Functional molecular mass of a vertebrate hyaluronan synthase as determined by radiation inactivation analysis

Advances in hyaluronic acid production: Biosynthesis and genetic engineering strategies based on Streptococcus - A review

Hyaluronic acid (HA), which is a highly versatile glycosaminoglycan, is widely applied across the fields of food, cosmetics, and pharmaceuticals. It is primary produced through Streptococcus fermentation, but the product presents inherent challenges concerning consistency and potential pathogenicity. However, recent strides in molecular biology have paved the way for genetic engineering, which facilitates the creation of high-yield, nonpathogenic strains adept at synthesizing HA with specific molecular weights. This comprehensive review extensively explores the molecular biology underpinning pivotal HA synthase genes, which elucidates the intricate mechanisms governing HA synthesis. Moreover, it delineates various strategies employed in engineering HA-producing strains.

Hyaluronan synthases; mechanisms, myths, & mysteries of three types of unique bifunctional glycosyltransferases

Hyaluronan (HA), the essential [-3-GlcNAc-1-β-4-GlcA-1-β-]n matrix polysaccharide in vertebrates and molecular camouflage coating in select pathogens, is polymerized by "HA synthase" (HAS) enzymes. The first HAS identified three decades ago opened the window for new insights and biotechnological tools. This review discusses current understanding of HA biosynthesis, its biotechnological utility, and addresses some misconceptions in the literature. HASs are fascinating enzymes that polymerize two different UDP-activated sugars via different glycosidic linkages. Therefore, these catalysts were the first examples to break the "one enzyme/one sugar transferred" dogma. Three distinct types of these bifunctional glycosyltransferases (GTs) with disparate architectures and reaction modes are known. Based on biochemical and structural work, we present an updated classification system. Class I membrane-integrated HASs employ a processive chain elongation mechanism and secrete HA across the plasma membrane. This complex operation is accomplished by functionally integrating a cytosolic catalytic domain with a channel-forming transmembrane region. Class I enzymes, containing a single GT family-2 (GT-2) module that adds both monosaccharide units to the nascent chain, are further subdivided into two groups that construct the polymer with opposite molecular directionalities: Class I-R and I-NR elongate the HA polysaccharide at either the reducing or the non-reducing end, respectively. In contrast, Class II HASs are membrane-associated peripheral synthases with a non-processive, non-reducing end elongation mechanism using two independent GT-2 modules (one for each type of monosaccharide) and require a separate secretion system for HA export. We discuss recent mechanistic insights into HA biosynthesis that promise biotechnological benefits and exciting engineering approaches.

Hyaluronan Synthase: The Mechanism of Initiation at the Reducing End and a Pendulum Model for Polysaccharide Translocation to the Cell Exterior

Hyaluronan (HA) biosynthesis has been studied for over six decades, but our understanding of the biochemical details of how HA synthase (HAS) assembles HA is still incomplete. Class I family members include mammalian and streptococcal HASs, the focus of this review, which add new intracellular sugar-UDPs at the reducing end of growing hyaluronyl-UDP chains. HA-producing cells typically create extracellular HA coats (capsules) and also secrete HA into the surrounding space. Since HAS contains multiple transmembrane domains and is lipid-dependent, we proposed in 1999 that it creates an intraprotein HAS-lipid pore through which a growing HA-UDP chain is translocated continuously across the cell membrane to the exterior. We review here the evidence for a synthase pore-mediated polysaccharide translocation process and describe a possible mechanism (the Pendulum Model) and potential energy sources to drive this ATP-independent process. HA synthases also synthesize chitin oligosaccharides, which are created by cleavage of novel oligo-chitosyl-UDP products. The synthesis of chitin-UDP oligomers by HAS confirms the reducing end mechanism for sugar addition during HA assembly by streptococcal and mammalian Class I enzymes. These new findings indicate the possibility that HA biosynthesis is initiated by the ability of HAS to use chitin-UDP oligomers as self-primers.

Fluorescence resonance energy transfer (FRET) and proximity ligation assays reveal functionally relevant homo- and heteromeric complexes among hyaluronan synthases HAS1, HAS2, and HAS3

In vertebrates, hyaluronan is produced in the plasma membrane from cytosolic UDP-sugar substrates by hyaluronan synthase 1-3 (HAS1-3) isoenzymes that transfer N-acetylglucosamine (GlcNAc) and glucuronic acid (GlcUA) in alternative positions in the growing polysaccharide chain during its simultaneous extrusion into the extracellular space. It has been shown that HAS2 immunoprecipitates contain functional HAS2 homomers and also heteromers with HAS3 (Karousou, E., Kamiryo, M., Skandalis, S. S., Ruusala, A., Asteriou, T., Passi, A., Yamashita, H., Hellman, U., Heldin, C. H., and Heldin, P. (2010) The activity of hyaluronan synthase 2 is regulated by dimerization and ubiquitination. J. Biol. Chem. 285, 23647-23654). Here we have systematically screened in live cells, potential interactions among the HAS isoenzymes using fluorescence resonance energy transfer (FRET) and flow cytometric quantification. We show that all HAS isoenzymes form homomeric and also heteromeric complexes with each other. The same complexes were detected both in Golgi apparatus and plasma membrane by using FRET microscopy and the acceptor photobleaching method. Proximity ligation assays with HAS antibodies confirmed the presence of HAS1-HAS2, HAS2-HAS2, and HAS2-HAS3 complexes between endogenously expressed HASs. C-terminal deletions revealed that the enzymes interact mainly via uncharacterized N-terminal 86-amino acid domain(s), but additional binding site(s) probably exist in their C-terminal parts. Of all the homomeric complexes HAS1 had the lowest and HAS3 the highest synthetic activity. Interestingly, HAS1 transfection reduced the synthesis of hyaluronan obtained by HAS2 and HAS3, suggesting functional cooperation between the isoenzymes. These data indicate a general tendency of HAS isoenzymes to form both homomeric and heteromeric complexes with potentially important functional consequences on hyaluronan synthesis.

Hyaluronan synthase polymerizing activity and control of product size are discrete enzyme functions that can be uncoupled by mutagenesis of conserved cysteines

Streptococcus equisimilis hyaluronan (HA) synthase (SeHAS) contains four cysteines (C226, C262, C281 and C367) that are conserved in the mammalian HAS family. Previous studies of single Cys-to-Ser and all possible Cys-to-Ala mutants of SeHAS found that: the Cys-null mutant is active, Cys modification inhibits HAS activity and the conserved cysteines are clustered at the membrane-enzyme interface in substrate-binding sites (Kumari K, Weigel PH. 2005. Identification of a membrane-localized cysteine cluster near the substrate binding sites of the Streptococcus equisimilis hyaluronan synthase. Glycobiology. 15:529-539). We re-examined these Cys mutants using a single technique (size exclusion chromatography-multi-angle laser light scattering) that allows simultaneous assays on the same sample for both HA synthesis activity and HA product size. Among 18 mutants compared with wild type, 4 showed no change in either function and 3 showed changes in both (decreased activity and HA size). Only one of the two functions was altered in 11 other mutants, which showed either decreased polymerizing activity or product size. No mutants made larger HA, 8 made smaller HA and 10 showed no change in HA size. Nine mutants showed no change in activity and nine were less active. The mutants fell into four of nine possible groups in terms of changes in HA size or synthesis rate (i.e. none, increased or decreased). Specific Cys residues were associated with each mutant group and the pattern of effects on both functions. Thus, the four conserved Cys residues, individually and in specific combinations, influence the rate of sugar assembly by HAS and HA product size, but their participation in one function is independent of the other.

Inhibition of hyaluronan export reduces collagen degradation in interleukin-1 treated cartilage

Background

Osteoarthrosis is characterized by cartilage erosion, proteolysis of aggrecan and collagen, and disturbed rates of synthesis of aggrecan and hyaluronan by chondrocytes, with hyaluronan over-production being an early reaction. We considered that inhibition of hyaluronan export might prevent subsequent proteoglycan loss and collagen degradation.

Methods

To test this hypothesis, we studied a tissue culture model using bovine cartilages explants activated with IL-1α to induce osteoarthritic reactions using the phosphodiesterase-5 inhibitors tadalafil, zaprinast and vardenafil.

Results

These drugs inhibited hyaluronan export, but they did not inhibit hyaluronan synthase activity. Simultaneously, they inhibited proteoglycan loss and collagen degradation, but not their synthesis. They also reduced the release of gelatinases into the culture media and diffusion of the indicator protein horseradish peroxidase through the cartilage explants. The mechanism of action of these compounds may be through inhibition of hyaluronan exporter multidrug resistance-associated protein 5 (MRP5), because the effective drug concentrations were much higher than required for phosphodiesterase-5 inhibition and intracellular cGMP accumulation.

Conclusion

Inhibition of hyaluronan over-production may be an appropriate target to attenuate IL-1-induced reactions in osteoarthritic cartilage.

Hyaluronan synthases: a decade-plus of novel glycosyltransferases

Hyaluronan synthases (HASs) are glycosyltransferases that catalyze polymerization of hyaluronan found in vertebrates and certain microbes. HASs transfer two distinct monosaccharides in different linkages and, in certain cases, participate in polymer transfer out of the cell. In contrast, the vast majority of glycosyltransferases form only one sugar linkage. Although our understanding of HAS biochemistry is still incomplete, very good progress has been made since the first genetic identification of a HAS in 1993. New enzymes have been discovered, and some molecular details have emerged. Important findings are the lipid dependence of Class I HASs, the function of HASs as protein monomers, and the elucidation of mechanisms of synthesis by Class II HAS. We propose three classes of HASs based on differences in protein sequences, predicted membrane topologies, potential architectures, mechanisms, and direction of polymerization.

Functional molecular mass of Escherichia coli K92 polysialyltransferase as determined by radiation target analysis

The polysialyltransferase of Escherichia coli K92 catalyzes the transfer of sialic acid from CMP-sialic acid to a growing chain of polysialic acid at the nonreducing end. The enzyme encoded by the neuS gene is membrane-associated and has been suggested to be organized within a complex of several proteins encoded by the K92 gene cluster. Attempts to prepare a soluble active NeuS enzyme have been unsuccessful. Recent results suggest that de novo synthesis of polysialic acid requires coexpression of four genes from the cluster: neuS, neuE, kpsC, and kpsS. However, elongation of preexisting polysialic acid chains only requires expression of neuS. The molecular organization of the catalytic unit of bacterial polysialyltransferases has not been described. We used radiation inactivation to measure the size of the minimum functional unit catalyzing the polysialyltransferase chain extension and de novo reactions. Membranes harboring NeuS in the presence and absence of other products of the K92 gene cluster were exposed to high-energy electrons. The rate of loss of polysialyltransferase activity reveals the mass of the molecules essential for catalytic activity. We observed that the transfer of neuNAc from CMP-neuNAc to a polysialic acid acceptor is catalyzed by a complex with a target size larger than that of monomeric NeuS. The target size of the unit catalyzing the extension of existing polysialic acid chains does not differ significantly from the size of the unit catalyzing transfer of sialic acid to the endogenous acceptor. Parallel samples of membranes containing NeuS and a green fluorescent protein (GFP) chimera were compared by target analysis. The target size of this structural unit was estimated by analysis of the rate of decay of the GFP-NeuS chimera band migrating in the immunoblots. The target size of the structural unit is larger than expected for a monomer. The results of these experiments show that while the target size of the catalytic activity for K92 polysialyltransferase is larger than a monomer of NeuS, a large complex is not required for catalysis.

Phospholipid Dependence and Liposome Reconstitution of Purified Hyaluronan Synthase

Previous radiation inactivation and enzyme characterization studies demonstrated that the Streptococcus equisimilis hyaluronan synthase (seHAS) is phospholipid-dependent and that cardiolipin (CL) is the best phospholipid for enzyme activation. Here we investigated the ability of seHAS, purified in the absence of added lipid, to be activated by synthetic phosphatidic acid (PA), phosphatidylserine, or CL lipids containing fatty acyl chains of different length or different numbers of double bonds. The most effective lipid was tetraoleoyl CL (TO-CL), whereas tetramyristoyl CL (TM-CL) was ineffective. None of the phosphatidylserine species tested gave significant activation. PAs containing C10 to C18 saturated acyl chains were not effective activators, and neither were oleoyl lyso PA, dilinoleoyl PA, or PA containing one oleoyl chain and either a palmitoyl or stearoyl chain. In contrast, dioleoyl PA stimulated seHAS approximately 10-fold, to approximately 20% of the activity observed with TO-CL. The tested acidic lipids such as PA and CL activated the enzyme most efficiently if they contained only oleic acid. Mixing experiments showed that the enzyme interacts preferentially with TO-CL in the presence of TM-CL. Similarly, seHAS incorporated into phosphotidylcholine-based liposomes showed increasing activity with increasing TO-CL, but not TM-CL, content. Inactivation of membrane-bound seHAS by solubilization with Nonidet P-40 was prevented by TO-CL, but not TM-CL. The pH dependence of seHAS in the presence of synthetic or naturally occurring CLs showed the same pattern of lipid preference between pH 6 and 10.5. Unexpectedly, HAS showed lipid-independent activity at pH 11.5. The results suggest that Class I HAS enzymes are lipid-dependent and that assembly of active seHAS-lipid complexes has high specificity for the phospholipid head group and the nature of the fatty acyl chains.

The functional molecular mass of the Pasteurella hyaluronan synthase is a monomer

Hyaluronan (HA), a linear polysaccharide composed of beta1,3-GlcNAc-beta1,4-GlcUA repeats, is found in the extracellular matrix of vertebrate tissues as well as the capsule of several pathogenic bacteria. All known HA synthases (HASs) are dual-action glycosyltransferases that catalyze the addition of two different sugars from UDP-linked precursors to the growing HA chain. The bacterial hyaluronan synthase, PmHAS from Gram-negative Pasteurella multocida, is a 972-residue membrane-associated protein. Previously, the Gram-positive Streptococcus pyogenes enzyme, SpHAS (419 residues), and the vertebrate enzyme, XlHAS1 (588 residues), were found to function as monomers of protein, but the PmHAS is not similar at the protein sequence level and has quite different enzymological properties. We have utilized radiation inactivation to measure the target size of recombinant full-length and truncated PmHAS. The target size of HAS activity was confirmed using internal enzyme standards of known molecular weight. We found that the Pasteurella HA synthase protein functions catalytically as a monomer. Functional truncated soluble PmHAS also behaves as a polypeptide monomer as assessed by gel filtration chromatography and light scattering.

Hyaluronan synthesis induces microvillus-like cell surface protrusions

Hyaluronan synthases (HASs) are plasma membrane enzymes that simultaneously elongate, bind, and extrude the growing hyaluronan chain directly into extracellular space. In cells transfected with green fluorescent protein (GFP)-tagged Has3, the dorsal surface was decorated by up to 150 slender, 3-20-microm-long microvillus-type plasma membrane protrusions, which also contained filamentous actin, the hyaluronan receptor CD44, and lipid raft microdomains. Enzymatic activity of HAS was required for the growth of the microvilli, which were not present in cells transfected with other GFP proteins or inactive GFP-Has3 mutants or in cells incubated with exogenous soluble hyaluronan. The microvilli induced by HAS3 were gradually withered by introduction of an inhibitor of hyaluronan synthesis and rapidly retracted by hyaluronidase digestion, whereas they were not affected by competition with hyaluronan oligosaccharides and disruption of the CD44 gene, suggesting independence of hyaluronan receptors. The data bring out the novel concept that the glycocalyx created by dense arrays of hyaluronan chains, tethered to HAS during biosynthesis, can induce and maintain prominent microvilli.

Mutation of Two Intramembrane Polar Residues Conserved within the Hyaluronan Synthase Family Alters Hyaluronan Product Size

We identified two conserved polar amino acids within different membrane domains (MD) of Streptococcus equisimilis hyaluronan synthase (seHAS), Lys48 in MD2 and Glu327 in MD4. In eukaryotic HASs, the position of the Glu is very similar and the Lys is replaced by a conserved polar Gln. To assess whether Lys48 and Glu327 interact or influence seHAS activity, we investigated the effects of changing Lys48 to Arg or Glu and Glu327 to Lys, Asp, or Gln. Mutants, including a double switch variant with Lys48 and Glu327 exchanged, were expressed and assayed in Escherichia coli membranes. SeHASE327Q and seHASE327K were expressed at low levels, whereas seHASE327D and the Lys48 mutants were expressed well. The specific enzyme activities (relative to wild type) were 17 and 7% for the K48R and K48E mutants and 26 and 38% for the E327Q and E327D mutants, respectively. In contrast, seHAS(E327K) showed only 0.16% of wild-type activity but was rescued over 46-fold by changing Lys48 to Glu. Expression of the seHASE327K,K48E protein was also rescued to near wild-type levels. Based on size exclusion chromatography coupled to multiangle laser light scattering analysis, all the variants synthesized hyaluronan (HA) of smaller weight-average molar mass than wild-type enzyme (3.6 MDa); the smallest HA (approximately 0.6 MDa) was made by seHASE327K,K48E and seHASK48E. The results indicate that Glu327 within MD4 is a critical residue for the stability of seHAS, that it may interact with Lys48 within MD2, and that these residues are involved in the ability of HAS to synthesize very large HA.

Target size analysis by radiation inactivation: the use of free radical scavengers

Several model systems were employed to assess indirect effects that occur in the process of using radiation inactivation analysis to determine protein target sizes. In the absence of free radical scavengers, such as mannitol and benzoic acid, protein functional unit sizes can be drastically overestimated. In the case of glutamate dehydrogenase, inclusion of free radical scavengers reduced the apparent target size from that of a hexamer to that of a trimer based on enzyme activity determinations. For glucose-6-phosphate dehydrogenase, the apparent target size was reduced from a dimer to a monomer. The target sizes for both glutamate dehydrogenase and glucose-6-phosphate dehydrogenase in the presence of free radical scavengers corresponded to subunit sizes when determinations of protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis or immunoblotting were done rather than enzyme activity. The free radical scavengers appear to compete with proteins for damage by secondary radiation products, since irradiation of these compounds can result in production of inhibitory species. Addition of benzoic acid/mannitol to samples undergoing irradiation was more effective in eliminating secondary damage than were 11 other potential free radical scavenging systems. Addition of a free radical scavenging system enables more accurate functional unit size determinations to be made using radiation inactivation analysis.

Identification of a membrane-localized cysteine cluster near the substrate-binding sites of the Streptococcus equisimilis hyaluronan synthase

The membrane-bound hyaluronan synthase (HAS) from Streptococcus equisimilis (seHAS), which is the smallest Class I HAS, has four cysteine residues (positions 226, 262, 281, and 367) that are generally conserved within this family. Although Cys-null seHAS is still active, chemical modification of cysteine residues causes inhibition of wild-type enzyme. Here we studied the effects of N-ethylmaleimide (NEM) treatment on a panel of seHAS Cys-mutants to examine the structural and functional roles of the four cysteine residues in the activity of the enzyme. We found that Cys226, Cys262, and Cys281 are reactive with NEM, but Cys367 is not. Substrate protection studies of wild-type seHAS and a variety of Cys-mutants revealed that binding of UDP-GlcUA, UDP-GlcNAc, or UDP can protect Cys226 and Cys262 from NEM inhibition. Inhibition of the six double Cys-mutants of seHAS by sodium arsenite, which can cross-link vicinyl sulfhydryl groups, also supported the conclusion that Cys262 and Cys281 are close enough to be cross-linked. Similar results indicated that Cys281 and Cys367 are also very close in the active enzyme. We conclude that three of the four Cys residues in seHAS (Cys262, Cys281, and Cys367) are clustered very close together, that these Cys residues and Cys226 are located at the inner surface of the cell membrane, and that Cys226 and Cys262 are located in or near a UDP binding site.

Hyaluronan and morphogenesis

In the past decade, there has been an explosion of interest in hyaluronan, an often misunderstood, biochemically simple, yet functionally complex carbohydrate polymer that is a resident of many extracellular matrices. Previously thought of as a passive, space-filling component of the extracellular matrix, the so-called "goo" concept, hyaluronan has risen to a much higher regard in recent years, even being called "magic glue" in a recent perspective. Hyaluronan is likely to be the common thread in many morphogenetic processes, including condensation events and epithelial-to-mesenchymal transformation. Hyaluronan is comparatively unique as a component of the extracellular matrix as it is solely composed of carbohydrate. In order to truly understand this biopolymer, one must first understand its biosynthesis, then understand its uptake and turnover, then identify its binding proteins and receptors. Major advances have been made in all of these arenas within the past decade. Hyaluronan synthases, hyaluronidases, and the hyaladherins have been molecularly identified and cloned. Furthermore, many have now been inactivated, employing gene targeting strategies, to create mice deficient in the respective gene product function. Collectively, huge strides have been made in our understanding of the diverse biological functions for this fascinating molecule. Hyaluronan appeared in metazoans immediately prior to the arrival of the vertebrates, and may be required for the differentiation, development, and/or function of most cell lineages, structures, and tissues that we associate with vertebrates, such as the neural crest, the skeleton, including the teeth, skin, and hair, and the chambered heart. In this review, we will update the reader on the advances of the past decade and provide insight into those morphogenetic processes through which hyaluronan regulates vertebrate development.

Inhibition of hyaluronan export from human fibroblasts by inhibitors of multidrug resistance transporters

In a previous report we described the export of hyaluronan from Streptococcus pyogenes by an ABC transporter. Extending these findings a sequence homology search against human proteins revealed a strong homology to the multidrug resistance transporter ABC-B (MDR-1) and ABC-C (MRP 5). Using several inhibitors directed against these and other transporters, a decreased hyaluronan production in cell culture as well as in hyaluronan synthase activity in purified membrane fractions was observed. The inhibitory capacity (IC(50) concentrations) was compared the with reported IC(50)- or the K(i)-concentrations for individual transporters. These analyses revealed that hyaluronan is synthesized within the cytoplasm of mammalian cells and actively secreted into the pericellular space by energy dependent transport proteins. While inhibition of several transport proteins resulted in a decrease of hyaluronan export, inhibition of the MRP5 transporter was the most effective one to decrease hyaluronan in the cell culture supernatant indicating that hyaluronan export is one physiological role of this transport protein.

Characterization of the purified hyaluronan synthase from Streptococcus equisimilis

Hyaluronan synthase (HAS) utilizes UDP-GlcUA and UDP-GlcNAc in the presence of Mg(2+) to form the GAG hyaluronan (HA). The purified HAS from Streptococcus equisimilis (seHAS) shows high fidelity in that it only polymerizes the native substrates, UDP-GlcNAc and UDP-GlcUA. However, other uridinyl nucleotides and UDP-sugars inhibited enzyme activity, including UDP-GalNAc, UDP-Glc, UDP-Gal, UDP-GalUA, UMP, UDP, and UTP. Purified seHAS was approximately 40% more active in 25 mM, compared to 50 mM, PO(4) in the presence of either 50 mM NaCl or KCl, and displayed a slight preference for KCl over NaCl. The pH profile was surprisingly broad, with an effective range of pH 6.5-11.5 and the optimum between pH 9 and 10. SeHAS displayed two apparent pK(a) values at pH 6.6 and 11.8. As the pH was increased from approximately 6.5, both K(m) and V(max) increased until pH approximately 10.5, above which the kinetic constants gradually declined. Nonetheless, the overall catalytic constant (120/s) was essentially unchanged from pH 6.5 to 10.5. The enzyme is temperature labile, but more stable in the presence of substrate and cardiolipin. Purified seHAS requires exogenous cardiolipin for activity and is very sensitive to the fatty acyl composition of the phospholipid. The enzyme was inactive or highly activated by synthetic cardiolipins containing, respectively, C14:0 or C18:1(Delta9) fatty acids. The apparent E(act) for HA synthesis is 40 kJ (9.5 kcal/mol) disaccharide. Increasing the viscosity by increasing concentrations of PEG, ethylene glycol, glycerol, or sucrose inhibited seHAS activity. For PEGs, the extent of inhibition was proportional to their molecular mass. PEGs with average masses of 2.7, 11.7, and 20 kg/mol caused 50% inhibition of V(max) at 21, 6.5, and 3.5 mM, respectively. The apparent K(i) values for ethylene glycol, glycerol, and sucrose were, respectively, 4.5, 3.3, and 1.2 mM.

An Engineered Hyaluronan Synthase

The Class I hyaluronan synthase (HAS) is a unique glycosyltransferase synthesizing hyaluronan (HA), a polysaccharide composed of GlcUA and GlcNAc, by using one catalytic domain that elongates two different monosaccharides. As for the synthetic mechanism, there are two alternative manners for the sugar elongation process. Some bacterial HASs add new sugars to the non-reducing end of the acceptor to grow polymers. On the other hand, some vertebrate enzymes seem to transfer sugars to the reducing end. Expression of vertebrate HASs as active and soluble proteins will accelerate further precise insight into mechanisms of sugar elongation reactions by natural HASs. Since large scale production of HA polymers and oligomers would become powerful tools both for basic studies and new biotechnology to create functional carbohydrates in medicinal purposes, advent of an efficient method for the expression of HASs in Escherichia coli is strongly expected. Here we communicate the first success of the production of recombinant human HAS2 proteins composed of only the catalytic region in E. coli as the active form. It was demonstrated that an engineered HAS2 expressed in E. coli exhibited significant activity to synthesize a mixture of HAS oligomers from 8-mer (HA8) to 16-mer (HA16). Engineered HAS2 prepared herein elongated sugars from exogenous tetrasaccharide to form polymers with a direction to the non-reducing end. According to the present results, large scale production of engineered recombinant HASs is to be performed using E. coli that will provide practical and economic advantages in manufacturing enzymes for use in the synthesis of various oligomeric HA molecules and their industrial applications.

Hyaluronan: genetic insights into the complex biology of a simple polysaccharide

It is appropriate that this review should appear in a volume dedicated to Mert Bernfield. Much of my interest in the cell biology of the extracellular matrix, particularly during development, echoes Mert's pioneering studies. His kind but provocative questioning during meetings is especially missed. The glycosaminoglycan hyaluronan is ubiquitous, and is especially abundant during embryogenesis. Hydrated matrices rich in hyaluronan expand the extracellular space, facilitating cell migration. The viscoelastic properties of hyaluronan are also essential for proper function of cartilage and joints. Recent understanding of hyaluronan biology has benefited from the identification of genes encoding hyaluronan synthases and hyaluronidases, genetic analysis of the roles of hyaluronan during development, elucidation of the biochemical mechanisms of hyaluronan synthesis, and by studies of human genetics and tumors. This review focuses on recent studies utilizing hyaluronan-deficient, gene targeted mice with null alleles for the principal source of hyaluronan during mid-gestation, hyaluronan synthase-2 (has-2).