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1.
Sialyltransferases of the GT‐80 glycosyltransferase family are considered multifunctional because of the array of activities detected. They exhibit glycosyl transfer, trans‐sialylation, and hydrolysis activities. How these enzymes utilize their active‐site residues in balancing the different enzymatic activities is not well understood. In this study of Pasteurella dagmatis α2,3sialyltransferase, we show that the conserved His85 controls efficiency and selectivity of the sialyl transfer. A His85→Asn variant was 200 times less efficient than wild‐type for sialylation of lactose, and exhibited relaxed site selectivity to form not only the α2,3‐ but also the α2,6‐sialylated product (21 %). The H85N variant was virtually inactive in trans‐sialylation but showed almost the same CMP‐Neu5Ac hydrolase activity as wild‐type. The competition between sialyl transfer and hydrolysis in the conversion of CMP‐Neu5Ac was dependent on the lactose concentration; this was characterized by a kinetic partition ratio of 85 m ?1 for the H85N variant, compared to 17 000 m ?1 for the wild‐type enzyme. His85 promotes the productive sialyl transfer to lactose and so prevents hydrolysis of CMP‐Neu5Ac in the reaction.  相似文献   

2.
A water tunnel in Candida antarctica lipase B that provides the active site with substrate water is hypothesized. A small, focused library created in order to prevent water from entering the active site through the tunnel was screened for increased transacylation over hydrolysis activity. A single mutant, S47L, in which the inner part of the tunnel was blocked, catalysed the transacylation of vinyl butyrate to 20 mM butanol 14 times faster than hydrolysis. The single mutant Q46A, which has a more open outer end of the tunnel, showed an increased hydrolysis rate and a decreased hydrolysis to transacylation ratio compared to the wild‐type lipase. Mutants with a blocked tunnel could be very useful in applications in which hydrolysis is unwanted, such as the acylation of highly hydrophilic compounds in the presence of water.  相似文献   

3.
Random mutagenesis targeted at hotspots of noncatalytic active‐site residues of potato epoxide hydrolase StEH1 combined with an enzyme‐activity screen allowed the isolation of enzyme variants displaying altered enantiopreference in the catalyzed hydrolysis of (2,3‐epoxypropyl)benzene. The wild‐type enzyme favored the S enantiomer with a ratio of 2.5:1, whereas the variant displaying the most radical functional changes showed a 15:1 preference for the R enantiomer. This mutant had accumulated four substitutions distributed over two out of four mutated hotspots: W106L, L109Y, V141K, and I151V. The underlying causes of the enantioselectivity were a decreased catalytic efficiency in the catalyzed hydrolysis of the S enantiomer combined with retained activity with the R enantiomer. The results demonstrate the feasibility of molding the stereoselectivity of this biocatalytically relevant enzyme.  相似文献   

4.
Pseudozyma antarctica lipase B (CALB) shows activity in the acrylation of hydroxypropylcarbamate, a racemic mixture of enantiomers of primary and secondary alcohols. However, full conversion is hampered by the slowly reacting S enantiomer of the secondary alcohol. The same is true for a wide range of secondary alcohols, for example, octan‐2‐ and ‐3‐ol. In order to get high conversion in these reactions in a short time, the stereospecificity pocket of CALB was redesigned by using predictions from molecular modeling. Positions 278, 104, and 47 were targeted, and a library for two‐site saturation mutagenesis at positions 104 and 278 was constructed. The library was then screened for hydrolysis of acrylated hydroxypropylcarbamates. The best mutants L278A, L278V, L278A/W104F, and L278A/W104F/S47A showed an increased conversion in hydrolysis and transesterification of more than 30 %. While the wild‐type showed only 73 % conversion in the acrylation of hydroxypropylcarbamate after 6 h, 97 % conversion was achieved by L278A in this time. Besides this, L278A/W104F reached >96 % conversion in the acrylation of octan‐2‐ and ‐3‐ol within 48 h and showed a significant decrease in stereoselectivity, while the wild‐type reached only 68 and 59 % conversion, respectively. Thus the new biocatalysts can be used for efficient transformation of racemic alcohols and esters with high activity when the high stereoselectivity of the wild‐type hampers complete conversion of racemic substrates in a short time.  相似文献   

5.
The chemical synthesis of 3‐substituted tyrosine derivatives requires a minimum of four steps to access optically enriched material starting from commercial precursors. Attempting to short‐cut the cumbersome chemical synthesis of 3‐substituted tyrosine derivatives, a single step biocatalytic approach was identified employing the tyrosine phenol lyase from Citrobacter freundii. The enzyme catalyses the hydrolysis of tyrosine to phenol, pyruvate and ammonium as well as the reverse reaction, thus the formation of tyrosine from phenol, pyruvate and ammonium. Since the wild‐type enzyme possessed a very narrow substrate spectrum, structure‐guided, site‐directed mutagenesis was required to change the substrate specificity of this C C bond forming enzyme. The best variant M379V transformed, for example, o‐cresol, o‐methoxyphenol and o‐chlorophenol efficiently to the corresponding tyrosine derivatives without any detectable side‐product. In contrast, all three phenol compounds were non‐substrates for the wild‐type enzyme. Employing the mutant, various L ‐tyrosine derivatives (3‐Me, 3‐OMe, 3‐F, 3‐Cl) were obtained with complete conversion and excellent enantiomeric excess (>97%) in just a single ‘green’ step starting from pyruvate and commercially available phenol derivatives.  相似文献   

6.
Perillyl alcohol is the terminal hydroxylation product of the cheap and readily available terpene, limonene. It has high potential as an anti‐tumor substance, but is of limited availability. In principle, cytochrome P450 monooxygenases, such as the self‐sufficient CYP102A1, are promising catalysts for the oxidation of limonene or other inert hydrocarbons. The wild‐type enzyme converts (4R)‐limonene to four different oxidation products; however, terminal hydroxylation at the allylic C7 is not observed. Here we describe a generic strategy to engineer this widely used enzyme to hydroxylate exclusively the exposed, but chemically less reactive, primary C7 in the presence of other reactive positions. The approach presented here turns CYP102A1 into a highly selective catalyst with a shifted product spectra by successive rounds of modeling, the design of small focused libraries, and screening. In the first round a minimal CYP102A1 mutant library was rationally designed. It contained variants with improved or strongly shifted regio‐, stereo‐ and chemoselectivity, compared to wild‐type. From this library the variant with the highest perillyl alcohol ratio was fine‐tuned by two additional rounds of molecular modeling, diversification, and screening. In total only 29 variants needed to be screened to identify the triple mutant A264V/A238V/L437F that converts (4R)‐limonene to perillyl alcohol with a selectivity of 97 %. Focusing mutagenesis on a small number of relevant positions identified by computational approaches is the key for efficient screening for enzyme selectivity.  相似文献   

7.
A few lipases, such as Candida antarctica lipase A (CAL‐A), are known to possess acyltransferase activity. This enables the enzyme to synthesize fatty acid esters from natural oils and alcohols even in the presence of bulk water. Unfortunately, fatty acids are still formed in these reactions as undesired side‐products. To reduce the amount of fatty acids, several CAL‐A variants were rationally designed based on its crystal structure. These variants were expressed in Escherichia coli and Pichia pastoris, purified, and their acyltransferase/hydrolase activities were investigated by various biocatalytic approaches. Among the investigated variants, mutant Asp122Leu showed a significant decrease in the hydrolytic activity, thus reducing the side‐product yield during acylation. As desired, this variant retained wild‐type process‐relevant features like pH profile and thermostability.  相似文献   

8.
Baeyer–Villiger monooxygenase (BVMO)‐mediated regiodivergent conversions of asymmetric ketones can lead to the formation of “normal” or “abnormal” lactones. In a previous study, we were able to change the regioselectivity of a BVMO by mutation of the active‐site residues to smaller amino acids, which thus created more space. In this study, we demonstrate that this method can also be used for other BVMO/substrate combinations. We investigated the regioselectivity of 2‐oxo‐Δ3‐4,5,5‐trimethylcyclopentenylacetyl‐CoA monooxygenase from Pseudomonas putida (OTEMO) for cis‐bicyclo[3.2.0]hept‐2‐en‐6‐one ( 1 ) and trans‐dihydrocarvone ( 2 ), and we were able to switch the regioselectivity of this enzyme for one of the substrate enantiomers. The OTEMO wild‐type enzyme converted (?)‐ 1 into an equal (50:50) mixture of the normal and abnormal products. The F255A/F443V variant produced 90 % of the normal product, whereas the W501V variant formed up to 98 % of the abnormal product. OTEMO F255A exclusively produced the normal lactone from (+)‐ 2 , whereas the wild‐type enzyme was selective for the production of the abnormal product. The positions of these amino acids were equivalent to those mutated in the cyclohexanone monooxygenases from Arthrobacter sp. and Acinetobacter sp. (CHMOArthro and CHMOAcineto) to switch their regioselectivity towards (+)‐ 2 , which suggests that there are hot spots in the active site of BVMOs that can be targeted with the aim to change the regioselectivity.  相似文献   

9.
In order to improve the efficiency of directed evolution experiments, in silico multiple‐substrate clustering was combined with an analysis of the variability of natural enzymes within a protein superfamily. This was applied to a Pseudomonas fluorescens esterase (PFE I) targeting the enantioselective hydrolysis of 3‐phenylbutyric acid esters. Data reported in the literature for nine substrates were used for the clustering meta‐analysis of the docking conformations in wild‐type PFE I, and this highlighted a tryptophan residue (W28) as an interesting target. Exploration of the most frequently, naturally occurring amino acids at this position suggested that the reduced flexibility observed in the case of the W28F variant leads to enhancement of the enantioselectivity. This mutant was subsequently combined with mutations identified in a library based on analysis of a correlated mutation network. By interrogation of <80 variants a mutant with 15‐fold improved enantioselectivity was found.  相似文献   

10.
Chemical modification has been used to introduce the unnatural amino acid γ‐thialysine in place of the catalytically important Lys165 in the enzyme N‐acetylneuraminic acid lyase (NAL). The Staphylococcus aureus nanA gene, encoding NAL, was cloned and expressed in E. coli. The protein, purified in high yield, has all the properties expected of a class I NAL. The S. aureus NAL which contains no natural cysteine residues was subjected to site‐directed mutagenesis to introduce a cysteine in place of Lys165 in the enzyme active site. Subsequently chemical mutagenesis completely converted the cysteine into γ‐thialysine through dehydroalanine (Dha) as demonstrated by ESI‐MS. Initial kinetic characterisation showed that the protein containing γ‐thialysine regained 17 % of the wild‐type activity. To understand the reason for this lower activity, we solved X‐ray crystal structures of the wild‐type S. aureus NAL, both in the absence of, and in complex with, pyruvate. We also report the structures of the K165C variant, and the K165‐γ‐thialysine enzyme in the presence, or absence, of pyruvate. These structures reveal that γ‐thialysine in NAL is an excellent structural mimic of lysine. Measurement of the pH‐activity profile of the thialysine modified enzyme revealed that its pH optimum is shifted from 7.4 to 6.8. At its optimum pH, the thialysine‐containing enzyme showed almost 30 % of the activity of the wild‐type enzyme at its pH optimum. The lowered activity and altered pH profile of the unnatural amino acid‐containing enzyme can be rationalised by imbalances of the ionisation states of residues within the active site when the pKa of the residue at position 165 is perturbed by replacement with γ‐thialysine. The results reveal the utility of chemical mutagenesis for the modification of enzyme active sites and the exquisite sensitivity of catalysis to the local structural and electrostatic environment in NAL.  相似文献   

11.
The Escherichia coli esterase YbfF displays high activity towards 1,2‐O‐isopropylideneglycerol (IPG) butyrate and IPG caprylate, and prefers the R‐enantiomer of these substrates, producing the S‐enantiomer of the IPG product in excess. To improve the potential of the enzyme for the kinetic resolution of racemic esters of IPG, an enhancement of the activity and enantioselectivity would be highly desirable. Molecular docking of the R‐enantiomer of both IPG esters into the active site of YbfF allowed the identification of proximal YbfF active site residues. Four residues (25, 124, 185 and 235) were selected as targets for mutagenesis, in order to enhance YbfF activity and enantioselectivity towards IPG esters. Random mutagenesis at positions 25, 124, 185 and 235 yielded several best YbfF variants with enhanced activity and enantioselectivity towards IPG esters. The best YbfF mutant, W235I, exhibited a 2‐fold higher enantioselectivity than wild‐type YbfF, with an E=38 for IPG butyrate and an E=77 for IPG caprylate. Molecular docking experiments further support the enhanced enantioselectivity shown experimentally and the structural effects of this amino acid substitution on the active site of YbfF are provided. The engineered W235I mutant is an attractive catalyst for practical applications in the kinetic resolution of IPG esters.  相似文献   

12.
A recombinant Escherichia coli strain was constructed which efficiently expressed the enantioselective nitrilase from Alcaligenes faecalis DSMZ 30030 as a hisitidine‐tagged enzyme variant under the control of a rhamnose inducible promoter. The enzyme was purified from cell extracts and used for the preparation of cross‐linked enzyme aggregates (CLEAs). The conditions for the preparation of the CLEAs were optimized using various organic solvents and cross‐linking agents and a procedure was developed which combined a precipitation with 85 % (v/v) isopropyl alcohol and a cross‐linking with 30 mM glutaraldehyde. Thus, about 80 % of the initial nitrilase activity could be incorporated into the CLEAs. The hydrolysis of racemic mandelonitrile to (R)‐mandelic acid was compared between the soluble nitrilase preparations and their CLEAs (nit‐CLEAs). The nitrilase activity in the CLEAs was at 30 °C and 60 °C about 5 times more stable than in the soluble preparations. The CLEAs could be reused 5 times with only about 10 % reduction in activity. The enantioselectivity of the nitrilase for the formation of (R)‐mandelic acid from racemic mandelonitrile decreased for both preparations with increasing temperatures (10 °C to 50 °C), but this effect was significantly less pronounced for the CLEAs. A detailed analysis of solvent effects on nitrilase enantioselectivity allowed thermodynamic insights into contributions from free energy component (activation enthalpy and entropy) to chiral preference of nitrilase in such non conventional media.  相似文献   

13.
Sortase‐catalyzed transacylation reactions are widely used for the construction of non‐natural protein derivatives. However, the most commonly used enzyme for these strategies (sortase A from Staphylococcus aureus) is limited by its narrow substrate scope. To expand the range of substrates compatible with sortase‐mediated reactions, we characterized the in vitro substrate preferences of eight sortase A homologues. From these studies, we identified sortase A enzymes that recognize multiple substrates that are unreactive toward sortase A from S. aureus. We further exploited the ability of sortase A from Streptococcus pneumoniae to recognize an LPATS substrate to perform a site‐specific modification of the N‐terminal serine residue in the naturally occurring antimicrobial peptide DCD‐1L. Finally, we unexpectedly observed that certain substrates (LPATXG, X=Nle, Leu, Phe, Tyr) were susceptible to transacylation at alternative sites within the substrate motif, and sortase A from S. pneumoniae was capable of forming oligomers. Overall, this work provides a foundation for the further development of sortase enzymes for use in protein modification.  相似文献   

14.
The crystal structures of the haem domains of Ala330Pro and Ile401Pro, two single‐site proline variants of CYP102A1 (P450BM3) from Bacillus megaterium, have been solved. In the A330P structure, the active site is constricted by the relocation of the Pro329 side chain into the substrate access channel, providing a basis for the distinctive C? H bond oxidation profiles given by the variant and the enhanced activity with small molecules. I401P, which is exceptionally active towards non‐natural substrates, displays a number of structural similarities to substrate‐bound forms of the wild‐type enzyme, notably an off‐axial water ligand, a drop in the proximal loop, and the positioning of two I‐helix residues, Gly265 and His266, the reorientation of which prevents the formation of several intrahelical hydrogen bonds. Second‐generation I401P variants gave high in vitro oxidation rates with non‐natural substrates as varied as fluorene and propane, towards which the wild‐type enzyme is essentially inactive. The substrate‐free I401P haem domain had a reduction potential slightly more oxidising than the palmitate‐bound wild‐type haem domain, and a first electron transfer rate that was about 10 % faster. The electronic properties of A330P were, by contrast, similar to those of the substrate‐free wild‐type enzyme.  相似文献   

15.
The two homologous proteins ribonuclease A and onconase fold through conserved initial contacts but differ significantly in their thermodynamic stability. A disulfide bond is located in the folding initiation site of onconase (the C‐terminal part of the protein molecule) that is missing in ribonuclease A, whereas the other three disulfide bonds of onconase are conserved in ribonuclease A. Consequently, the deletion of this C‐terminal disulfide bond (C87–C104) allows the impact of the contacts in this region on the folding of onconase to be studied. We found the C87A/C104A‐onconase variant to be less active and less stable than the wild‐type protein, whereas the tertiary structure, which was determined by both X‐ray crystallography and NMR spectroscopy, was only marginally affected. The folding kinetics of the variant, however, were found to be changed considerably in comparison to wild‐type onconase. Proton exchange experiments in combination with two‐dimensional NMR spectroscopy revealed differences in the native‐state dynamics of the two proteins in the folding initiation site, which are held responsible for the changed folding mechanism. Likewise, the molecular dynamics simulation of the unfolding reaction indicated disparities for both proteins. Our results show that the high stability of onconase is based on the efficient stabilization of the folding initiation site by the C‐terminal disulfide bond. The formation of the on‐pathway intermediate, which is detectable during the folding of the wild‐type protein and promotes the fast and efficient refolding reaction, requires the presence of this covalent bond.  相似文献   

16.
To enhance the phosphorylating activity of the bacterial nonspecific acid phosphatase from Salmonella enterica ser. typhimurium LT2 towards dihydroxyacetone (DHA), a mutant library was generated from the native enzyme. Three different variants that showed enhanced activity were identified after one round of epPCR. The single mutant V78L was the most active and showed an increase in the maximal DHAP concentration to 25 % higher than that of the wild‐type enzyme at pH 6.0. This variant is 17 times more active than the wild‐type acid phosphatase from Salmonella enterica ser. typhimurium LT2 in the acid phosphatase/aldolase cascade reaction at pH 6.0 and is also six times more active than the phosphatase from Shigella flexneri that we previously used.  相似文献   

17.
The ring‐opening polymerization of ε‐caprolactone was carried out with poly(ethylene‐co‐vinyl alcohol) as a macroinitiator to synthesize poly(ethylene‐co‐vinyl alcohol)‐graft‐polycaprolactone (EVOH‐g‐PCL). A simple low‐density polyethylene (LDPE)/polycaprolactone (PCL) (64/36) blend lost 5.3 wt % of its original weight after 90 days of a soil burial test. However, the elongation at break of the LDPE/PCL blend remained almost invariable even after the solid burial test because the tensile properties depended mostly on the LDPE phase on account of the poor interaction between the continuous LDPE matrix and the dispersed PCL phase. For EVOH‐g‐PCL, the elongation at break decreased drastically as a result of the soil burial test, and the reduction of the elongation at break was more pronounced for EVOH‐g‐PCL with a higher PCL concentration, even though the weight loss of EVOH‐g‐PCL after the soil burial test was as low as 1.2–1.3% and was nearly independent of the PCL concentration. Few holes were observed in EVOH‐g‐PCL when the PCL concentration was less than 26 wt % after an accelerated hydrolysis experiment at 60°C for 7 days in a 0.1M KOH solution. In contrast, the hydrolysis formed small holes in EVOH‐g‐PCL with a PCL concentration of 36 wt %. The LDPE/PCL blend was much better percolated, as a result of the hydrolysis, than EVOH‐g‐PCL with the same PCL concentration; the soil burial test showed the same results. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1064–1071, 2005  相似文献   

18.
Two highly engineered halohydrin dehalogenase variants were characterized in terms of their performance in dehalogenation and epoxide cyanolysis reactions. Both enzyme variants outperformed the wild‐type enzyme in the cyanolysis of ethyl (S)‐3,4‐epoxybutyrate, a conversion yielding ethyl (R)‐4‐cyano‐3‐hydroxybutyrate, an important chiral building block for statin synthesis. One of the enzyme variants, HheC2360, displayed catalytic rates for this cyanolysis reaction enhanced up to tenfold. Furthermore, the enantioselectivity of this variant was the opposite of that of the wild‐type enzyme, both for dehalogenation and for cyanolysis reactions. The 37‐fold mutant HheC2360 showed an increase in thermal stability of 8 °C relative to the wild‐type enzyme. Crystal structures of this enzyme were elucidated with chloride and ethyl (S)‐3,4‐epoxybutyrate or with ethyl (R)‐4‐cyano‐3‐hydroxybutyrate bound in the active site. The observed increase in temperature stability was explained in terms of a substantial increase in buried surface area relative to the wild‐type HheC, together with enhanced interfacial interactions between the subunits that form the tetramer. The structures also revealed that the substrate binding pocket was modified both by substitutions and by backbone movements in loops surrounding the active site. The observed changes in the mutant structures are partly governed by coupled mutations, some of which are necessary to remove steric clashes or to allow backbone movements to occur. The importance of interactions between substitutions suggests that efficient directed evolution strategies should allow for compensating and synergistic mutations during library design.  相似文献   

19.
Polyunsaturated fatty acids (PUFA), especially docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), can be concentrated in glycerides by hydrolyzing tuna oil withGeotrichum candidum lipase, the main components in the resulting oil being triglycerides. The reaction mechanism of this selective hydrolysis was investigated. Although the lipase acted well on the esters of oleic, linoleic, and α-linolenic acids, it did not affect the esters of γ-linolenic acid, arachidonic acid, EPA, and DHA as much. The action of PUFA-glycerides was mono-> di- > triglycerides. Furthermore, the condensation of PUFA-partial glycerides and PUFA occurred even in the presence of a large amount of water, and the partial glycerides converted to the triglycerides by transacylation. These results suggested that the PUFA-rich triglycerides were accumulated in the glyceride fraction by the following mechanism: The PUFA-partial glycerides generated by the hydrolysis were converted to PUFA-triglycerides by condensation and transacylation reactions. As the PUFA-triglycerides formed were the poor substrates of lipase, they were accumulated in the reaction mixture.  相似文献   

20.
The aim of this study is to pursue the identification and characterization of different CAL‐A variants displaying higher specificity toward erucic acid than CAL‐A wild type (wt). A careful analysis of the data generated from previously created site‐directed saturation libraries reveals several variants that display a higher preference for the hydrolysis of p‐nitrophenyl (pNP)‐erucate over pNP‐oleate than the wt. The best three candidates (CAL‐A V238D, V238Y, and V286N) are applied in biocatalysis using both Crambe oil and ethyl ester derivatives. When acting on Crambe oil, these CAL‐A variants are as efficient as CAL‐A wt in terms of C22:1 enrichment and product recovery independently of the temperature (enrichment and recovery values between 70–76% and 67–79% at 37 °C, and between 71–73% and 61–75% at 50 °C). In contrast, hydrolysis of Crambe ethyl esters leads to substantially increased accumulations of C22:1 and recovery values (V238Y: 78% enrichment and 92% recovery; V286N: 83% enrichment and 91% recovery) when using CAL‐A V238Y and CAL‐A V286N compared to CAL‐A wt (78% enrichment, 60% recovery) in the free fatty acid fraction. Practical Applications: This study describes the enhancement of lipase CAL‐A selectivity for the isolation and recovery of erucic acid (C22:1) from plant oil or its ethyl ester derivatives. Hence, this approach could represent a more eco‐friendly alternative for its application in processes where the erucic acid is used as building block, such as the production of surfactants or polymers.  相似文献   

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