Crystal Structures of the Catalytic Domain of Arabidopsis thaliana Starch Synthase IV, of Granule Bound Starch Synthase From CLg1 and of Granule Bound Starch Synthase I of Cyanophora paradoxa Illustrate Substrate Recognition in Starch Synthases.

Starch synthases (SSs) are responsible for depositing the majority of glucoses in starch. Structural knowledge on these enzymes that is available from the crystal structures of rice granule bound starch synthase (GBSS) and barley SSI provides incomplete information on substrate binding and active site architecture. Here we report the crystal structures of the catalytic domains of SSIV from Arabidopsis thaliana, of GBSS from the cyanobacterium CLg1 and GBSSI from the glaucophyte Cyanophora paradoxa, with all three bound to ADP and the inhibitor acarbose. The SSIV structure illustrates in detail the modes of binding for both donor and acceptor in a plant SS. CLg1GBSS contains, in the same crystal structure, examples of molecules with and without bound acceptor, which illustrates the conformational changes induced upon acceptor binding that presumably precede catalytic activity. With structures available from several isoforms of plant and non-plant SSs, as well as the closely related bacterial glycogen synthases, we analyze, at the structural level, the common elements that define a SS, the elements that are necessary for substrate binding and singularities of the GBSS family that could underlie its processivity. While the phylogeny of the SSIII/IV/V has been recently discussed, we now further report the detailed evolutionary history of the GBSS/SSI/SSII type of SSs enlightening the origin of the GBSS enzymes used in our structural analysis.

Conserved residues Arg188 and Asp302 are critical for active site organization and catalysis in human ABO(H) blood group A and B glycosyltransferases.

Homologous glycosyltransferases GTA and GTB perform the final step in human ABO(H) blood group A and B antigen synthesis by transferring the sugar moiety from donor UDP-GalNAc/UDP-Gal to the terminal H antigen disaccharide acceptor. Like other GT-A fold family 6 glycosyltransferases, GTA and GTB undergo major conformational changes in two mobile regions, the C-terminal tail and internal loop, to achieve the closed, catalytic state. These changes are known to establish a salt bridge network among conserved active site residues Arg188, Asp211 and Asp302, which move to accommodate a series of discrete donor conformations while promoting loop ordering and formation of the closed enzyme state. However, the individual significance of these residues in linking these processes remains unclear. Here, we report the kinetics and high-resolution structures of GTA/GTB mutants of residues 188 and 302. The structural data support a conserved salt bridge network critical to mobile polypeptide loop organization and stabilization of the catalytically competent donor conformation. Consistent with the X-ray crystal structures, the kinetic data suggest that disruption of this salt bridge network has a destabilizing effect on the transition state, emphasizing the importance of Arg188 and Asp302 in the glycosyltransfer reaction mechanism. The salt bridge network observed in GTA/GTB structures during substrate binding appears to be conserved not only among other Carbohydrate Active EnZyme family 6 glycosyltransferases but also within both retaining and inverting GT-A fold glycosyltransferases. Our findings augment recently published crystal structures, which have identified a correlation between donor substrate conformational changes and mobile loop ordering.

High-resolution crystal structures and STD NMR mapping of human ABO(H) blood group glycosyltransferases in complex with trisaccharide reaction products suggest a molecular basis for product release.

The human ABO(H) blood group A- and B-synthesizing glycosyltransferases GTA and GTB have been structurally characterized to high resolution in complex with their respective trisaccharide antigen products. These findings are particularly timely and relevant given the dearth of glycosyltransferase structures collected in complex with their saccharide reaction products. GTA and GTB utilize the same acceptor substrates, oligosaccharides terminating with α-l-Fucp-(1→2)-ß-d-Galp-OR (where R is a glycolipid or glycoprotein), but use distinct UDP donor sugars, UDP-N-acetylgalactosamine and UDP-galactose, to generate the blood group A (α-l-Fucp-(1→2)[α-d-GalNAcp-(1→3)]-ß-d-Galp-OR) and blood group B (α-l-Fucp-(1→2)[α-d-Galp-(1→3)]-ß-d-Galp-OR) determinant structures, respectively. Structures of GTA and GTB in complex with their respective trisaccharide products reveal a conflict between the transferred sugar monosaccharide and the ß-phosphate of the UDP donor. Mapping of the binding epitopes by saturation transfer difference NMR measurements yielded data consistent with the X-ray structural results. Taken together these data suggest a mechanism of product release where monosaccharide transfer to the H-antigen acceptor induces active site disorder and ejection of the UDP leaving group prior to trisaccharide egress.

Functional and structural characterization of plastidic starch phosphorylase during barley endosperm development.

The production of starch is essential for human nutrition and represents a major metabolic flux in the biosphere. The biosynthesis of starch in storage organs like barley endosperm operates via two main pathways using different substrates: starch synthases use ADP-glucose to produce amylose and amylopectin, the two major components of starch, whereas starch phosphorylase (Pho1) uses glucose-1-phosphate (G1P), a precursor for ADP-glucose production, to produce α-1,4 glucans. The significance of the Pho1 pathway in starch biosynthesis has remained unclear. To elucidate the importance of barley Pho1 (HvPho1) for starch biosynthesis in barley endosperm, we analyzed HvPho1 protein production and enzyme activity levels throughout barley endosperm development and characterized structure-function relationships of HvPho1. The molecular mechanisms underlying the initiation of starch granule biosynthesis, that is, the enzymes and substrates involved in the initial transition from simple sugars to polysaccharides, remain unclear. We found that HvPho1 is present as an active protein at the onset of barley endosperm development. Notably, purified recombinant protein can catalyze the de novo production of α-1,4-glucans using HvPho1 from G1P as the sole substrate. The structural properties of HvPho1 provide insights into the low affinity of HvPho1 for large polysaccharides like starch or amylopectin. Our results suggest that HvPho1 may play a role during the initiation of starch biosynthesis in barley.

Protein NMR Studies of Substrate Binding to Human Blood Group†A and B Glycosyltransferases.

Donor and acceptor substrate binding to human blood group A and B glycosyltransferases (GTA, GTB) has been studied by a variety of protein NMR experiments. Prior crystallographic studies had shown these enzymes to adopt an open conformation in the absence of substrates. Binding either of the donor substrate UDP-Gal or of UDP induces a semiclosed conformation. In the presence of both donor and acceptor substrates, the enzymes shift towards a closed conformation with ordering of an internal loop and the C-terminal residues, which then completely cover the donor-binding pocket. Chemical-shift titrations of uniformly 2 H,15 N-labeled GTA or GTB with UDP affected about 20 % of all crosspeaks in 1 H,15 N TROSY-HSQC spectra, reflecting substantial plasticity of the enzymes. On the other hand, it is this conformational flexibility that impedes NH backbone assignments. Chemical-shift-perturbation experiments with δ1-[13 C]methyl-Ile-labeled samples revealed two Ile residues-Ile123 at the bottom of the UDP binding pocket, and Ile192 as part of the internal loop-that were significantly disturbed upon stepwise addition of UDP and H-disaccharide, also revealing long-range perturbations. Finally, methyl TROSY-based relaxation dispersion experiments do not reveal micro- to millisecond timescale motions. Although this study reveals substantial conformational plasticity of GTA and GTB, the matter of how binding of substrates shifts the enzymes into catalytically competent states remains enigmatic.

Waxy and non-waxy barley cultivars exhibit differences in the targeting and catalytic activity of GBSS1a.

Amylose synthesis is strictly associated with activity of granule-bound starch synthase (GBSS) enzymes. Among several crops there are cultivars containing starch types with either little or no amylose known as near-waxy or waxy. This (near) amylose-free phenotype is associated with a single locus (waxy) which has been mapped to GBSS-type genes in different crops. Most waxy varieties are a result of either low or no expression of a GBSS gene. However, there are some waxy cultivars where the GBSS enzymes are expressed normally. For these types, single nucleotide polymorphisms have been hypothesized to represent amino-acid substitutions leading to loss of catalytic activity. We here confirm that the HvGBSSIa enzyme from one such waxy barley variety, CDC_Alamo, has a 90% reduction in catalytic activity. We also engineered plants with expression of transgenic C-terminal green fluorescent protein-tagged HvGBSSIa of both the non-waxy type and of the CDC_Alamo type to monitor their subcellular localization patterns in grain endosperm. HvGBSSIa from non-waxy cultivars was found to localize in discrete concentric spheres strictly within starch granules. In contrast, HvGBSSIa from waxy CDC_Alamo showed deficient starch targeting mostly into unknown subcellular bodies of 0.5-3 µm in size, indicating that the waxy phenotype of CDC_Alamo is associated with deficient targeting of HvGBSSIa into starch granules.

Glycosyltransfer in mutants of putative catalytic residue Glu303 of the human ABO(H) A and B blood group glycosyltransferases GTA and GTB proceeds through a labile active site.

The homologous glycosyltransferases α-1,3-N-acetylgalactosaminyltransferase (GTA) and α-1,3-galactosyltransferase (GTB) carry out the final synthetic step of the closely related human ABO(H) blood group A and B antigens. The catalytic mechanism of these model retaining enzymes remains under debate, where Glu303 has been suggested to act as a putative nucleophile in a double displacement mechanism, a local dipole stabilizing the intermediate in an orthogonal associative mechanism or a general base to stabilize the reactive oxocarbenium ion-like intermediate in an SNi-like mechanism. Kinetic analysis of GTA and GTB point mutants E303C, E303D, E303Q and E303A shows that despite the enzymes having nearly identical sequences, the corresponding mutants of GTA/GTB have up to a 13-fold difference in their residual activities relative to wild type. High-resolution single crystal X-ray diffraction studies reveal, surprisingly, that the mutated Cys, Asp and Gln functional groups are no more than 0.8 Å further from the anomeric carbon of donor substrate compared to wild type. However, complicating the analysis is the observation that Glu303 itself plays a critical role in maintaining the stability of a strained "double-turn" in the active site through several hydrogen bonds, and any mutation other than E303Q leads to significantly higher thermal motion or even disorder in the substrate recognition pockets. Thus, there is a remarkable juxtaposition of the mutants E303C and E303D, which retain significant activity despite disrupted active site architecture, with GTB/E303Q, which maintains active site architecture but exhibits zero activity. These findings indicate that nucleophilicity at position 303 is more catalytically valuable than active site stability and highlight the mechanistic elasticity of these enzymes.

Structural insights and membrane binding properties of MGD1, the major galactolipid synthase in plants.

Monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are the major lipid components of photosynthetic membranes, and hence the most abundant lipids in the biosphere. They are essential for assembly and function of the photosynthetic apparatus. In Arabidopsis, the first step of galactolipid synthesis is catalyzed by MGDG synthase 1 (MGD1), which transfers a galactosyl residue from UDP-galactose to diacylglycerol (DAG). MGD1 is a monotopic protein that is embedded in the inner envelope membrane of chloroplasts. Once produced, MGDG is transferred to the outer envelope membrane, where DGDG synthesis occurs, and to thylakoids. Here we present two crystal structures of MGD1: one unliganded and one complexed with UDP. MGD1 has a long and flexible region (approximately 50 amino acids) that is required for DAG binding. The structures reveal critical features of the MGD1 catalytic mechanism and its membrane binding mode, tested on biomimetic Langmuir monolayers, giving insights into chloroplast membrane biogenesis. The structural plasticity of MGD1, ensuring very rapid capture and utilization of DAG, and its interaction with anionic lipids, possibly driving the construction of lipoproteic clusters, are consistent with the role of this enzyme, not only in expansion of the inner envelope membrane, but also in supplying MGDG to the outer envelope and nascent thylakoid membranes.

Novel UDP-GalNAc Derivative Structures Provide Insight into the Donor Specificity of Human Blood Group Glycosyltransferase.

Two closely related glycosyltransferases are responsible for the final step of the biosynthesis of ABO(H) human blood group A and B antigens. The two enzymes differ by only four amino acid residues, which determine whether the enzymes transfer GalNAc from UDP-GalNAc or Gal from UDP-Gal to the H-antigen acceptor. The enzymes belong to the class of GT-A folded enzymes, grouped as GT6 in the CAZy database, and are characterized by a single domain with a metal dependent retaining reaction mechanism. However, the exact role of the four amino acid residues in the specificity of the enzymes is still unresolved. In this study, we report the first structural information of a dual specificity cis-AB blood group glycosyltransferase in complex with a synthetic UDP-GalNAc derivative. Interestingly, the GalNAc moiety adopts an unusual yet catalytically productive conformation in the binding pocket, which is different from the "tucked under" conformation previously observed for the UDP-Gal donor. In addition, we show that this UDP-GalNAc derivative in complex with the H-antigen acceptor provokes the same unusual binding pocket closure as seen for the corresponding UDP-Gal derivative. Despite this, the two derivatives show vastly different kinetic properties. Our results provide a important structural insight into the donor substrate specificity and utilization in blood group biosynthesis, which can very likely be exploited for the development of new glycosyltransferase inhibitors and probes.

The Role of Cysteine Residues in Redox Regulation and Protein Stability of Arabidopsis thaliana Starch Synthase 1.

Starch biosynthesis in Arabidopsis thaliana is strictly regulated. In leaf extracts, starch synthase 1 (AtSS1) responds to the redox potential within a physiologically relevant range. This study presents data testing two main hypotheses: 1) that specific thiol-disulfide exchange in AtSS1 influences its catalytic function 2) that each conserved Cys residue has an impact on AtSS1 catalysis. Recombinant AtSS1 versions carrying combinations of cysteine-to-serine substitutions were generated and characterized in vitro. The results demonstrate that AtSS1 is activated and deactivated by the physiological redox transmitters thioredoxin f1 (Trxf1), thioredoxin m4 (Trxm4) and the bifunctional NADPH-dependent thioredoxin reductase C (NTRC). AtSS1 displayed an activity change within the physiologically relevant redox range, with a midpoint potential equal to -306 mV, suggesting that AtSS1 is in the reduced and active form during the day with active photosynthesis. Cys164 and Cys545 were the key cysteine residues involved in regulatory disulfide formation upon oxidation. A C164S_C545S double mutant had considerably decreased redox sensitivity as compared to wild type AtSS1 (30% vs 77%). Michaelis-Menten kinetics and molecular modeling suggest that both cysteines play important roles in enzyme catalysis, namely, Cys545 is involved in ADP-glucose binding and Cys164 is involved in acceptor binding. All the other single mutants had essentially complete redox sensitivity (98-99%). In addition of being part of a redox directed activity "light switch", reactivation tests and low heterologous expression levels indicate that specific cysteine residues might play additional roles. Specifically, Cys265 in combination with Cys164 can be involved in proper protein folding or/and stabilization of translated protein prior to its transport into the plastid. Cys442 can play an important role in enzyme stability upon oxidation. The physiological and phylogenetic relevance of these findings is discussed.

A diverse range of bacterial and eukaryotic chitinases hydrolyzes the LacNAc (Galß1-4GlcNAc) and LacdiNAc (GalNAcß1-4GlcNAc) motifs found on vertebrate and insect cells.

There is emerging evidence that chitinases have additional functions beyond degrading environmental chitin, such as involvement in innate and acquired immune responses, tissue remodeling, fibrosis, and serving as virulence factors of bacterial pathogens. We have recently shown that both the human chitotriosidase and a chitinase from Salmonella enterica serovar Typhimurium hydrolyze LacNAc from Galß1-4GlcNAcß-tetramethylrhodamine (LacNAc-TMR (Galß1-4GlcNAcß(CH2)8CONH(CH2)2NHCO-TMR)), a fluorescently labeled model substrate for glycans found in mammals. In this study we have examined the binding affinities of the Salmonella chitinase by carbohydrate microarray screening and found that it binds to a range of compounds, including five that contain LacNAc structures. We have further examined the hydrolytic specificity of this enzyme and chitinases from Sodalis glossinidius and Polysphondylium pallidum, which are phylogenetically related to the Salmonella chitinase, as well as unrelated chitinases from Listeria monocytogenes using the fluorescently labeled substrate analogs LacdiNAc-TMR (GalNAcß1-4GlcNAcß-TMR), LacNAc-TMR, and LacNAcß1-6LacNAcß-TMR. We found that all chitinases examined hydrolyzed LacdiNAc from the TMR aglycone to various degrees, whereas they were less active toward LacNAc-TMR conjugates. LacdiNAc is found in the mammalian glycome and is a common motif in invertebrate glycans. This substrate specificity was evident for chitinases of different phylogenetic origins. Three of the chitinases also hydrolyzed the ß1-6 bond in LacNAcß1-6LacNAcß-TMR, an activity that is of potential importance in relation to mammalian glycans. The enzymatic affinities for these mammalian-like structures suggest additional functional roles of chitinases beyond chitin hydrolysis.

Design of Glycosyltransferase Inhibitors: Serine Analogues as Pyrophosphate Surrogates?

Mimicking the diphosphate moiety of nucleotide diphosphate sugars with serine analogues provided modest glycosyltransferase inhibitors. The synthetic strategy employed a combination of glycosylation, amide bond formation and azide-alkyne "click" chemistry. Inhibition constants (Ki ) in the high micromolar range were obtained with a selection of five galactosyltransferases. Cocrystals of three inhibitors bound at the active site of a blood group A/B synthesizing glycosyltransferase were analysed. The structures and inhibitory patterns of the analogues demonstrate the flexibility of the enzymes which complicates the rational design of glycosyltransferase inhibitors.

In vitro Biochemical Characterization of All Barley Endosperm Starch Synthases.

Starch is the main storage polysaccharide in cereals and the major source of calories in the human diet. It is synthesized by a panel of enzymes including five classes of starch synthases (SSs). While the overall starch synthase (SS) reaction is known, the functional differences between the five SS classes are poorly understood. Much of our knowledge comes from analyzing mutant plants with altered SS activities, but the resulting data are often difficult to interpret as a result of pleitropic effects, competition between enzymes, overlaps in enzyme activity and disruption of multi-enzyme complexes. Here we provide a detailed biochemical study of the activity of all five classes of SSs in barley endosperm. Each enzyme was produced recombinantly in E. coli and the properties and modes of action in vitro were studied in isolation from other SSs and other substrate modifying activities. Our results define the mode of action of each SS class in unprecedented detail; we analyze their substrate selection, temperature dependence and stability, substrate affinity and temporal abundance during barley development. Our results are at variance with some generally accepted ideas about starch biosynthesis and might lead to the reinterpretation of results obtained in planta. In particular, they indicate that granule bound SS is capable of processive action even in the absence of a starch matrix, that SSI has no elongation limit, and that SSIV, believed to be critical for the initiation of starch granules, has maltoligosaccharides and not polysaccharides as its preferred substrates.

Structures of a human blood group glycosyltransferase in complex with a photo-activatable UDP-Gal derivative reveal two different binding conformations.

Glycosyltransferases (GTs) catalyse the sequential addition of monosaccharides to specific acceptor molecules and play major roles in key biological processes. GTs are classified into two main families depending on the inverted or retained stereochemistry of the glycosidic bond formed during the reaction. While the mechanism of inverting enzymes is well characterized, the precise nature of retaining GTs is still a matter of much debate. In an attempt to clarify this issue, studies were initiated to identify reaction-intermediate states by using a crystallographic approach based on caged substrates. In this paper, two distinct structures of AA(Gly)B, a dual-specificity blood group synthase, are described in complex with a UDP-galactose derivative in which the O6'' atom is protected by a 2-nitrobenzyl group. The distinct conformations of the caged substrate in both structures of the enzyme illustrate the highly dynamic nature of its active site. An attempt was also made to photolyse the caged compound at low temperature, which unfortunately is not possible without damaging the uracil group as well. These results pave the way for kinetic crystallography experiments aiming at trapping and characterizing reaction-intermediate states in the mechanism of enzymatic glycosyl transfer.

Flexibility and mutagenic resiliency of glycosyltransferases.

The human blood group A and B antigens are synthesized by two highly homologous enzymes, glycosyltransferase A (GTA) and glycosyltransferase B (GTB), respectively. These enzymes catalyze the transfer of either GalNAc or Gal from their corresponding UDP-donors to αFuc1-2ßGal-R terminating acceptors. GTA and GTB differ at only four of 354 amino acids (R176G, G235S, L266M, G268A), which alter the donor specificity from UDP-GalNAc to UDP-Gal. Blood type O individuals synthesize truncated or non-functional enzymes. The cloning, crystallization and X-ray structure elucidations for GTA and GTB have revealed key residues responsible for donor discrimination and acceptor binding. Structural studies suggest that numerous conformational changes occur during the catalytic cycle. Over 300 ABO alleles are tabulated in the blood group antigen mutation database (BGMUT) that provides a framework for structure-function studies. Natural mutations are found in all regions of GTA and GTB from the active site, flexible loops, stem region and surfaces remote from the active site. Our characterizations of natural mutants near a flexible loop (V175M), on a remote surface site (P156L), in the metal binding motif (M212V) and near the acceptor binding site (L232P) demonstrate the resiliency of GTA and GTB to mutagenesis.

Crystal structure of the Chlamydomonas starch debranching enzyme isoamylase ISA1 reveals insights into the mechanism of branch trimming and complex assembly.

The starch debranching enzymes isoamylase 1 and 2 (ISA1 and ISA2) are known to exist in a large complex and are involved in the biosynthesis and crystallization of starch. It is suggested that the function of the complex is to remove misplaced branches of growing amylopectin molecules, which would otherwise prevent the association and crystallization of adjacent linear chains. Here, we investigate the function of ISA1 and ISA2 from starch producing alga Chlamydomonas. Through complementation studies, we confirm that the STA8 locus encodes for ISA2 and sta8 mutants lack the ISA1·ISA2 heteromeric complex. However, mutants retain a functional dimeric ISA1 that is able to partly sustain starch synthesis in vivo. To better characterize ISA1, we have overexpressed and purified ISA1 from Chlamydomonas reinhardtii (CrISA1) and solved the crystal structure to 2.3 Å and in complex with maltoheptaose to 2.4 Å. Analysis of the homodimeric CrISA1 structure reveals a unique elongated structure with monomers connected end-to-end. The crystal complex reveals details about the mechanism of branch binding that explains the low activity of CrISA1 toward tightly spaced branches and reveals the presence of additional secondary surface carbohydrate binding sites.

Human chitotriosidase CHIT1 cross reacts with mammalian-like substrates.

Humans do not synthesize chitin, yet they produce a number of active and inactive chitinases. One of the active enzymes is chitotriosidase whose serum levels are elevated in a number of diseases such as Gaucher's disease and upon fungal infection. Since the biological role of chitotriosidase in disease pathogenesis is not understood we screened a panel of mammalian GlcNAc-containing glycoconjugates as alternate substrates. LacNAc and LacdiNAc-terminating substrates are hydrolyzed, the latter with a turnover comparable to that of pNP-chitotriose. Glycolipids or glycoproteins with LacNAc and LacdiNAc represent potential chitinase substrates and the subsequent alteration of glycosylation pattern could be a factor in disease pathogenesis.

Design of glycosyltransferase inhibitors: pyridine as a pyrophosphate surrogate.

A series of ten glycosyltransferase inhibitors has been designed and synthesized by using pyridine as a pyrophosphate surrogate. The series was prepared by conjugation of carbohydrate, pyridine, and nucleoside building blocks by using a combination of glycosylation, the Staudinger-Vilarrasa amide-bond formation, and azide-alkyne click chemistry. The compounds were evaluated as inhibitors of five metal-dependent galactosyltransferases. Crystallographic analyses of three inhibitors complexed in the active site of one of the enzymes confirmed that the pyridine moiety chelates the Mn(2+) ion causing a slight displacement (2 Å) from its original position. The carbohydrate head group occupies a different position than in the natural uridine diphosphate (UDP)-Gal substrate with little interaction with the enzyme.

Base-modified donor analogues reveal novel dynamic features of a glycosyltransferase.

Glycosyltransferases (GTs) are enzymes that are involved, as Nature's "glycosylation reagents," in many fundamental biological processes including cell adhesion and blood group biosynthesis. Although of similar importance to that of other large enzyme families such as protein kinases and proteases, the undisputed potential of GTs for chemical biology and drug discovery has remained largely unrealized to date. This is due, at least in part, to a relative lack of GT inhibitors and tool compounds for structural, mechanistic, and cellular studies. In this study, we have used a novel class of GT donor analogues to obtain new structural and enzymological information for a representative blood group GT. These analogues interfere with the folding of an internal loop and the C terminus, which are essential for catalysis. Our experiments have led to the discovery of an entirely new active site folding mode for this enzyme family, which can be targeted in inhibitor development, similar to the DFG motif in protein kinases. Taken together, our results provide new insights into substrate binding, dynamics, and utilization in this important enzyme family, which can very likely be harnessed for the rational development of new GT inhibitors and probes.

Thermodynamic signature of substrates and substrate analogs binding to human blood group B galactosyltransferase from isothermal titration calorimetry experiments.

It has been observed earlier that human blood group B galactosyltransferase (GTB) hydrolyzes its donor substrate UDP-Galactose (UDP-Gal) in the absence of acceptor substrate, and that this reaction is promoted by the presence of an acceptor substrate analog, α-L-Fuc-(1,2)-ß-D-3-deoxy-Gal-O-octyl (3DD). This acceleration of enzymatic hydrolysis of UDP-Gal was traced back to an increased affinity of GTB toward the donor substrate in the presence of 3DD. Herein, we present new thermodynamic data from isothermal titration calorimetry (ITC) on the binding of donor and acceptor substrates and analogs to GTB. ITC data are supplemented by surface plasmon resonance and STD-NMR titration experiments. These new data validate mutual allosteric control of binding of donor and acceptor substrates to GTB. It is of note that ITC experiments reveal significant differences in enthalpic and entropic contributions to binding of the natural donor substrate UDP-Gal, when compared with its analog UDP-Glucose (UDP-Glc). This may reflect different degrees of ordering of an internal loop (amino acids 176-194) and the C-terminus (amino acids 346-354), which close the binding pocket on binding of UDP-Gal or UDP-Glc. As both ligands have rather similar dissociation constants KD and almost identical modes of binding this finding is unexpected. Another surprising finding is that an acceptor analog, α-L-Fuc-(1,2)-ß-D-3-amino-3-deoxy-Gal-O-octyl (3AD) as well as the constituent monosaccharide ß-D-3-amino-3-deoxy-Gal-O-octyl (3AM) effectively inhibit enzymatic hydrolysis of UDP-Gal. This is unexpected, too, because in analogy to the effects of 3DD one would have predicted acceleration of enzymatic hydrolysis of UDP-Gal. It is difficult to explain these observations based on structural data alone. Therefore, our results highlight that there is an urgent need of experimental studies into the dynamic properties of GTB.