Bacterial glycosidases for the production of universal red blood cells

Application of α-N-acetylgalactosaminidase and α-galactosidase in AB to O red blood cells conversion

BACKGROUND

Enzymatical conversion of A or B RBCs into group O RBCs (ECORBCs) was achieved by using α-N-acetylgalactosaminidase and α-galactosidase, respectively. Now, we initiated AB to O-RBC conversion by using these two enzymes together. But α-N-acetylgalactosaminidase and α-galactosidase's preserving and their reaction buffer were quite different. The aim of this study is to confirm an available system for converting AB to O RBCs, especially to study the maximal permission amount of PCS which was brought to the system-accompanied enzyme addition.

METHOD

Enzyme activity was detected by using GalNAc-pNp or Gal-pNp as substrates. The efficiency of the conversion of A or B antigen was evaluated by routine method and measured by fluorescence-activated cell sorting analysis. The optimal buffer component and the doses of α-N-acetylgalactosaminidase and α-galactosidase were confirmed according to A and B antigen epitope removal efficiency.

RESULTS

The activity of α-N-acetylgalactosaminidase and α-galactosidase was not decreased drastically when they were kept in PCS Buffer in 4°C. The optimal reaction buffer composed of glycine 250 mM and NaCl 3 mM, pH 6.8 and PCS less than 10%(v/v). For converting A(1)B to O RBCs completely, the doses of α-N-acetylgalactosaminidase and α-galactosidase were confirmed as 0.015 mg/ml packed RBCs(pRBCs) for A(1) antigen epitopes and 0.005 mg/ml pRBCs for B epitopes. Approximately 0.004 mg α-N-acetylgalactosaminidase and 0.005 mg α-galactosidase were required to convert 1 ml pRBCs.

CONCLUSION

Our studies indicated that α-N-acetylgalactosaminidase and α-galactosidase were stable in PCS buffer and a modified protocol which was propitious to converting AB to O RBCs was provided.

N-acetylgalactosamine utilization pathway and regulon in proteobacteria: genomic reconstruction and experimental characterization in Shewanella

We used a comparative genomics approach to reconstruct the N-acetyl-d-galactosamine (GalNAc) and galactosamine (GalN) utilization pathways and transcriptional regulons in Proteobacteria. The reconstructed GalNAc/GalN utilization pathways include multiple novel genes with specific functional roles. Most of the pathway variations were attributed to the amino sugar transport, phosphorylation, and deacetylation steps, whereas the downstream catabolic enzymes in the pathway were largely conserved. The predicted GalNAc kinase AgaK, the novel variant of GalNAc-6-phosphate deacetylase AgaA(II) and the GalN-6-phosphate deaminase AgaS from Shewanella sp. ANA-3 were validated in vitro using individual enzymatic assays and reconstitution of the three-step pathway. By using genetic techniques, we confirmed that AgaS but not AgaI functions as the main GalN-6-P deaminase in the GalNAc/GalN utilization pathway in Escherichia coli. Regulons controlled by AgaR repressors were reconstructed by bioinformatics in most proteobacterial genomes encoding GalNAc pathways. Candidate AgaR-binding motifs share a common sequence with consensus CTTTC that was found in multiple copies and arrangements in regulatory regions of aga genes. This study provides comprehensive insights into the common and distinctive features of the GalNAc/GalN catabolism and its regulation in diverse Proteobacteria.

α-N-acetylgalactosaminidase from infant-associated bifidobacteria belonging to novel glycoside hydrolase family 129 is implicated in alternative mucin degradation pathway

Bifidobacteria inhabit the lower intestine of mammals including humans where the mucin gel layer forms a space for commensal bacteria. We previously identified that infant-associated bifidobacteria possess an extracellular membrane-bound endo-α-N-acetylgalactosaminidase (EngBF) that may be involved in degradation and assimilation of mucin-type oligosaccharides. However, EngBF is highly specific for core-1-type O-glycan (Galβ1-3GalNAcα1-Ser/Thr), also called T antigen, which is mainly attached onto gastroduodenal mucins. By contrast, core-3-type O-glycans (GlcNAcβ1-3GalNAcα1-Ser/Thr) are predominantly found on the mucins in the intestines. Here, we identified a novel α-N-acetylgalactosaminidase (NagBb) from Bifidobacterium bifidum JCM 1254 that hydrolyzes the Tn antigen (GalNAcα1-Ser/Thr). Sialyl and galactosyl core-3 (Galβ1-3/4GlcNAcβ1-3(Neu5Acα2-6)GalNAcα1-Ser/Thr), a major tetrasaccharide structure on MUC2 mucin primarily secreted from goblet cells in human sigmoid colon, can be serially hydrolyzed into Tn antigen by previously identified bifidobacterial extracellular glycosidases such as α-sialidase (SiaBb2), lacto-N-biosidase (LnbB), β-galactosidase (BbgIII), and β-N-acetylhexosaminidases (BbhI and BbhII). Because NagBb is an intracellular enzyme without an N-terminal secretion signal sequence, it is likely involved in intracellular degradation and assimilation of Tn antigen-containing polypeptides, which might be incorporated through unknown transporters. Thus, bifidobacteria possess two distinct pathways for assimilation of O-glycans on gastroduodenal and intestinal mucins. NagBb homologs are conserved in infant-associated bifidobacteria, suggesting a significant role for their adaptation within the infant gut, and they were found to form a new glycoside hydrolase family 129.

Generation and characterization of erythroid cells from human embryonic stem cells and induced pluripotent stem cells: an overview

Because of the imbalance in the supply and demand of red blood cells (RBCs), especially for alloimmunized patients or patients with rare blood phenotypes, extensive research has been done to generate therapeutic quantities of mature RBCs from hematopoietic stem cells of various sources, such as bone marrow, peripheral blood, and cord blood. Since human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) can be maintained indefinitely in vitro, they represent potentially inexhaustible sources of donor-free RBCs. In contrast to other ex vivo stem-cell-derived cellular therapeutics, tumorigenesis is not a concern, as RBCs can be irradiated without marked adverse effects on in vivo function. Here, we provide a comprehensive review of the recent publications relevant to the generation and characterization of hESC- and iPSC-derived erythroid cells and discuss challenges to be met before the eventual realization of clinical usage of these cells.

ABO research in the modern era of genomics

Research on ABO has advanced significantly in recent years. A database was established to manage the sequence information of an increasing number of novel alleles. Genome sequencings have identified ABO orthologues and paralogues in various organisms and enhanced the knowledge on the evolution of the ABO and related genes. The most prominent advancements include clarification of the association between ABO and different disease processes. For instance, ABO status affects the infectivity of certain strains of Helicobacter pylori and Noroviruses as well as the sequestration and rosetting of red blood cells infected with Plasmodium falciparum. Genome-wide association studies have conclusively linked the ABO locus to pancreatic cancer, venous thromboembolism, and myocardial infarction in the presence of coronary atherosclerosis. These findings suggest ABO's important role in determining an individual's susceptibility to such diseases. Furthermore, our understanding of the structures of A and B transferases and their enzymology has been dramatically improved. ABO has also become a research subject in neurobiology and the preparation of artificial/universal blood and became a topic in the pseudoscience of "blood type diets." With such new progress, it has become evident that ABO is a critical player in the modern era of genomic medicine. This article provides the most up-to-date information regarding ABO genomics.

Oxidoreductive cellulose depolymerization by the enzymes cellobiose dehydrogenase and glycoside hydrolase 61

Several members of the glycoside hydrolase 61 (GH61) family of proteins have recently been shown to dramatically increase the breakdown of lignocellulosic biomass by microbial hydrolytic cellulases. However, purified GH61 proteins have neither demonstrable direct hydrolase activity on various polysaccharide or lignacious components of biomass nor an apparent hydrolase active site. Cellobiose dehydrogenase (CDH) is a secreted flavocytochrome produced by many cellulose-degrading fungi with no well-understood biological function. Here we demonstrate that the binary combination of Thermoascus aurantiacus GH61A (TaGH61A) and Humicola insolens CDH (HiCDH) cleaves cellulose into soluble, oxidized oligosaccharides. TaGH61A-HiCDH activity on cellulose is shown to be nonredundant with the activities of canonical endocellulase and exocellulase enzymes in microcrystalline cellulose cleavage, and while the combination of TaGH61A and HiCDH cleaves highly crystalline bacterial cellulose, it does not cleave soluble cellodextrins. GH61 and CDH proteins are coexpressed and secreted by the thermophilic ascomycete Thielavia terrestris in response to environmental cellulose, and the combined activities of T. terrestris GH61 and T. terrestris CDH are shown to synergize with T. terrestris cellulose hydrolases in the breakdown of cellulose. The action of GH61 and CDH on cellulose may constitute an important, but overlooked, biological oxidoreductive system that functions in microbial lignocellulose degradation and has applications in industrial biomass utilization.

Redox mechanism of glycosidic bond hydrolysis catalyzed by 6-phospho-alpha-glucosidase: a DFT study

Glycosidic bonds are remarkably resistant to cleavage by chemical hydrolysis. Glycoside hydrolases catalyze their selective hydrolysis in oligosaccharides, polysaccharides, and glycoconjugates by following nonredox catalytic pathways or a net redox-neutral catalytic pathway using NAD(+) and divalent metal ions as cofactors. GlvA (6-phospho-alpha-glucosidase) is a glycosidase belonging to family GH4 and follows a regioselective redox-neutral mechanism of glycosidic-bond hydrolysis that favors alpha- over beta-glycosides. Its proposed catalytic mechanism can be divided into two half-reactions: the first one activates the glucopyranose ring by successively forming intermediates that are oxidized at the 3-, 2-, and 1-positions of the ring, which ultimately facilitate the heterolytic deglycosylation. The second half-reaction is essentially the reverse of the first half-reaction, beginning with the pyranose ring hydroxylation at the anomeric carbon, and it is followed by 3-reduction and regeneration of the active forms of the catalytic site and its cofactors. We investigated the NAD(+)-dependent redox mechanism of glycosidic bond hydrolysis as catalyzed by GlvA through the combined application of density functional theory and a self-consistent reaction field to a large active-site model obtained from the crystallographic structure of the enzyme, then we applied natural bond orbital and second-order perturbation analyses to monitor the electron flow and change in oxidation state on each atomic center along the reaction coordinate to rationalize the energetics and regioselectivity of this catalytic mechanism. We find that in GlvA, the redox catalytic mechanism of hydrolysis is driven by the gradual strengthening of the axial endo-anomeric component within the hexose ring along the reaction coordinate to facilitate the heterolytic dissociation of the axial C1-O bond. In addition, the combined influence of specific components of the generalized anomeric effect fully explains the regioselectivity observed in the catalytic activity of GlvA.

Glycoside hydrolases: catalytic base/nucleophile diversity

Recent studies have shown that a number of glycoside hydrolase families do not follow the classical catalytic mechanisms, as they lack a typical catalytic base/nucleophile. A variety of mechanisms are used to replace this function, including substrate-assisted catalysis, a network of several residues, and the use of non-carboxylate residues or exogenous nucleophiles. Removal of the catalytic base/nucleophile by mutation can have a profound impact on substrate specificity, producing enzymes with completely new functions.

Viewing the human microbiome through three-dimensional glasses: integrating structural and functional studies to better define the properties of myriad carbohydrate-active enzymes

Recent studies have provided an unprecedented view of the trillions of microbes associated with the human body. The human microbiome harbors tremendous diversity at multiple levels: the species that colonize each individual and each body habitat; the genes that are found in each organism's genome; the expression of these genes and the interactions and activities of their protein products. The sources of this diversity are wide-ranging and reflect both environmental and host factors. A major challenge moving forward is defining the precise functions of members of various families of proteins represented in our microbiomes, including the highly diverse carbohydrate-active enzymes (CAZymes) involved in numerous biologically important chemical transformations, such as the degradation of complex dietary polysaccharides. Coupling metagenomic analyses to structural genomics initiatives and to biochemical and other functional assays of CAZymes will be essential for determining how these as well as other microbiome-encoded proteins operate to shape the properties of microbial communities and their human hosts.

Brief report: a new profile of terminal N-acetyllactosamines glycans on pig red blood cells and different expression of alpha-galactose on Sika deer red blood cells and nucleated cells

It has been reported that: (1) large variations were found in the number of sialic acid (SA) capped with N-acetyllactosamines (SA-Galbeta1-4GlcNAc-R) and alpha-Gal epitopes (Galalpha1-3Galbeta1-4GlcNAc-R) or uncapped N-acetyllactosamines (Galbeta1-4GlcNAc-R) on different mammalian red blood cells, and on nucleated cells originating from a given tissue in various species; (2) goat, sheep, horse and mouse red blood cells lack alpha-Gal epitopes, despite the expression of this epitope on a variety of nucleated cells in these species, including lymphocytes differentiated from the same hematopoietic origin. In this study, flow cytometry and Western blot analyses of pig red blood cells showed that alpha-Gal epitopes on pig red cells developed concomitantly after treatment with neuraminidase, suggesting that the terminal N-acetyllactosaminide glycans were capped with SA-alpha-Gal epitopes. Whereas, the expression of the alpha-Gal epitopes on red blood cells from Sika deer (Cevus nippon hortulorum) were found to be absent even though the epitopes were present on their white blood cells. Thus, these results add new data not only for the terminal carbohydrate structures on cell surface glycans of various mammalian cells, but also for wide variety of epitope expression on the cells from different tissues, which might be useful for understanding their unique states resulting from differentiation and evolution.

The IgG-specific endoglycosidase EndoS inhibits both cellular and complement-mediated autoimmune hemolysis

EndoS from Streptococcus pyogenes is an immunomodulating enzyme that specifically hydrolyzes glycans from human immunoglobulin G and thereby affects antibody effector functions. Autoimmune hemolytic anemia is caused by antibody-mediated red blood cell (RBC) destruction and often resists treatment with corticosteroids that also cause frequent adverse effects. We show here that anti-RhD (anti-D) and rabbit anti-human-RBC antibodies (anti-RBC) mediated destruction of RBC, ie, phagocytosis, complement activation, and hemolysis in vitro and in vivo was inhibited by EndoS. Phagocytosis by monocytes in vitro was inhibited by pretreatment of anti-D with EndoS before sensitization of RBCs and abrogated by direct addition of EndoS to blood containing sensitized RBCs. The toxic effects of monocytes stimulated with anti-D-sensitized RBCs, as measured by interleukin-8 secretion and oxygen metabolite production, was restrained by EndoS. Agglutination of RBCs and complement-mediated hemolysis in vitro in whole human blood caused by rabbit anti-RBCs was inhibited by EndoS. Development of anemia in mice caused by a murine anti-RBC immunoglobulin G2a monoclonal autoantibody and complement activation and erythrophagocytosis by Kupffer cells in the liver were reduced by EndoS. Our data indicate that EndoS is a potential therapeutic agent that might be evaluated as an alternative to current treatment regimens against antibody-mediated destruction of RBCs.

Catalytic mechanism of human alpha-galactosidase

The enzyme alpha-galactosidase (alpha-GAL, also known as alpha-GAL A; E.C. 3.2.1.22) is responsible for the breakdown of alpha-galactosides in the lysosome. Defects in human alpha-GAL lead to the development of Fabry disease, a lysosomal storage disorder characterized by the buildup of alpha-galactosylated substrates in the tissues. alpha-GAL is an active target of clinical research: there are currently two treatment options for Fabry disease, recombinant enzyme replacement therapy (approved in the United States in 2003) and pharmacological chaperone therapy (currently in clinical trials). Previously, we have reported the structure of human alpha-GAL, which revealed the overall structure of the enzyme and established the locations of hundreds of mutations that lead to the development of Fabry disease. Here, we describe the catalytic mechanism of the enzyme derived from x-ray crystal structures of each of the four stages of the double displacement reaction mechanism. Use of a difluoro-alpha-galactopyranoside allowed trapping of a covalent intermediate. The ensemble of structures reveals distortion of the ligand into a (1)S(3) skew (or twist) boat conformation in the middle of the reaction cycle. The high resolution structures of each step in the catalytic cycle will allow for improved drug design efforts on alpha-GAL and other glycoside hydrolase family 27 enzymes by developing ligands that specifically target different states of the catalytic cycle. Additionally, the structures revealed a second ligand-binding site suitable for targeting by novel pharmacological chaperones.

Glycosidase inhibition: assessing mimicry of the transition state

Glycoside hydrolases, the enzymes responsible for hydrolysis of the glycosidic bond in di-, oligo- and polysaccharides, and glycoconjugates, are ubiquitous in Nature and fundamental to existence. The extreme stability of the glycosidic bond has meant these enzymes have evolved into highly proficient catalysts, with an estimated 10(17) fold rate enhancement over the uncatalysed reaction. Such rate enhancements mean that enzymes bind the substrate at the transition state with extraordinary affinity; the dissociation constant for the transition state is predicted to be 10(-22) M. Inhibition of glycoside hydrolases has widespread application in the treatment of viral infections, such as influenza and HIV, lysosomal storage disorders, cancer and diabetes. If inhibitors are designed to mimic the transition state, it should be possible to harness some of the transition state affinity, resulting in highly potent and specific drugs. Here we examine a number of glycosidase inhibitors which have been developed over the past half century, either by Nature or synthetically by man. A number of criteria have been proposed to ascertain which of these inhibitors are true transition state mimics, but these features have only be critically investigated in a very few cases.

The 1.9 a structure of human alpha-N-acetylgalactosaminidase: The molecular basis of Schindler and Kanzaki diseases

alpha-N-acetylgalactosaminidase (alpha-NAGAL; E.C. 3.2.1.49) is a lysosomal exoglycosidase that cleaves terminal alpha-N-acetylgalactosamine residues from glycopeptides and glycolipids. In humans, a deficiency of alpha-NAGAL activity results in the lysosomal storage disorders Schindler disease and Kanzaki disease. To better understand the molecular defects in the diseases, we determined the crystal structure of human alpha-NAGAL after expressing wild-type and glycosylation-deficient glycoproteins in recombinant insect cell expression systems. We measured the enzymatic parameters of our purified wild-type and mutant enzymes, establishing their enzymatic equivalence. To investigate the binding specificity and catalytic mechanism of the human alpha-NAGAL enzyme, we determined three crystallographic complexes with different catalytic products bound in the active site of the enzyme. To better understand how individual defects in the alpha-NAGAL glycoprotein lead to Schindler disease, we analyzed the effect of disease-causing mutations on the three-dimensional structure.

ABO blood group glycans modulate sialic acid recognition on erythrocytes

ABH(O) blood group polymorphisms are based on well-known intraspecies variations in structures of neutral blood cell surface glycans in humans and other primates. Whereas natural antibodies against these glycans can act as barriers to blood transfusion and transplantation, the normal functions of this long-standing evolutionary polymorphism remain largely unknown. Although microbial interactions have been suggested as a selective force, direct binding of lethal pathogens to ABH antigens has not been reported. We show in this study that ABH antigens found on human erythrocytes modulate the specific interactions of 3 sialic acid-recognizing proteins (human Siglec-2, 1918SC influenza hemagglutinin, and Sambucus nigra agglutinin) with sialylated glycans on the same cell surface. Using specific glycosidases that convert A and B glycans to the underlying H(O) structure, we show ABH antigens stabilize sialylated glycan clusters on erythrocyte membranes uniquely for each blood type, generating differential interactions of the 3 sialic acid-binding proteins with erythrocytes from each blood type. We further show that by stabilizing such structures ABH antigens can also modulate sialic acid-mediated interaction of pathogens such as Plasmodium falciparum malarial parasite. Thus, ABH antigens can noncovalently alter the presentation of other cell surface glycans to cognate-binding proteins, without themselves being a direct ligand.

Evolution and biochemistry of family 4 glycosidases: implications for assigning enzyme function in sequence annotations

Glycosyl hydrolase Family 4 (GH4) is exceptional among the 114 families in this enzyme superfamily. Members of GH4 exhibit unusual cofactor requirements for activity, and an essential cysteine residue is present at the active site. Of greatest significance is the fact that members of GH4 employ a unique catalytic mechanism for cleavage of the glycosidic bond. By phylogenetic analysis, and from available substrate specificities, we have assigned a majority of the enzymes of GH4 to five subgroups. Our classification revealed an unexpected relationship between substrate specificity and the presence, in each subgroup, of a motif of four amino acids that includes the active-site Cys residue: alpha-glucosidase, CHE(I/V); alpha-galactosidase, CHSV; alpha-glucuronidase, CHGx; 6-phospho-alpha-glucosidase, CDMP; and 6-phospho-beta-glucosidase, CN(V/I)P. The question arises: Does the presence of a particular motif sufficiently predict the catalytic function of an unassigned GH4 protein? To test this hypothesis, we have purified and characterized the alpha-glucoside-specific GH4 enzyme (PalH) from the phytopathogen, Erwinia rhapontici. The CHEI motif in this protein has been changed by site-directed mutagenesis, and the effects upon substrate specificity have been determined. The change to CHSV caused the loss of all alpha-glucosidase activity, but the mutant protein exhibited none of the anticipated alpha-galactosidase activity. The Cys-containing motif may be suggestive of enzyme specificity, but phylogenetic placement is required for confidence in that specificity. The Acholeplasma laidlawii GH4 protein is phylogenetically a phospho-beta-glucosidase but has a unique SSSP motif. Lacking the initial Cys in that motif it cannot hydrolyze glycosides by the normal GH4 mechanism because the Cys is required to position the metal ion for hydrolysis, nor can it use the more common single or double-displacement mechanism of Koshland. Several considerations suggest that the protein has acquired a new function as the consequence of positive selection. This study emphasizes the importance of automatic annotation systems that by integrating phylogenetic analysis, functional motifs, and bioinformatics data, may lead to innovative experiments that further our understanding of biological systems.

Differential recognition and hydrolysis of host carbohydrate antigens by Streptococcus pneumoniae family 98 glycoside hydrolases

The presence of a fucose utilization operon in the Streptococcus pneumoniae genome and its established importance in virulence indicates a reliance of this bacterium on the harvesting of host fucose-containing glycans. The identities of these glycans, however, and how they are harvested is presently unknown. The biochemical and high resolution x-ray crystallographic analysis of two family 98 glycoside hydrolases (GH98s) from distinctive forms of the fucose utilization operon that originate from different S. pneumoniae strains reveal that one enzyme, the predominant type among pneumococcal isolates, has a unique endo-beta-galactosidase activity on the LewisY antigen. Altered active site topography in the other species of GH98 enzyme tune its endo-beta-galactosidase activity to the blood group A and B antigens. Despite their different specificities, these enzymes, and by extension all family 98 glycoside hydrolases, use an inverting catalytic mechanism. Many bacterial and viral pathogens exploit host carbohydrate antigens for adherence as a precursor to colonization or infection. However, this is the first evidence of bacterial endoglycosidase enzymes that are known to play a role in virulence and are specific for distinct host carbohydrate antigens. The strain-specific distribution of two distinct types of GH98 enzymes further suggests that S. pneumoniae strains may specialize to exploit host-specific antigens that vary from host to host, a factor that may feature in whether a strain is capable of colonizing a host or establishing an invasive infection.

A genetic strategy to control expression of human blood group antigens in red blood cells generated in vitro

BACKGROUND

The ability to generate red blood cells of a chosen blood group phenotype would be a major advance in transfusion when considering low- and high-frequency blood group antigens.

STUDY DESIGN AND METHODS

Cord blood CD34+ cells undergoing erythroid differentiation in vitro were genetically manipulated with human immunodeficiency virus Type 1-derived lentiviral vectors expressing hUT-B1 cDNA (overexpression strategy) or bicistronic vectors expressing both enhanced green fluorescent protein and a short-hairpin RNA (shRNA) designed to silence SLC14A1(JK) gene that encodes hUT-B1 protein (silencing strategy). Resulting cell populations were analyzed by fluorescent-activated cell sorting and gel affinity column assay.

RESULTS

When transduced with hUT-B1 cDNA lentiviral vectors encoding JK*B and JK*A alleles, respectively, CD34+ cell-derived erythroid populations from Jk(a+b-) and Jk(a-b+) donors exhibited a Jk(a+b+) phenotype different from the original phenotype. In concomitant tests, Jk(a+b+) donor cells transduced with lentiviral vectors carrying a shRNA designed to interfere with hUT-B1 transcription showed a marked decrease in hUT-B1 expression and were assessed as null for Jk antigen by a routine assay.

CONCLUSION

In this work focusing on the Kidd blood group system that relies on expression of hUT-B1 glycoprotein under the Jk(a) or Jk(b) antigenic configurations, we demonstrated that hematopoietic progenitors could be genetically modified to exhibit a chosen Kidd phenotype. Beyond production of atypical Kidd phenotypes, this genetic strategy could allow generation of rare blood phenotypes from hematopoietic stem cells regardless of initial donor phenotype. Potential applications for genetically modified blood include production of control samples for immunohematologic testing and for resolution of antibody detection in multiply transfused patients.

Glycans and glycosylation of platelets: current concepts and implications for transfusion

PURPOSE OF REVIEW

Platelet products are currently stored at room temperature, because refrigeration causes their rapid clearance from the circulation upon transfusion. Glycans have recently been emphasized as important determinants for the clearance of refrigerated platelets. The present review addresses the current knowledge of platelet glycans and the potential of glycosylation for improving platelet storage.

RECENT FINDINGS

Removal of refrigerated platelets from the circulation is partly mediated by recognition of clustered beta-N-acetylglucosamine on platelet surface glycoproteins by the alphaMbeta2 hepatic lectin receptor. Capping the exposed beta-N-acetylglucosamine residues by enzymatic galactosylation restored the circulation of short-term chilled murine platelets, introducing a novel method that allows for cold storage of platelet. Recent studies have, however, shown that galactosylation is not sufficient to restore circulation of long-term refrigerated platelets. Additional data indicate that differential carbohydrate-mediated mechanisms may exist for clearance of short-term and long-term cold-stored platelets.

SUMMARY

Room temperature storage of platelet products increases the risk of transfusion-mediated sepsis and accelerates platelet deterioration, limiting platelet shelf life. Recent evidence suggests that glycoengineering of platelets might allow for their cold storage, significantly improving the quality of platelet products.

Mechanistic insights into glycosidase chemistry

The enzymatic hydrolysis of the glycosidic bond continues to gain importance, reflecting the critically important roles complex glycans play in health and disease as well as the rekindled interest in enzymatic biomass conversion. Recent advances include the broadening of our understanding of enzyme reaction coordinates, through both computational and structural studies, improved understanding of enzyme inhibition through transition state mimicry and fascinating insights into mechanism yielded by physical organic chemistry approaches.

Glycosidases: a key to tailored carbohydrates

In recent years, carbohydrate-processing enzymes have become the enzymes of choice in many applications thanks to their stereoselectivity and efficiency. This review presents recent developments in glycosidase-catalyzed synthesis via two complementary approaches: the use of wild-type enzymes with engineered substrates, and mutant glycosidases. Genetic engineering has recently produced glucuronyl synthases, an inverting xylosynthase and the first mutant endo-beta-N-acetylglucosaminidase. A thorough selection of enzyme strains and aptly modified substrates have resulted in rare glycostructures, such as N-acetyl-beta-galactosaminuronates, beta1,4-linked mannosides and alpha1,4-linked galactosides. The efficient selection of mutant enzymes is facilitated by high-throughput screening assays involving the co-expression of coupled enzymes or chemical complementation. Selective glycosidase inhibitors and highly specific glycosidases are finding attractive applications in biomedicine, biology and proteomics.

Non-Stick Sugars: Synthesis of Difluorosugar Fluorides as Potential Glycosidase Inactivators

Four new difluorosugar fluorides, 2-deoxy-2,5-difluoro-α-l-idopyranosyl fluoride, 1,5-difluoro-d-glucopyranosyl fluoride, 1,5-difluoro-l-idopyranosyl fluoride, and 2-deoxy-1,2-difluoro-d-glucopyranosyl fluoride, were synthesized from known precursors by a radical bromination/fluoride displacement sequence, followed by deprotection. The compounds were tested as time-dependent inactivators of the β-glucosidase from Agrobacterium sp. (Abg, EC 3.2.1.21) and, while they were shown to bind to the enzyme active site as reversible competitive inhibitors, the only time-dependent inactivation observed was traced to the presence of an extremely small amount (<0.1%) of a highly reactive contaminating impurity.

Removal of blood group A/B antigen in organs by ex vivo and in vivo administration of endo-beta-galactosidase (ABase) for ABO-incompatible transplantation

BACKGROUND

ABO incompatibility in organ transplantation is still a high risk factor for antibody-mediated rejection, despite the progress in effective treatments. We have explored the possibility of using the enzyme to remove the blood type A/B antigen in organs.

METHODS

Recombinant endo-beta-galactosidase (ABase), which releases A/B antigen, was produced in E. coli BL-21. Human A/B red blood cells (RBC) were digested with ABase, and subjected to flow cytometric analysis after incubation with human sera. Purified recombinant ABase was intravenously administered to a baboon. Biopsies were taken from kidney and liver before and 1, 4 and 24 h after in vivo administration. Excised baboon kidneys were perfused with cold UW solution+/-purified recombinant ABase and preserved at 4 degrees C. Biopsies were taken before and 1 and 4 h after ex vivo perfusion. The change in A/B antigen expression was analyzed by immunohistochemical study.

RESULTS

ABase removed 82% of A antigen and 95% of B antigen in human A/B red blood cells, and suppressed anti-A/B antibody binding and complement activation effectively. ABase was also found to remain active at 4 degrees C. In vivo infusion of ABase into a blood type A baboon demonstrated a marked reduction of A antigen expression in the glomeruli of kidney (85% at 1 h, 9% at 4 h and 13% at 24 h) and the sinusoids of liver (47% at 1 h, 1% at 4 h and 3% at 24 h) without serious adverse effects. After ex vivo perfusion and cold storage of excised baboon kidney (blood type B) with ABase, the expression levels of B antigen in glomeruli were reduced to 49% at 1 h and 6% at 4 h.

CONCLUSIONS

This alternative approach might be useful for minimizing antibody removal and anti-B cell immunosuppression as an adjuvant therapy in ABO-incompatible kidney, liver and possibly heart transplantation.

Glycosyltransferases: structures, functions, and mechanisms

Glycosyltransferases catalyze glycosidic bond formation using sugar donors containing a nucleoside phosphate or a lipid phosphate leaving group. Only two structural folds, GT-A and GT-B, have been identified for the nucleotide sugar-dependent enzymes, but other folds are now appearing for the soluble domains of lipid phosphosugar-dependent glycosyl transferases. Structural and kinetic studies have provided new insights. Inverting glycosyltransferases utilize a direct displacement S(N)2-like mechanism involving an enzymatic base catalyst. Leaving group departure in GT-A fold enzymes is typically facilitated via a coordinated divalent cation, whereas GT-B fold enzymes instead use positively charged side chains and/or hydroxyls and helix dipoles. The mechanism of retaining glycosyltransferases is less clear. The expected two-step double-displacement mechanism is rendered less likely by the lack of conserved architecture in the region where a catalytic nucleophile would be expected. A mechanism involving a short-lived oxocarbenium ion intermediate now seems the most likely, with the leaving phosphate serving as the base.

Identification of a GH110 subfamily of alpha 1,3-galactosidases: novel enzymes for removal of the alpha 3Gal xenotransplantation antigen

In search of alpha-galactosidases with improved kinetic properties for removal of the immunodominant alpha1,3-linked galactose residues of blood group B antigens, we recently identified a novel prokaryotic family of alpha-galactosidases (CAZy GH110) with highly restricted substrate specificity and neutral pH optimum (Liu, Q. P., Sulzenbacher, G., Yuan, H., Bennett, E. P., Pietz, G., Saunders, K., Spence, J., Nudelman, E., Levery, S. B., White, T., Neveu, J. M., Lane, W. S., Bourne, Y., Olsson, M. L., Henrissat, B., and Clausen, H. (2007) Nat. Biotechnol. 25, 454-464). One member of this family from Bacteroides fragilis had exquisite substrate specificity for the branched blood group B structure Galalpha1-3(Fucalpha1-2)Gal, whereas linear oligosaccharides terminated by alpha1,3-linked galactose such as the immunodominant xenotransplantation epitope Galalpha1-3Galbeta1-4GlcNAc did not serve as substrates. Here we demonstrate the existence of two distinct subfamilies of GH110 in B. fragilis and thetaiotaomicron strains. Members of one subfamily have exclusive specificity for the branched blood group B structures, whereas members of a newly identified subfamily represent linkage specific alpha1,3-galactosidases that act equally well on both branched blood group B and linear alpha1,3Gal structures. We determined by one-dimensional (1)H NMR spectroscopy that GH110 enzymes function with an inverting mechanism, which is in striking contrast to all other known alpha-galactosidases that use a retaining mechanism. The novel GH110 subfamily offers enzymes with highly improved performance in enzymatic removal of the immunodominant alpha3Gal xenotransplantation epitope.

Modulating the red cell membrane to produce universal/stealth donor red cells suitable for transfusion

Two approaches have been used to produce universal group O donor red blood cells (RBCs) from groups A, B, and AB RBCs. The first involves cleavage of the terminal immunodominant sugars from carbohydrate chains on the RBC membrane, using specific enzymes, to produce so-called enzyme-converted group O (ECO) RBCs. ECO RBCs have been produced from whole units of B RBCs and transfused successfully to humans. Group A RBCs (especially A(1) RBCs) have been more difficult. New sources of enzymes have produced ECO RBCs from A(1) and A(2) RBCs that do not react with powerful monoclonal anti-A. Unfortunately, there are still problems encountered with polyclonal human antibodies (i.e. cross-matching). The second approach interferes with an antibody reaching its specific antigen on the RBC membrane by bonding polyethylene glycol (PEG) to the RBC. PEG will attract water molecules, yielding a combination that may block most RBC antigens, including A and B antigens. Initial excitement generated by preliminary reports of the possibility of producing 'stealth' PEG-RBCs were tempered by the findings of in vitro serological problems and possible reduced in vivo RBC survival. Many of these problems were solved, but recent findings that PEG is immunogenic in animals and humans, and that PEG antibodies can shorten the survival of PEG-RBCs (in rabbits) and pegylated proteins (e.g. PEG-asparaginase) in humans, are disturbing, suggesting that 'stealth' RBCs may never become a reality.