W. Bruce Turnbull

Efficient N‐Terminal Labeling of Proteins by Use of Sortase

Protein labeling is a pivotal technique in molecular and cell biology. Strategies include derivatization of cysteine residues, labeling lysine or N-terminal amino groups with activated esters, periodate or PLP-mediated oxidation of the N-terminus for oxime ligation, and native-chemical ligation. Each of these methods has its own associated challenges: Selective labeling of a single cysteine residue frequently requires rounds of site-directed mutagenesis to introduce the labeling site and/or remove other cysteine residues, and selective labeling of the N-terminal amino group requires careful control of pH to ensure lysine residues are not also modified. Other modern methods for chemoselective labeling often require the introduction of specific recognition sequences or nonnatural amino acids into the protein to be labeled. In most cases, a substantial excess of the labeling reagent is necessary to ensure complete conversion to the product. We report a method for chemoselective N-terminal labeling of recombinant proteins in quantitative yield using depsipeptide substrates for the transpeptidase sortase A. The method does not require engineering of the protein sequence beyond that typically used in contemporary recombinant protein purification strategies. Unlike previous approaches, the method requires only a single N-terminal glycine residue in a sterically unhindered position, a minimal excess of the labeling reagent and substoichiometric quantities of transpeptidase. Sortase A (SrtA) catalyzes the reversible attachment of virulence factors to the cell walls of Gram positive bacteria by C-terminal modification of proteins at an LPXTG recognition sequence. The enzyme catalyzes the covalent attachment of the LPXT motif to a cysteine residue in the catalytic site to form a thioester intermediate. An N-terminal oligoglycine motif in the peptidoglycan can then react with this intermediate to covalently attach the substrate to the cell wall. SrtA has been exploited extensively for the C-terminal modification of proteins. 9] However, this method has certain constraints: the LPXTG sequence must be engineered into the protein and excess nucleophilic labeling reagent is required to push the equilibrium toward formation of product as the transpeptidase reaction is reversible (Scheme 1 a).

The influence of ligand valency on aggregation mechanisms for inhibiting bacterial toxins.

Divalent and tetravalent analogues of ganglioside GM1 are potent inhibitors of cholera toxin and Escherichia coli heat‐labile toxin. However, they show little increase in inherent affinity when compared to the corresponding monovalent carbohydrate ligand. Analytical ultracentrifugation and dynamic light scattering have been used to demonstrate that the multivalent inhibitors induce protein aggregation and the formation of space‐filling networks. This aggregation process appears to arise when using ligands that do not match the valency of the protein receptor. While it is generally accepted that multivalency is an effective strategy for increasing the activity of inhibitors, here we show that the valency of the inhibitor also has a dramatic effect on the kinetics of aggregation and the stability of intermediate protein complexes. Structural studies employing atomic force microscopy have revealed that a divalent inhibitor induces head‐to‐head dimerization of the protein toxin en route to higher aggregates.

Large Oligosaccharide‐Based Glycodendrimers

Carbohydrate-based dendritic structures composed of 21 and 27 monosaccharide residues have been synthesized in a convergent manner from trisaccharide building blocks. The oligosaccharide AB2 monomers are based on a maltosyl beta(1-->6)galactose structure, which has been modified to include two methylamino groups at the primary positions of the glucosyl residues. Reductive alkylation of the secondary amino groups, with the innate formyl function of a second oligosaccharide monomer, allows for the chemoselective construction of dendritic wedges, while employing a minimal number of protecting groups. The first-generation dendron can be coupled either to another AB2 monomer, to give a second-generation dendron, or to a tris[2-(methylamino)ethyl]amine-based core moiety, to provide a carbohydrate-based dendrimer. Alternating alpha- and beta-glucosyl residues in the monomers and dendrons, simplifies 1H NMR spectra as a consequence of spreading out the anomeric proton signals. Monomers and dendrons were characterized by extensive one- and two-dimensional NMR spectroscopy in addition to FAB, electrospray, and MALDI-TOF mass spectrometry. Molecular dynamics simulations revealed similar conformations in the dendrons as in the isolated trisaccharide repeating units.

Do Glycosyl Sulfonium Ions Engage in Neighbouring-Group Participation? A Study of Oxathiane Glycosyl Donors and the Basis for their Stereoselectivity

Neighbouring-group participation has long been used to control the synthesis of 1,2-trans-glycosides. More recently there has been a growing interest in the development of similar strategies for the synthesis of 1,2-cis-glycosides, in particular the use of auxiliary groups that generate sulfonium ion intermediates. However, there has been some debate over the role of sulfonium ion intermediates in these reactions: do sulfonium ions actually engage in neighbouring-group participation, or are they a resting state of the system prior to reaction through an oxacarbenium ion intermediate? Herein, we describe the reactivities and stereoselectivities of a family of bicyclic thioglycosides in which an oxathiane ring is fused to the sugar to form a trans-decalin-like structure. A methyl sulfonium ion derived from one such glycosyl donor is so stable that it can be crystallised from ethanol, yet it reacts with complete stereoselectivity at high temperature. The importance of a ketal group in the oxathiane ring for maintaining this high stereoselectivity is investigated using a combination of experiment and ab initio calculations. The data are discussed in terms of S(N)1 and S(N)2 type mechanisms. Trends in stereoselectivity across a series of compounds are more consistent with selective addition to oxacarbenium ions rather than a shift between S(N)1 and S(N)2 mechanisms.

A Protein-Based Pentavalent Inhibitor of the Cholera Toxin B-Subunit

Protein toxins produced by bacteria are the cause of many life-threatening diarrheal diseases. Many of these toxins, including cholera toxin (CT), enter the cell by first binding to glycolipids in the cell membrane. Inhibiting these multivalent protein/carbohydrate interactions would prevent the toxin from entering cells and causing diarrhea. Here we demonstrate that the site-specific modification of a protein scaffold, which is perfectly matched in both size and valency to the target toxin, provides a convenient route to an effective multivalent inhibitor. The resulting pentavalent neoglycoprotein displays an inhibition potency (IC50) of 104 pm for the CT B-subunit (CTB), which is the most potent pentavalent inhibitor for this target reported thus far. Complexation of the inhibitor and CTB resulted in a protein heterodimer. This inhibition strategy can potentially be applied to many multivalent receptors and also opens up new possibilities for protein assembly strategies.

Hydrolase and sialyltransferase activities of Trypanosoma cruzi trans-sialidase towards NeuAc-α-2,3-Gal-β-O-PNP ☆

NeuAc-α-2,3-Gal-β-O-PNP has been synthesised and its ability to act as a substrate for the hydrolase and transferase activities of Trypanosoma cruzi trans-sialidase have been investigated. The turn-over of this compound shows marked differences from the behaviour of NeuAc-MU. In addition, distinct differences in the action of T. cruzi trans-sialidase and Clostridium perfringens neuraminidase on NeuAc-α-2,3-Gal-β-O-PNP were apparent.

Depsipeptide substrates for sortase-mediated N-terminal protein ligation

Technologies that allow the efficient chemical modification of proteins under mild conditions are widely sought after. Sortase-mediated peptide ligation provides a strategy for modifying the N or C terminus of proteins. This protocol describes the use of depsipeptide substrates (containing an ester linkage) with sortase A (SrtA) to completely modify proteins carrying a single N-terminal glycine residue under mild conditions in 4–6 h. The SrtA-mediated ligation reaction is reversible, so most labeling protocols that use this enzyme require a large excess of both substrate and sortase to produce high yields of ligation product. In contrast, switching to depsipeptide substrates effectively renders the reaction irreversible, allowing complete labeling of proteins with a small excess of substrate and catalytic quantities of sortase. Herein we describe the synthesis of depsipeptide substrates that contain an ester linkage between a threonine and glycolic acid residue and an N-terminal FITC fluorophore appended via a thiourea linkage. The synthesis of the depsipeptide substrate typically takes 2–3 d.

Mechanistic Studies on a Sulfoxide Transfer Reaction Mediated by Diphenyl Sulfoxide/Triflic Anhydride

Sulfoxides are frequently used in organic synthesis as chiral auxiliaries and reagents to mediate a wide variety of chemical transformations. For example, diphenyl sulfoxide and triflic anhydride can be used to activate a wide range of glycosyl donors including hemiacetals, glycals and thioglycosides. In this way, an alcohol, enol or sulfide is converted into a good leaving group for subsequent reaction with an acceptor alcohol. However, reaction of diphenyl sulfoxide and triflic anhydride with oxathiane-based thioglycosides, and other oxathianes, leads to a different process in which the thioglycoside is oxidised to a sulfoxide. This unexpected oxidation reaction is very stereoselective and proceeds under anhydrous conditions in which the diphenyl sulfoxide acts both as oxidant and as the source of the oxygen atom. Isotopic labelling experiments support a reaction mechanism that involves the formation of oxodisulfonium (S-O-S) dication intermediates. These intermediates undergo oxygen-exchange reactions with other sulfoxides and also allow interconversion of axial and equatorial sulfoxides in oxathiane rings. The reversibility of the oxygen-exchange reaction suggests that the stereochemical outcome of the oxidation reaction may be under thermodynamic control, which potentially presents a novel strategy for the stereoselective synthesis of sulfoxides.

Observations on chemical and enzymatic approaches to α-2,3-sialylated octyl β-lactoside

A comparison of chemical and chemo-enzymatic syntheses of α-2,3-sialylated octyl lactoside is reported. The chemical approach, starting from lactose and sialic acid, required 14 steps and proceeded in 5% overall yield; poor α-selectivity in the sialylation step necessitated a difficult and low yielding separation of anomers. A chemoenzymatic approach, employing recombinant Trypanosoma cruzi trans-sialidase to effect the key sialylation reaction, required 10 steps and gave a similar overall yield. Whereas the chemo-enzymatic synthesis required only three chromatographic purification steps overall, the chemical synthesis required at least nine.

Stereoselective glycosylations using oxathiane spiroketal glycosyl donors

Novel oxathiane spiroketal donors have been synthesised and activated via an umpolung S-arylation strategy using 1,3,5-trimethoxybenzene and 1,3-dimethoxybenzene. The comparative reactivity of the resulting 2,4,6-trimethoxyphenyl (TMP)- and 2,4-dimethoxyphenyl (DMP)-oxathiane spiroketal sulfonium ions is discussed, and their α-stereoselectivity in glycosylation reactions is compared to the analogous TMP- and DMP-sulfonium ions derived from an oxathiane glycosyl donor bearing a methyl ketal group. The results show that the stereoselectivity of the oxathiane glycosyl donors is dependent on the structure of the ketal group and reactivity can be tuned by varying the substituent on the sulfonium ion.