Carrier-based immobilization of Aerococcus viridansl-lactate oxidase.

l-Lactate oxidase has important applications in biosensing and finds increased use in biocatalysis. The enzyme has been characterized well, yet its immobilization has not been explored in depth. Here, we studied immobilization of Aerococcus viridansl-lactate oxidase on porous carriers of variable matrix material (polymethacrylate, polyurethane, agarose) and surface functional group (amine, Ni2+-loaded nitrilotriacetic acid (NiNTA), epoxide). Carrier activity (Ac) and immobilized enzyme effectiveness (ɳ) were evaluated in dependence of protein loading. Results show that efficient immobilization (Ac: up to 1450 U/g carrier; ɳ: up to 65%) requires a hydrophilic carrier (agarose) equipped with amine groups. The value of ɳ declines sharply as Ac increases, probably due to transition into diffusional regime. Untagged l-lactate oxidase binds to NiNTA carrier similarly as N-terminally His-tagged enzyme. Lixiviation studies reveal quasi-irreversible enzyme adsorption on NiNTA carrier while partial release of activity (≤ 25%) is shown from amine carrier. The desorbed enzyme exhibits the same specific activity as the original l-lactate oxidase. Collectively, our study identifies basic requirements of l-lactate oxidase immobilization on solid carrier and highlights the role of ionic interactions in enzyme-surface adsorption.

Enzymes revolutionize the bioproduction of value-added compounds: From enzyme discovery to special applications.

Competitive sustainable production in industry demands new and better biocatalysts, optimized bioprocesses and cost-effective product recovery. Our review sheds light on the progress made for the individual steps towards these goals, starting with the discovery of new enzymes and their corresponding genes. The enzymes are subsequently engineered to improve their performance, combined in reaction cascades to expand the reaction scope and integrated in whole cells to provide an optimal environment for the bioconversion. Strain engineering using synthetic biology methods tunes the host for production, reaction design optimizes the reaction conditions and downstream processing ensures the efficient recovery of commercially viable products. Selected examples illustrate how modified enzymes can revolutionize future-oriented applications ranging from the bioproduction of bulk-, specialty- and fine chemicals, active pharmaceutical ingredients and carbohydrates, over the conversion of the greenhouse-gas CO2 into valuable products and biocontrol in agriculture, to recycling of synthetic polymers and recovery of precious metals.

Product solubility control in cellooligosaccharide production by coupled cellobiose and cellodextrin phosphorylase.

Soluble cellodextrins (linear ß-1,4-d-gluco-oligosaccharides) have interesting applications as ingredients for human and animal nutrition. Their bottom-up synthesis from glucose is promising for bulk production, but to ensure a completely water-soluble product via degree of polymerization (DP) control (DP ≤ 6) is challenging. Here, we show biocatalytic production of cellodextrins with DP centered at 3 to 6 (~96 wt.% of total product) using coupled cellobiose and cellodextrin phosphorylase. The cascade reaction, wherein glucose was elongated sequentially from α-d-glucose 1-phosphate (αGlc1-P), required optimization and control at two main points. First, kinetic and thermodynamic restrictions upon αGlc1-P utilization (200 mM; 45°C, pH 7.0) were effectively overcome (53% → ≥90% conversion after 10 hrs of reaction) by in situ removal of the phosphate released via precipitation with Mg2+ . Second, the product DP was controlled by the molar ratio of glucose/αGlc1-P (∼0.25; 50 mM glucose) used in the reaction. In optimized conversion, soluble cellodextrins in a total product concentration of 36 g/L were obtained through efficient utilization of the substrates used (glucose: 98%; αGlc1-P: ∼80%) after 1 hr of reaction. We also showed that, by keeping the glucose concentration low (i.e., 1-10 mM; 200 mM αGlc1-P), the reaction was shifted completely towards insoluble product formation (DP ∼9-10). In summary, this study provides the basis for an efficient and product DP-controlled biocatalytic synthesis of cellodextrins from expedient substrates.

Production of glucosyl glycerol by immobilized sucrose phosphorylase: Options for enzyme fixation on a solid support and application in microscale flow format.

2-O-(α-d-Glucopyranosyl)-sn-glycerol (αGG) is a natural osmolyte. αGG is produced industrially for application as an active cosmetic ingredient. The biocatalytic process involves a selective transglucosylation from sucrose to glycerol catalyzed by sucrose phosphorylase (SPase). Here we examined immobilization of SPase (from Leuconostoc mesenteroides) on solid support with the aim of enabling continuous production of αGG. By fusing SPase to the polycationic binding module Zbasic2 we demonstrated single-step noncovalent immobilization of the enzyme chimera to different porous supports offering an anionic surface. We showed that immobilization facilitated by Zbasic2 was similarly efficient as immobilization by multipoint covalent attachment on epoxy-activated supports in terms of production of αGG. Enzyme loadings of up to 90mg enzyme g-1 support were obtained and the immobilized SPase was about half as effective as the enzyme in solution. The high regio- and chemo-selectivity of soluble SPase in αGG synthesis was retained in the immobilized enzyme and product yields of >85% were obtained at titers of ∼800mM. The Zbasic2-SPase immobilizates were fully recyclable: besides reuse of the enzyme activity, easy recovery of the solid support for fresh immobilizations was facilitated by the reversible nature of the enzyme attachment. Application of immobilized Zbasic2-SPase for continuous production of αGG in a microstructured flow reactor was demonstrated. Space-time yields of 500mmol αGG L-1h-1 were obtained at product titers of ∼200mM. The continuous microreactor was operated for 16days and an operational half-life of about 10days was determined.

Two N-terminally truncated variants of human ß-galactoside α2,6 sialyltransferase I with distinct properties for in vitro protein glycosylation.

Sialic acid groups of protein N-glycans are important determinants of biological activity. Exposed at the end of the glycan chain, they are potential targets for glycan remodeling. Sialyltransferases (STs; EC 2.4.99) are the enzymes that catalyze the sialic acid transfer from a CMP-activated donor on to a carbohydrate acceptor in vivo. Recombinant expression of the full-length human ß-galactoside α2,6 sialyltransferase I (ST6Gal-I) was hampered and therefore variants with truncated N-termini were investigated. We report on the distinct properties of two N-terminally truncated versions of ST6Gal-I, namely Δ89ST6Gal-I and Δ108ST6Gal-I, which were successfully expressed in human embryonic kidney cells. The different properties of these enzymes result most probably from the loss of interactions from helix α1 in the Δ108ST6Gal-I variant, which plays a role in acceptor substrate binding. The Km for N-acetyl-d-lactosamine was 10-fold increased for Δ108ST6Gal-I (84 mM) as compared to Δ89ST6Gal-I (8.3 mM). The two enzyme variants constitute a suitable tool box for the terminal modification of N-glycans. While the enzyme Δ89ST6Gal-I exhibited both ST (di-sialylation) and sialidase activity on a monoclonal antibody, the enzyme Δ108ST6Gal-I showed only ST activity with specificity for mono-sialylation.

Combining expression and process engineering for high-quality production of human sialyltransferase in Pichia pastoris.

The human ß-galactoside α2,6-sialyltransferase I, ST6Gal-I has drawn considerable interest for its use as biocatalyst for in-vitro glycoengineering of recombinantly produced therapeutic proteins. By attaching sialic acid onto the terminal galactoses of biantennary protein N-glycans, ST6Gal-I facilitates protein remodeling towards a humanized glycosylation and thus optimized efficacy in pharmacological use. Secreted expression of ST6Gal-I in Pichia pastoris is promising, but proteolysis restricts both the yield and the quality of the enzyme produced. Focusing on an N-terminally truncated (Δ108) variant of ST6Gal-I previously shown to represent a minimally sized, still active form of ST6Gal-I, we show here that protein expression engineering and optimization of bioreactor cultivation of P. pastoris KM71H (pPICZαB) synergized to enhance the maximum enzyme titer about 57-fold to 17units/L. N-Terminal fusion to the Flag-tag plus deletion of a potential proteolytic site (Lys(114)-Asn→Gln(114)-Asn) improved the intrinsic resistance of Δ108ST6Gal-I to degradation in P. pastoris culture. A mixed glycerol/methanol feeding protocol for P. pastoris growth and induction was key for enzyme production in high yield and quality. The sialyltransferase was recovered from the bioreactor culture in a yield of 70% using a single step of anion-exchange chromatography. Its specific activity was 0.05units/mg protein.

Comparison of broad-scope assays of nucleotide sugar-dependent glycosyltransferases.

Glycosyltransferases (GTs) are abundant in nature and diverse in their range of substrates. Application of GTs is, however, often complicated by their narrow substrate specificity. GTs with tailored specificities are highly demanded for targeted glycosylation reactions. Engineering of such GTs is, however, restricted by lack of practical and broad-scope assays currently available. Here we present an improvement of an inexpensive and simple assay that relies on the enzymatic detection of inorganic phosphate cleaved from nucleoside phosphate products released in GT reactions. This phosphatase-coupled assay (PCA) is compared with other GT assays: a pH shift assay and a commercially available immunoassay in Escherichia coli cell-free extract (CE). Furthermore, we probe PCA with three GTs with different specificities. Our results demonstrate that PCA is a versatile and apparently general GT assay with a detection limit as low as 1 mU. The detection limit of the pH shift assay is roughly 4 times higher. The immunoassay, by contrast, detected only nucleoside diphosphates (NDPs) but had the lowest detection limit. Compared with these assays, PCA showed superior robustness and, therefore, appears to be a suitable general screening assay for nucleotide sugar-dependent GTs.

All-in-one assay for ß-d-galactoside sialyltransferases: Quantification of productive turnover, error hydrolysis, and site selectivity.

Sialyltransferases are important enzymes of glycobiology and the related biotechnologies. The development of sialyltransferases calls for access to quick, inexpensive, and robust analytical tools. We have established an assay for simultaneous characterization of sialyltransferase activity, error hydrolysis, and site selectivity. The described assay does not require expensive substrates, is very sensitive (limit of detection=0.3 µU), and is easy to perform. It is based on sialylation of nitrophenyl galactosides; the products thereof are separated and quantified by ion pair reversed phase high-performance liquid chromatography with ultraviolet detection.

Complete switch from α-2,3- to α-2,6-regioselectivity in Pasteurella dagmatis ß-D-galactoside sialyltransferase by active-site redesign.

Structure-guided active-site redesign of a family GT-80 ß-D-galactoside sialyltransferase (from Pasteurella dagmatis) to change enzyme regioselectivity from α-2,3 in the wild type to α-2,6 in a P7H-M117A double mutant is reported. Biochemical data for sialylation of lactose together with protein crystal structures demonstrate highly precise enzyme engineering.

High-quality production of human α-2,6-sialyltransferase in Pichia pastoris requires control over N-terminal truncations by host-inherent protease activities.

BACKGROUND: α-2,6-sialyltransferase catalyzes the terminal step of complex N-glycan biosynthesis on human glycoproteins, attaching sialic acid to outermost galactosyl residues on otherwise fully assembled branched glycans. This "capping" of N-glycans is critical for therapeutic efficacy of pharmaceutical glycoproteins, making the degree of sialylation an important parameter of glycoprotein quality control. Expression of recombinant glycoproteins in mammalian cells usually delivers heterogeneous N-glycans, with a minor degree of sialylation. In-vitro chemo-enzymatic glycoengineering of the N-glycans provides an elegant solution to increase the degree of sialylation for analytical purposes but also possibly for modification of therapeutic proteins. RESULTS: Human α-2,6-sialyltransferase (ST6Gal-I) was secretory expressed in P.pastoris KM71H. ST6Gal-I featuring complete deletion of both the N-terminal cytoplasmic tail and the transmembrane domain, and also partial truncation of the stem region up to residue 108 were expressed N-terminally fused to a His or FLAG-Tag. FLAG-tagged proteins proved much more resistant to proteolysis during production than the corresponding His-tagged proteins. Because volumetric transferase activity measured on small-molecule and native glycoprotein acceptor substrates did not correlate to ST6Gal-I in the supernatant, enzymes were purified and characterized in their action on non-sialylated protein-linked and released N-glycans, and the respective N-terminal sequences were determined by automated Edman degradation. Irrespective of deletion construct used (Δ27, Δ48, Δ62, Δ89), isolated proteins showed N-terminal processing to a highly similar degree, with prominent truncations at residue 108 - 114, whereby only Δ108ST6Gal-I retained activity. FLAG-tagged Δ108ST6Gal-I was therefore produced and obtained with a yield of 4.5 mg protein/L medium. The protein was isolated and shown by MS to be intact. Purified enzyme exhibited useful activity (0.18 U/mg) for sialylation of different substrates. CONCLUSIONS: Functional expression of human ST6Gal-I as secretory protein in P.pastoris necessitates that N-terminal truncations promoted by host-inherent proteases be tightly controlled. N-terminal FLAG-Tag contributes extra stability to the N-terminal region as compared to N-terminal His-Tag. Proteolytic degradation proceeds up to residues 108 - 114 and of the resulting short-form variants, only Δ108ST6Gal-I seems to be active. FLAG-Δ108ST6Gal-I transfers sialic acids to monoclonal antibody substrate with sufficient yields, and because it is stably produced in P.pastoris, it is identified here as an interesting glycoengineering catalyst.

Mechanistic study of CMP-Neu5Ac hydrolysis by α2,3-sialyltransferase from Pasteurella dagmatis.

Bacterial sialyltransferases of the glycosyltransferase family GT-80 exhibit pronounced hydrolase activity toward CMP-activated sialyl donor substrates. Using in situ proton NMR, we show that hydrolysis of CMP-Neu5Ac by Pasteurella dagmatis α2,3-sialyltransferase (PdST) occurs with axial-to-equatorial inversion of the configuration at the anomeric center to release the α-Neu5Ac product. We propose a catalytic reaction through a single displacement-like mechanism where water replaces the sugar substrate as a sialyl group acceptor. PdST variants having His(284) in the active site replaced by Asn, Asp or Tyr showed up to 10(4)-fold reduced activity, but catalyzed CMP-Neu5Ac hydrolysis with analogous inverting stereochemistry. The proposed catalytic role of His(284) in the PdST hydrolase mechanism is to facilitate the departure of the CMP leaving group.

Characterization of a multifunctional α2,3-sialyltransferase from Pasteurella dagmatis.

A new multifunctional α2,3-sialyltransferase has been discovered in Pasteurella dagmatis. The enzyme, in short PdST, was identified from the P. dagmatis genome by sequence similarity with sialyltransferases of glycosyltransferase family GT-80. In addition to its regioselective sialyltransferase activity (5.9 U/mg; pH 8.0), purified PdST is alternatively active at low pH as α2,3-sialidase (0.5 U/mg; pH 4.5) and α2,3-trans-sialidase (1.0 U/mg; pH 4.5). It also shows cytidine-5'-monophosphate N-acetyl-neuraminic (CMP-Neu5Ac) hydrolase activity (3.7 U/mg; pH 8.0) when no sialyl acceptor substrate is present in the reaction. After sialyltransferase PmST1 from P. multocida, PdST is the second member of family GT-80 to display this remarkable catalytic promiscuity. A unique feature of PdST, however, is a naturally occurring Ser-to-Thr substitution within a highly conserved Y(112)DDGS(116) sequence motif. In PmST1, the equivalent Ser(143) is involved in binding of the CMP-Neu5Ac donor substrate. Reversion of the natural mutation in a T116S-PdST variant resulted in a marked increase in α2,3-trans-sialidase side activity (4.0 U/mg; pH 4.5), whereas the major sialyltransferase activity was lowered (3.8 U/mg; pH 8.0). The Michaelis-Menten constant for CMP-Neu5Ac was decreased 4-fold in T116S mutant when compared with wild-type PdST (KM=1.1 mM), indicating that residue 116 of PdST contributes to a delicate balance between substrate binding and catalytic activity. D-Galactose and various ß-D-galactosides function as sialyl acceptors from CMP-Neu5Ac, whereas other hexoses (e.g. D-glucose) are inactive. Structure comparison was used to rationalize the particular acceptor substrate specificity of PdST in relation to other GT-80 sialyltransferases that show strict α2,3-regioselectivity, but are flexible in using α/ß-galactosides for sialylation.

Aromatic interactions at the catalytic subsite of sucrose phosphorylase: their roles in enzymatic glucosyl transfer probed with Phe52→Ala and Phe52→Asn mutants.

Mutants of Leuconostoc mesenteroides sucrose phosphorylase having active-site Phe(52) replaced by Ala (F52A) or Asn (F52N) were characterized by free energy profile analysis for catalytic glucosyl transfer from sucrose to phosphate. Despite large destabilization (≥3.5kcal/mol) of the transition states for enzyme glucosylation and deglucosylation in both mutants as compared to wild-type, the relative stability of the glucosyl enzyme intermediate was weakly affected by substitution of Phe(52). In reverse reaction where fructose becomes glucocylated, "error hydrolysis" was the preponderant path of breakdown of the covalent intermediate of F52A and F52N. It is proposed, therefore, that Phe(52) facilitates reaction of the phosphorylase through (1) positioning of the transferred glucosyl moiety at the catalytic subsite and (2) strong cation-π stabilization of the oxocarbenium ion-like transition states flanking the covalent enzyme intermediate.

Carbohydrate synthesis by disaccharide phosphorylases: reactions, catalytic mechanisms and application in the glycosciences.

Disaccharide phosphorylases are glycosyltransferases (EC 2.4.1.α) of specialized carbohydrate metabolism in microorganisms. They catalyze glycosyl transfer to phosphate using a disaccharide as donor substrate. Phosphorylases for the conversion of naturally abundant disaccharides including sucrose, maltose, α,α-trehalose, cellobiose, chitobiose, and laminaribiose have been described. Structurally, these disaccharide phosphorylases are often closely related to glycoside hydrolases and transglycosidases. Mechanistically, they are categorized according the stereochemical course of the reaction catalyzed, whereby the anomeric configuration of the disaccharide donor substrate may be retained or inverted in the sugar 1-phosphate product. Glycosyl transfer with inversion is thought to occur through a single displacement-like catalytic mechanism, exemplified by the reaction coordinate of cellobiose/chitobiose phosphorylase. Reaction via configurational retention takes place through the double displacement-like mechanism employed by sucrose phosphorylase. Retaining α,α-trehalose phosphorylase (from fungi) utilizes a different catalytic strategy, perhaps best described by a direct displacement mechanism, to achieve stereochemical control in an overall retentive transformation. Disaccharide phosphorylases have recently attracted renewed interest as catalysts for synthesis of glycosides to be applied as food additives and cosmetic ingredients. Relevant examples are lacto-N-biose and glucosylglycerol whose enzymatic production was achieved on multikilogram scale. Protein engineering of phosphorylases is currently pursued in different laboratories with the aim of broadening the donor and acceptor substrate specificities of naturally existing enzyme forms, to eventually generate a toolbox of new catalysts for glycoside synthesis.

Regioselective O-glucosylation by sucrose phosphorylase: a promising route for functional diversification of a range of 1,2-propanediols.

1,2-Propanediol and 3-aryloxy/alkyloxy derivatives thereof are bulk commodities produced directly from glycerol. Glycosylation is a promising route for their functional diversification into useful fine chemicals. Regioselective glucosylation of the secondary hydroxyl in different 1,2-propanediols was achieved by a sucrose phosphorylase-catalyzed transfer reaction where sucrose is the substrate and 2-O-alpha-d-glucopyranosyl products are exclusively obtained. Systematic investigation for optimization of the biocatalytic synthesis included prevention of sucrose hydrolysis, which occurs in the process as a side reaction of the phosphorylase. In addition to 'nonproductive' depletion of donor substrate, the hydrolysis also resulted in formation of maltose and kojibiose (up to 45%) due to secondary enzymatic glucosylation of the glucose thus produced. Using 3-ethoxy-1,2-propanediol as the acceptor substrate (1.0M), the desired transfer product was obtained in about 65% yield when employing a moderate (1.5-fold) excess of sucrose donor. Loss of the glucosyl substrate to 'glucobiose' by-products was minimal (<7.5%) under these conditions. The reactivity of other acceptors decreased in the order, 3-methoxy-1,2-propanediol>1,2-propanediol>3-allyloxy-1,2-propanediol>3-(o-methoxyphenoxy)-1,2-propanediol>3-tert-butoxy-1,2-propanediol. Glucosylated 1,2-propanediols were not detectably hydrolyzed by sucrose phosphorylase so that their synthesis by transglucosylation occurred simply under quasi-equilibrium reaction conditions.

Small-molecule glucosylation by sucrose phosphorylase: structure-activity relationships for acceptor substrates revisited.

Sucrose phosphorylase catalyzes the O-glucosylation of a wide range of acceptor substrates. Acceptors presenting a suitable 1,2-diol moiety are glucosylated exclusively at the secondary hydroxyl. Production of the naturally occurring compatible solute, 2-O-alpha-d-glucopyranosyl-sn-glycerol, from sucrose and glycerol is a notable industrial realization of the regio- and stereoselective biotransformation promoted by sucrose phosphorylase. The acceptor substrate specificity of sucrose phosphorylase was analyzed on the basis of recent high-resolution crystal structures of the enzyme. Interactions at the acceptor binding site, observed in the crystal (d-fructosyl) and suggested by results of docking experiments (glycerol), are used to rationalize experimentally determined efficiencies and regioselectivities of enzymatic glucosyl transfer.

Glucosylglycerol and glucosylglycerate as enzyme stabilizers.

Compatible solutes constitute a diverse class of low-molecular-mass organic molecules that are accumulated in high intracellular concentrations in response to the external stress of hyperosmolality or high temperature. Many of these compounds like alpha, alpha-trehalose are well known for their stabilizing effect on protein structure and could lead to development of more stable protein formulations. Negatively charged solutes like mannosylglycerate (R-2-O-alpha-D-mannopyranosyl-glycerate) are widespread among (hyper)thermophilic microorganisms and are thought to be exceptionally potent stabilizers of proteins under high-temperature denaturation conditions. To further inquire into the role of compound charge for protective function, we have compared two naturally occurring and structurally related solutes, glucosylglycerol (2-O-alpha-D-glucopyranosyl-sn-glycerol) and glucosylglycerate (R-2-O-alpha-D-glucopyranosyl-glycerate), as stabilizers of different enzymes undergoing inactivation through elevated temperature or freeze drying, and benchmarked their effects against that of alpha,alpha-trehalose. Glucosylglycerate in concentrations of >/=0.1 M was the most effective in preventing thermally induced loss of enzyme activity of lactate dehydrogenase, mannitol dehydrogenase, starch phosphorylase, and xylose reductase. alpha,alpha-Trehalose could usually be replaced by glucosylglycerol without compromising enzyme stability. Glucosylglycerol and glucosylglycerate afforded substantial (eightfold) protection to mannitol dehydrogenase during freeze drying.

Single-step enzymatic synthesis of (R)-2-O-alpha-D-glucopyranosyl glycerate, a compatible solute from micro-organisms that functions as a protein stabiliser.

Regioselective glucosylation of R-glycerate catalysed by sucrose phosphorylase in the presence of sucrose as the donor substrate provided the natural compatible solute (R)-2-O-alpha-D-glucopyranosyl glycerate with complete regioselectivity in an optimised synthetic yield of 90% R-glycerate converted and a concentration of about 270 mM.