Hydrogen/deuterium exchange-LC-MS approach to characterize the action of heparan sulfate C5-epimerase

Interacting polymer-modification enzymes in heparan sulfate biosynthesis

Glucuronyl 5-epimerase (Hsepi) converts D-glucuronic acid (GlcA) into L-iduronic acid (IdoA) units, through a mechanism involving reversible abstraction of a proton at C5 of hexuronic acid residues. Incubations of a [4GlcAβ1-4GlcNSO₃α1-]_n precursor substrate with recombinant enzymes in a D₂O/H₂O medium enabled an isotope exchange approach to the assessment of functional interactions of Hsepi with hexuronyl 2-O-sulfotransferase (Hs2st) and glucosaminyl 6-O-sulfotransferase (Hs6st), both involved in the final polymer-modification steps. Enzyme complexes were supported by computational modeling and homogeneous time resolved fluorescence. GlcA and IdoA D/H ratios related to product composition revealed kinetic isotope effects that were interpreted in terms of efficiency of the coupled epimerase and sulfotransferase reactions. Evidence for a functional Hsepi/Hs6st complex was provided by selective incorporation of D atoms into GlcA units adjacent to 6-O-sulfated glucosamine residues. The inability to achieve simultaneous 2-O- and 6-O-sulfation in vitro supported topologically separated reactions in the cell. These findings provide novel insight into the roles of enzyme interactions in heparan sulfate biosynthesis.

Methods to Analyze the Effect of Diet-Derived Metabolites on Endothelial Inflammation and Cell Surface Glycosaminoglycans

The glycocalyx is a biologically active barrier that covers the luminal side of the vascular endothelium and it is comprised of proteoglycans [core proteins with glycosaminoglycans (GAG) side chains], glycoproteins, and plasma proteins. Evidence shows that the disruption in the structure and function of the endothelial glycocalyx exacerbates vascular inflammation and atherosclerosis. The GAG components of the glycocalyx undergo remodeling in the setting of diabetes and these alterations in endothelial GAGs negatively impact the vascular function. Hence, the preservation and restoration of GAGs in altered vasculature may be a novel strategy to ameliorate vascular complications in diabetes and metabolic syndrome. Human studies support the beneficial vascular effects of flavonoids which are widely found in fruits and vegetables. Flavonoids are extensively metabolized by the intestinal microbiota and digestive enzymes in humans, suggesting that their biological activities may be mediated by their circulating metabolites. Studies indicate that counteracting the damage to GAGs using dietary compounds improve vascular complications. In this article, we describe the methods to analyze the effect of diet-derived metabolites such as metabolites of flavonoids on endothelial inflammation and cell surface glycosaminoglycans.

Elucidating the unusual reaction kinetics of D-glucuronyl C5-epimerase

The chemoenzymatic synthesis of heparin, through a multienzyme process, represents a critical challenge in providing a safe and effective substitute for this animal-sourced anticoagulant drug. D-glucuronyl C5-epimerase (C5-epi) is an enzyme acting on a heparin precursor, N-sulfoheparosan, catalyzing the reversible epimerization of D-glucuronic acid (GlcA) to L-iduronic acid (IdoA). The absence of reliable assays for C5-epi has limited elucidation of the enzymatic reaction and kinetic mechanisms. Real time and offline assays are described that rely on 1D 1H NMR to study the activity of C5-epi. Apparent steady-state kinetic parameters for both the forward and the pseudo-reverse reactions of C5-epi are determined for the first time using polysaccharide substrates directly relevant to the chemoenzymatic synthesis and biosynthesis of heparin. The forward reaction shows unusual sigmoidal kinetic behavior, and the pseudo-reverse reaction displays nonsaturating kinetic behavior. The atypical sigmoidal behavior of the forward reaction was probed using a range of buffer additives. Surprisingly, the addition of 25 mM each of CaCl2 and MgCl2 resulted in a forward reaction exhibiting more conventional Michaelis-Menten kinetics. The addition of 2-O-sulfotransferase, the next enzyme involved in heparin synthesis, in the absence of 3'-phosphoadenosine 5'-phosphosulfate, also resulted in C5-epi exhibiting a more conventional Michaelis-Menten kinetic behavior in the forward reaction accompanied by a significant increase in apparent Vmax. This study provides critical information for understanding the reaction kinetics of C5-epi, which may result in improved methods for the chemoenzymatic synthesis of bioengineered heparin.

Heavy Heparin: A Stable Isotope-Enriched, Chemoenzymatically-Synthesized, Poly-Component Drug

Heparin is a highly sulfated, complex polysaccharide and widely used anticoagulant pharmaceutical. In this work, we chemoenzymatically synthesized perdeuteroheparin from biosynthetically enriched heparosan precursor obtained from microbial culture in deuterated medium. Chemical de-N-acetylation, chemical N-sulfation, enzymatic epimerization, and enzymatic sulfation with recombinant heparin biosynthetic enzymes afforded perdeuteroheparin comparable to pharmaceutical heparin. A series of applications for heavy heparin and its heavy biosynthetic intermediates are demonstrated, including generation of stable isotope labeled disaccharide standards, development of a non-radioactive NMR assay for glucuronosyl-C5-epimerase, and background-free quantification of in vivo half-life following administration to rabbits. We anticipate that this approach can be extended to produce other isotope-enriched glycosaminoglycans.

Blueberry metabolites restore cell surface glycosaminoglycans and attenuate endothelial inflammation in diabetic human aortic endothelial cells

BACKGROUND

Glycosaminoglycan (GAG), a major component of the endothelial glycocalyx, is severely perturbed in diabetic vasculature leading to endothelial inflammation and vascular disease in diabetes. We tested the hypothesis that blueberry metabolites (BBM) ameliorate endothelial inflammation in diabetic endothelial cells (ECs) by restoring cell surface GAGs.

METHODS

ECs isolated from healthy individuals [human aortic ECs (HAECs)] and diabetic patients (diabetic HAECs) were treated with ±BBM (benzoic acid-4-sulfate, hippuric acid, hydroxyhippuric acid, isovanillic acid-3-sulfate, and vanillic acid-4-sulfate at concentrations known to circulate in human plasma following blueberry consumption) for 3 days, and indices for endothelial inflammation were measured. To analyze GAGs, ECs were incubated with sulfate-free medium supplemented with [₃₅S] Na₂SO₄ ± BBM. Total GAGs in ECs and medium were purified using DEAE-Sepharose column and were analyzed with high-pressure liquid chromatography coupled to an inline flow scintillation analyzer. Heparan sulfate/chondroitin sulfate ratio and disaccharide composition of GAGs from the medium were analyzed using DEAE-3SW column and Dionex CarboPac PA1 column, respectively.

RESULTS

BBM suppressed diabetes-induced monocyte binding to ECs, and reduced the expression of inflammatory markers in diabetic HAECs. Diabetic HAECs displayed a decrease in [³⁵S] sulfate incorporation into the cell surface GAGs indicating the dysregulation of sulfated GAGs. However, treatment with BBM restored the levels of GAGs in diabetic HAECs. The composition, heparan sulfate/chondroitin sulfate ratio, and disaccharide composition of GAGs from medium were similar among groups.

CONCLUSIONS

BBM restored cell surface GAGs and attenuated endothelial inflammation in diabetic HAECs. Blueberry might complement conventional therapies to improve vascular complications in diabetes.

Deciphering the Mode of Action of the Processive Polysaccharide Modifying Enzyme Dermatan Sulfate Epimerase 1 by Hydrogen-Deuterium Exchange Mass Spectrometry

Distinct from template-directed biosynthesis of nucleic acids and proteins, the enzymatic synthesis of heterogeneous polysaccharides is a complex process that is difficult to study using common analytical tools. Therefore, the mode of action and processivity of those enzymes are largely unknown. Dermatan sulfate epimerase 1 (DS-epi1) is the predominant enzyme during the formation of iduronic acid residues in the glycosaminoglycan dermatan sulfate. Using recombinant DS-epi1 as a model enzyme, we describe a tandem mass spectrometry-based method to study the mode of action of polysaccharide processing enzymes. The enzyme action on the substrate was monitored by hydrogen-deuterium exchange mass spectrometry and the sequence information was then fed into mathematical models with two different assumptions of the mode of action for the enzyme: processive reducing end to non-reducing end, and processive non-reducing end to reducing end. Model data was scored by correlation to experimental data and it was found that DS-epi1 attacks its substrate on a random position, followed by a processive mode of modification towards the non-reducing end and that the substrate affinity of the enzyme is negatively affected by each additional epimerization event. It could also be shown that the smallest active substrate was the reducing end uronic acid in a tetrasaccharide and that octasaccharides and longer oligosaccharides were optimal substrates. The method of using tandem mass spectrometry to generate sequence information of the complex enzymatic products in combination with in silico modeling can be potentially applied to study the mode of action of other enzymes involved in polysaccharide biosynthesis.

C5-epimerase and 2-O-sulfotransferase associate in vitro to generate contiguous epimerized and 2-O-sulfated heparan sulfate domains

Heparan sulfate (HS), a complex polysaccharide of the cell surface, is endowed with the remarkable ability to bind numerous proteins and, as such, regulates a large variety of biological processes. Protein binding depends on HS structure; however, in the absence of a template driving its biosynthesis, the mechanism by which protein binding sequences are assembled remains poorly known. Here, we developed a chemically defined 13C-labeled substrate and NMR based experiments to simultaneously follow in real time the activity of HS biosynthetic enzymes and characterize the reaction products. Using this new approach, we report that the association of C5-epimerase and 2-O-sulfotransferase, which catalyze the production of iduronic acid and its 2-O-sulfation, respectively, is necessary to processively generate extended sequences of contiguous IdoA2S-containing disaccharides, whereas modifications are randomly introduced when the enzymes are uncoupled. These data shed light on the mechanisms by which HS motifs are generated during biosynthesis. They support the view that HS structure assembly is controlled not only by the availability of the biosynthetic enzymes but also by their physical association, which in the case of the C5-epimerase and 2-O-sulfotransferase was characterized by an affinity of 80 nM as demonstrated by surface plasmon resonance experiments.

Disaccharide analysis of glycosaminoglycans using hydrophilic interaction chromatography and mass spectrometry

Heparan sulfate (HS) and chondroitin sulfate/dermatan sulfate (CS/DS) glycosaminoglycans (GAGs) participate in many important biological processes. Quantitative disaccharide analysis of HS and CS/DS is essential for the characterization of GAGs and enables modeling of the GAG domain structure. Methods involving enzymatic digestion and chemical depolymerization have been developed to determine the type and location of sulfation/acetylation modifications as well as uronic acid epimerization. Enzymatic digestion generates disaccharides with Δ-4,5-unsaturation at the nonreducing end. Chemical depolymerization with nitrous acid retains the uronic acid epimerization. This work shows the use of hydrophilic interaction liquid chromatography mass spectrometry (HILIC-MS) for quantification of both enzyme-derived and nitrous acid depolymerization products for structural analysis of HS and CS/DS. This method enables biomedical researchers to determine complete disaccharide profiles on GAG samples using a single LC-MS platform.

Mass spectrometric method for determining the uronic acid epimerization in heparan sulfate disaccharides generated using nitrous acid

Heparan sulfate (HS) glycosaminoglycans (GAGs) regulate a host of biological functions. To better understand their biological roles, it is necessary to gain understanding about the structure of HS, which requires identification of the sulfation pattern as well as the uronic acid epimerization. In order to model HS structure, it is necessary to quantitatively profile depolymerization products. To date, liquid chromatography-mass spectrometry (LC-MS) methods for profiling heparin lyase decomposition products have been shown. These enzymes, however, destroy information about uronic acid epimerization. Deaminative cleavage using nitrous acid (HONO) is a classic method for GAG depolymerization that retains uronic acid epimerization. Several chromatographic methods have been used for analysis of deaminative cleavage products. The chromatographic methods have the disadvantage that there is no direct readout on the structures producing the observed peaks. This report demonstrates a porous graphitized carbon (PGC)-MS method for the quantification of HONO generated disaccharides to obtain information about the sulfation pattern and uronic acid epimerization. Here, we demonstrate the separation and identification of uronic acid epimers as well as geometric sulfation isomers. The results are comparable to those expected for benchmark HS and heparin samples. The data demonstrate the utility of PGC-MS for quantification of HS nitrous acid depolymerization products for structural analysis of HS and heparin.

Recent advances in sulfotransferase enzyme activity assays

Sulfotransferases are enzymes that catalyze the transfer of sulfo groups from a donor, for example 3'-phosphoadenosine 5'-phosphosulfate, to an acceptor, for example the amino or hydroxyl groups of a small molecule, xenobiotic, carbohydrate, or peptide. These enzymes are important targets in the design of novel therapeutics for treatment of a variety of diseases. This review examines assays used for this important class of enzyme, paying particular attention to sulfotransferases acting on carbohydrates and peptides and the major challenges associated with their analysis.

Uncovering biphasic catalytic mode of C5-epimerase in heparan sulfate biosynthesis

Heparan sulfate (HS), a highly sulfated polysaccharide, is biosynthesized through a pathway involving several enzymes. C(5)-epimerase (C(5)-epi) is a key enzyme in this pathway. C(5)-epi is known for being a two-way catalytic enzyme, displaying a "reversible" catalytic mode by converting a glucuronic acid to an iduronic acid residue, and vice versa. Here, we discovered that C(5)-epi can also serve as a one-way catalyst to convert a glucuronic acid to an iduronic acid residue, displaying an "irreversible" catalytic mode. Our data indicated that the reversible or irreversible catalytic mode strictly depends on the saccharide substrate structures. The biphasic mode of C(5)-epi offers a novel mechanism to regulate the biosynthesis of HS with the desired biological functions.