplasmic solute-binding proteins (SBPs) in bacteria are involved in the active transport of nutrients into the cytoplasm. In marine bacteria of the genus Vibrio, a chitooligosaccharide-binding protein (CBP) is thought to be the major SBP controlling the rate of chitin uptake in these bacteria. However, the molecular mechanism of the CBP involvement in chitin metabolism has not been elucidated. Here, we report the structure and function of a recombinant chitooligosaccharide-binding protein from Vibrio harveyi, namely VhCBP, expressed in Escherichia coli. Isothermal titration calorimetry (ITC) revealed that VhCBP strongly binds shorter chitooligosaccharides [(GlcNAc)n, n = 2, 3, and 4] with affinities that are considerably greater than those for glycoside hydrolase family 18 (GH18) and GH19 chitinases, but does not bind longer ones, including insoluble chitin polysaccharides. We also found that VhCBP comprises two domains with flexible linkers and that the domain–domain interface forms the sugar-binding cleft, which is not long extended but forms a small cavity. (GlcNAc)2 bound to this cavity, apparently triggering a closed conformation of VhCBP. Trp-363 and Trp-513, which stack against the two individual GlcNAc rings, likely make a major contribution to the high affinity of VhCBP for (GlcNAc)2. The strong chitobiose binding, followed by the conformational change of VhCBP, may facilitate its interaction with an active-transport system in the inner membrane of Vibrio species.
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Feb 2018
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I04-1-Macromolecular Crystallography (fixed wavelength)
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Diamond Proposal Number(s):
[8425]
Abstract: The cellulosome is a remarkably intricate multienzyme nanomachine produced by anaerobic bacteria to degrade plant cell wall polysaccharides. Cellulosome assembly is mediated through binding of enzyme-borne dockerin modules to cohesin modules of the primary scaffoldin subunit. The anaerobic bacterium Acetivibrio cellulolyticus produces a highly intricate cellulosome comprising an adaptor scaffoldin, ScaB, whose cohesins interact with the dockerin of the primary scaffoldin (ScaA) that integrates the cellulosomal enzymes. The ScaB dockerin selectively binds to cohesin modules in ScaC that anchors the cellulosome onto the cell surface. Correct cellulosome assembly requires distinct specificities displayed by structurally related type I cohesin-dockerin pairs that mediate ScaC-ScaB and ScaA-enzyme assemblies. To explore the mechanism by which these two critical protein interactions display their required specificities, we determined the crystal structure of the dockerin of a cellulosomal enzyme in complex with a ScaA cohesin. The data revealed that the enzyme-borne dockerin binds to the ScaA cohesin in two orientations, indicating two identical cohesin-binding sites. Combined mutagenesis experiments served to identify amino acid residues that modulate type I cohesin-dockerin specificity in A. cellulolyticus. Rational design was used to test the hypothesis that the ligand-binding surfaces of ScaA- and ScaB-associated dockerins mediate cohesin recognition, independent of the structural scaffold. Novel specificities could thus be engineered into one, but not both of the ligand-binding sites of ScaB, while attempts at manipulating the specificity of the enzyme-associated dockerin were unsuccessful. These data indicate that dockerin specificity requires critical interplay between the ligand-binding surface and the structural scaffold of these modules.
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Jan 2018
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I04-1-Macromolecular Crystallography (fixed wavelength)
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Florian
Delbart
,
Marijke
Brams
,
Fabian
Gruss
,
Sam
Noppen
,
Steve
Peigneur
,
Sandro
Boland
,
Patrick
Chaltin
,
Jose
Brandao-neto
,
Frank
Von Delft
,
Wouter G.
Touw
,
Robbie
Joosten
,
Sandra
Liekens
,
Jan
Tytgat
,
Chris
Ulens
Abstract: Nicotinic acetylcholine receptors (nAChRs) belong to the family of pentameric ligand-gated ion channels and mediate fast excitatory transmission in the central and peripheral nervous systems. Among the different existing receptor subtypes, the homomeric & α7 nAChR has attracted considerable attention because of its possible implication in several neurological and psychiatric disorders, including cognitive decline associated with Alzheimer's disease or schizophrenia. Allosteric modulators of ligand-gated ion channels are of particular interest as therapeutic agents, as they modulate receptor activity without affecting normal fluctuations of synaptic neurotransmitter release. Here, we used X-ray crystallography and surface plasmon resonance spectroscopy of α7-acetylcholine binding protein (AChBP), a humanized chimera of a snail AChBP, which has 71% sequence similarity with the extracellular ligand-binding domain of the human α7 nAChR, to investigate the structural determinants of allosteric modulation. We extended previous observations that an allosteric site located in the vestibule of the receptor offers an attractive target for receptor modulation. We introduced seven additional humanizing mutations in the vestibule-located binding site of AChBP to improve its suitability as a model for studying allosteric binding. Using a fragment-based screening approach, we uncovered an allosteric binding site located near the β8-β9 loop, which critically contributes to coupling ligand binding to channel opening in human α7 nAChR. This work expands our understanding of the topology of allosteric binding sites in AChBP and, by extrapolation, in the human α7 nAChR as determined by electrophysiology measurements. Our insights pave the way for drug design strategies targeting nAChRs involved in ion channel-mediated disorders.
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Dec 2017
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I02-Macromolecular Crystallography
I03-Macromolecular Crystallography
I04-1-Macromolecular Crystallography (fixed wavelength)
I04-Macromolecular Crystallography
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Diamond Proposal Number(s):
[8997]
Abstract: The UbiD family of reversible decarboxylases act on aromatic, heteroaromatic, and unsaturated aliphatic acids and utilize a prenylated flavin mononucleotide (prFMN) as cofactor, bound adjacent to a conserved Glu-ArgGlu/Asp ionic network in the enzyme's active site. It is proposed that UbiD activation requires oxidative maturation of the cofactor, for which two distinct isomers, prFMNketimine and prFMNiminium have been observed. It also has been suggested that only the prFMNiminium form is relevant to catalysis, which requires transient cycloaddition between substrate and cofactor. Using Aspergillus niger Fdc1 as a model system, we reveal isomerization of prFMNiminium to prFMNketimine is a light-dependent process that is largely independent of the Glu277-Arg173-Glu282 network and accompanied by irreversible loss of activity. On the other hand, efficient catalysis was highly dependent on an intact Glu-Ar-Glu network, as only Glu to Asp substitutions retain activity. Surprisingly, oxidative maturation to form the prFMNiminium species is severely affected only for the R173A variant. In summary, the unusual irreversible isomerization of prFMN is light dependent and likely proceeds via high-energy intermediates, but is independent of the Glu-Arg-Glu network. Our results from mutagenesis, crystallographic, spectroscopic and kinetic experiments indicate a clear role for the Glu-Arg-Glu network in both catalysis and oxidative maturation.
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Dec 2017
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B21-High Throughput SAXS
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Diamond Proposal Number(s):
[14307]
Abstract: Eukaryotic and archaeal proteasomes are paradigms for self-compartmentalizing proteases. To a large extent, their function requires the interplay with hexameric ATPases associated with diverse cellular activities (AAA+) that act as substrate unfoldases. Bacteria have various types of self-compartmentalizing proteases; in addition to the proteasome itself, these include the proteasome homolog HslV, which functions together with the AAA+ ATPase HslU; the ClpP protease with its partner AAA+ ATPase ClpX; and Anbu, a recently characterized ancestral proteasome variant. Previous bioinformatic analysis has revealed a novel bacterial member of the proteasome family, BPH (Beta-proteobacteria proteasome homolog). Using cluster analysis, we here affirmed that BPH evolutionarily descends from HslV. Crystal structures of the Thiobacillus denitrificans and Cupriavidus metallidurans BPHs disclosed a homo-oligomeric double-ring architecture, in which the active sites face the interior of the cylinder. Using small-angle X-ray scattering (SAXS) and electron microscopy averaging, we found that BPH forms tetradecamers in solution, unlike the dodecamers seen in HslV. While the highly acidic inner surface of BPH was in striking contrast to the cavity characteristics of the proteasome and HslV, a classical proteasomal reaction mechanism could be inferred from the covalent binding of the proteasome-specific inhibitor epoxomicin to BPH. A ligand-bound structure implied that the elongated BPH inner pore loop may be involved in substrate recognition. The apparent lack of a partner unfoldase and other unique features, such as Ser replacing Thr as catalytic residue in certain BPH subfamilies, suggest a proteolytic function for BPH distinct from those of known bacterial self-compartmentalizing proteases.
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Nov 2017
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I04-Macromolecular Crystallography
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Yindi
Chu
,
Tao
Tu
,
Leena
Penttinen
,
Xianli
Xue
,
Xiaoyu
Wang
,
Zhuolin
Yi
,
Li
Gong
,
Juha
Rouvinen
,
Huiying
Luo
,
Nina
Hakulinen
,
Bin
Yao
,
Xiaoyun
Su
Abstract: Bifunctional glycoside hydrolases have potential for cost saving in enzymatic decomposition of plant cell wall polysaccharides for biofuels and bio-based chemicals. The N-terminal GH10 domain of a bifunctional multimodular enzyme CbXyn10C/Cel48B from Caldicellulosiruptor bescii, is an enzyme able to degrade xylan and cellulose simultaneously. However, the molecular mechanism underlying its substrate promiscuity has not been elucidated. Herein, we discovered that the binding cleft of CbXyn10C would have at least six sugar binding subsites by using isothermal titration calorimetry analysis of the inactive E140Q/E248Q mutant with xylo- and cellooligosaccharides. This was confirmed by determining the catalytic efficiency of the wild-type enzyme on these oligosaccharides. The free form and complex structures of CbXyn10C with xylose- or glucose-configured oligosaccharide ligands were further obtained by crystallographic analysis and molecular modeling and docking. CbXyn10C was found to have a typical (β/α)8-TIM barrel fold and "salad-bowl" shape of GH10 enzymes. In complex structure with xylo-oligosaccharides, seven sugar-binding subsites were found and many residues responsible for substrate interactions were identified. Site-directed mutagenesis indicated that six and ten amino acid residues were key residues for xylan and cellulose hydrolysis, respectively. The most important residues are centered on the subsites -2 and -1 near the cleavage site, while residues playing moderate roles could be located at more distal regions of the binding cleft. Manipulating the residues directly or indirectly interacting with substrates in the distal regions improved the activity of CbXyn10C on xylan and cellulose. Most of the key residues for cellulase activity are conserved across GH10 xylanases. Revisiting randomly selected GH10 enzymes revealed unreported cellulase activity, indicating that the dual function may be a more common phenomenon than has been expected.
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Oct 2017
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Abstract: Thioesterases catalyze the cleavage of thioester bonds within many activated fatty acids and acyl-CoA substrates. They are expressed ubiquitously in both prokaryotes and eukaryotes and are subdivided into 25 thioesterase families according to their catalytic active site, protein oligomerization, and substrate specificity. While many of these enzyme families are well characterized in terms of function and substrate specificity, regulation across most thioesterase families is poorly understood. Here, we characterized a TE6 thioesterase from the bacterium Neisseria meningitidis. Structural analysis with X-ray crystallographic diffraction data to 2.0 Å revealed that each protein subunit harbors a hot dog fold and that the TE6 enzyme forms a hexamer with D3 symmetry. An assessment of thioesterase activity against a range of acyl-CoA substrates revealed greatest activity against acetyl-CoA, and structure-guided mutagenesis of putative active site residues identified Asn-24 and Asp-39 as being essential for activity. Our structural analysis revealed that six GDP nucleotides bound the enzyme in close proximity to an intersubunit disulfide bond interactions that covalently link thioesterase domains in a double hot dog dimer. Structure-guided mutagenesis of residues within the GDP-binding pocket identified Arg-93 as playing a key role in the nucleotide interaction and revealed that GDP is required for activity. All mutations were confirmed to be specific and not to have resulted from structural perturbations by X-ray crystallography. This is the first report of a bacterial GDP-regulated thioesterase and of covalent linkage of thioesterase domains through a disulfide bond, revealing structural similarities with ADP regulation in the human ACOT12 thioesterase.
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Oct 2017
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I04-1-Macromolecular Crystallography (fixed wavelength)
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Abstract: One central goal in molecular evolution is to pinpoint the mechanisms and evolutionary forces that cause an enzyme to change its substrate specificity; however, these processes remain largely unexplored. Using the glycolytic ADP-dependent kinases of archaea, including the orders Thermococcales, Methanosarcinales, and Methanococcales, as a model and employing an approach involving paleoenzymology, evolutionary statistics, and protein structural analysis, we could track changes in substrate specificity during ADP-dependent kinase evolution along with the structural determinants of these changes. To do so, we studied five key resurrected ancestral enzymes as well as their extant counterparts. We found that a major shift in function from a bifunctional ancestor that could phosphorylate either glucose or fructose 6-phosphate (fructose-6P) as a substrate to a fructose-6P-specific enzyme was started by a single amino acid substitution resulting in negative selection with a ground-state mode against glucose and a subsequent 1,600-fold change in specificity of the ancestral protein. This change rendered the residual phosphorylation of glucose a promiscuous and physiologically irrelevant activity, highlighting how promiscuity may be an evolutionary vestige of ancestral enzyme activities, which have been eliminated over time. We also could reconstruct the evolutionary history of substrate utilization by using an evolutionary model of discrete binary characters, indicating that substrate uses can be discretely lost or acquired during enzyme evolution. These findings exemplify how negative selection and subtle enzyme changes can lead to major evolutionary shifts in function, which can subsequently generate important adaptive advantages, for example, in improving glycolytic efficiency in Thermococcales.
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Jul 2017
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I02-Macromolecular Crystallography
I04-Macromolecular Crystallography
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Diamond Proposal Number(s):
[9948, 13587]
Abstract: The human gut microbiota utilizes complex carbohydrates as major nutrients. The requirement for efficient glycan degrading systems exerts a major selective selection pressure on this microbial community. Thus, we propose that this microbial ecosystem represents a substantial resource for discovering novel carbohydrate active enzymes. To test this hypothesis we screened the potential enzymatic functions of hypothetical proteins encoded by genes of Bacteroides thetaiotaomicron that were upregulated by arabinogalactan arabinogalactan proteins or AGPs. Although AGPs are ubiquitous in plants, there is a paucity of information on their detailed structure, the function of these glycans in planta and the mechanisms by which they are depolymerized in microbial ecosystems. Here we have discovered a new polysaccharide lyase family that is specific for the L-rhamnose-alpha1,4-D-glucuronic acid linkage that caps the side chains of complex AGPs. The reaction product generated by the lyase, delta4,5-unsaturated uronic acid, is removed from AGP by a glycoside hydrolase located in family GH105, producing the final product 4-deoxy-β-L-threo-hex-4-enepyranosyl-uronic acid. The crystal structure of a member of the novel lyase family revealed a catalytic domain that displays an (alpha/alpha6)6 barrel fold. In the centre of the barrel is a deep pocket, which, based on mutagenesis data and amino acid conservation, comprises the active site of the lyase. A tyrosine is the proposed catalytic base in the beta-elimination reaction. This study illustrates how highly complex glycans can be used as a scaffold to discover new enzyme families within microbial ecosystems where carbohydrate metabolism is a major evolutionary driver.
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Jun 2017
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I02-Macromolecular Crystallography
I04-1-Macromolecular Crystallography (fixed wavelength)
I24-Microfocus Macromolecular Crystallography
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Diamond Proposal Number(s):
[6886, 7141]
Abstract: Bone morphogenetic proteins (BMPs) are secreted growth factors that promote differentiation processes in embryogenesis and tissue development. Regulation of BMP signalling involves binding to a variety of extracellular proteins, among which are many von Willebrand factor C (vWC) domain-containing proteins. While the crystal structure of the complex of crossveinless-2 (CV-2) vWC1 and BMP-2 previously revealed one mode of the vWC:BMP binding mechanism, other vWC domains may bind to BMP differently. Here, using X-ray crystallography, we present for the first time structures of the vWC domains of two proteins thought to interact with BMP-2 - collagen IIA and matricellular protein CCN3. We found that these two vWC domains share a similar N-terminal fold that differs greatly from that in CV-2 vWC, which comprises its BMP-2 binding site. We analysed the ability of these vWC domains to directly bind to BMP-2 and detected an interaction only between the collagen IIa vWC and BMP-2. Guided by the collagen IIa vWC domain crystal structure and conservation of surface residues among orthologous domains, we mapped the BMP-binding epitope on the subdomain 1 of the vWC domain. This binding site is different from that previously observed in the complex between CV-2 vWC and BMP-2, revealing an alternative mode of interaction between vWC domains and BMPs.
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Jun 2017
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