Patent Application
20140273154
Endo-β-D-glucanases are fibrolytic enzymes that play an important role in the hydrolysis of polysaccharide components. Several industrial applications of glucanase enzymes have been reported. For instance, 1,3-1,4-β-D-glucanases (EC 3.2.1.73, lichenase) can be used in the brewing industry and in the animal feeds industry. 1,3-β-D-Glucanases (laminarinases) have potential for use in commercial yeast extract production and for the conversion of algal biomass to fermentable sugars in the generation of bioenergy. The enzymes also have antimycotic activity for disease protection in plants.
Described herein is a fusion polypeptide that includes (a) a first segment containing a Fibrobacter succinogenes 1,3-1,4-β-D-glucanase (Fsβ-glucanase) or a fragment thereof; (b) a second segment containing a first Thermotoga maritima 1,3-β-D-glucanase (TmLam) or a fragment thereof; and (c) an optional third segment containing a second Thermotoga maritima 1,3-β-D-glucanase (TmLam) or a fragment thereof; wherein the fusion polypeptide has a glucanase activity. For example, the first segment can contain TFsW203F, the second segment and the optional third segment can each contain a first carbohydrate binding module of TmLam (CBM1), a second carbohydrate binding module of TmLam (CBM2), or a catalytic domain (CD) of TmLam. In one embodiment, the fusion polypeptide can have an amino acid sequence that is at least 90% identical to the amino acid sequence of SEQ ID NO:5, 7, 9, 11, or 13.
Also described herein is an isolated nucleic acid molecule that has a nucleic acid sequence encoding the above-described fusion polypeptide. An expression vector containing the nucleic acid molecule and a host cell harboring the expression vector are also described.
The fusion polypeptide, which has glucanase activity, can be used to degrade a substrate, e.g., lichenin and larmarine.
The details of one or more embodiments are set forth in the accompanying drawing and the description below. Other features, objects, and advantages will be apparent from the description and drawing, and from the claims.
Described herein are fusion polypeptides each containing at least a Fibrobacter succinogenes 1,3-1,4-β-D-glucanase (Fsβ-glucanase) or fragment thereof, and at least a Thermotoga maritima 1,3-β-D-glucanase (TmLam) or a fragment thereof. The fusion polypeptides exhibit dual substrate specificities toward lichenin and larmarine, and also increased catalytic efficiencies and thermotolerance as compared to the parental single domain enzymes.
Shown below are a TmLam amino acid sequence and a TmLam nucleic acid sequence.
The fusion polypeptides and nucleic acid molecules described herein can be generated using methods known in the art, e.g., recombinant technology.
The nucleic acid molecules can be used to express the polypeptides and fusion polypeptides described herein. Each nucleic acid molecule can be linked to suitable regulatory sequences to generate an expression vector.
Examples of the vector include a plasmid, cosmid, or viral vector. The vector includes a nucleic acid in a form suitable for expression of the nucleic acid in a host cell. Preferably the vector includes one or more regulatory sequences operatively linked to the nucleic acid sequence to be expressed. A regulatory sequence includes promoters, enhancers, and other expression control elements (e.g., T7 promoter, cauliflower mosaic virus 35S promoter sequences or polyadenylation signals). Regulatory sequences include those that direct constitutive expression of a nucleotide sequence, as well as tissue-specific regulatory and/or inducible sequences. The design of the expression vector can depend on such factors as the choice of the host cell to be transformed, the level of expression of protein desired, and the like. The expression vector can be introduced into host cells to produce the polypeptide or fusion protein of this invention.
Host cells include E. coli cells, insect cells (e.g., using baculovirus expression vectors), plant cells, yeast cells, and mammalian cells. See e.g., Goeddel, (1990) Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif.
Host cells containing an expression vector for expressing a fusion polypeptide described herein can be cultured under conditions allowing expression of the polypeptide. The expressed polypeptide can then be isolated from the host cells or culture medium.
The isolated fusion polypeptide can be used for various purposes. Thus, described herein are also methods of using the fusion polypeptides. For example, the fusion polypeptide can be used to degrade a substrate, e.g., a polysaccharide. The fusion polypeptide can also be used in industrial applications.
The specific example below is to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. Without further elaboration, it is believed that one skilled in the art can, based on the description herein, utilize the present disclosure to its fullest extent. All publications cited herein are herein incorporated by reference in their entirety.
A truncated and mutated 1,3-1,4-β-D-glucanase gene (TFsW203F) from Fibrobacter succinogenes, and a 1,3-β-D-glucanase gene (TmLam) from hyperthermophilic Thermotoga maritima were used to generate hybrid enzymes. The substrate binding domains (TmB1 and TmB2) and the catalytic domain (TmLamCD) of TmLam were linked to the N- or C-terminus of TFsW203F to create four hybrid enzymes, TmB1-TFsW203F, TFsW203F-TmB2, TmB1-TFsW203F-TmB2, and TFsW203F-TmLamCD. The results obtained from kinetic studies show that increased specific activities and turnover rate for lichenan and laminarin were observed in TmB1-TFsW203F-TmB2 and TFsW203F-TmLamCD, respectively. Furthermore, fluorescence and CD spectrometric analyses indicated that the hybrid TFsW203F-TmLamCD was structurally more stable and more thermal tolerant than the parental TFsW203F.
Thermotoga maritima MSB8 (Huber et al., 1986, Arch. Microbiol., 144, 324-333) was purchased from ATCC (DSM 3109) and used as the source for the cloning of the 1,3-β-D-glucanase (TmLam) gene. T. maritima cells were grown anaerobically at 70° C. in a growth medium (ATCC #2114 broth) composed of 25% artificial sea water (v/v) supplemented with the following components (g/L): soluble starch, 5.0; yeast extract, 0.5; NaCl, 20; KH2PO4, 0.5; NiCl2.6H2O, 0.002; Na2S.9H2O, 0.5; resazurin, 0.001; and Wolfe's mineral solution (1.5%, v/v). E. coli strain XL-1 Blue (Stratagene) was used for the purpose of cloning, and BL21(DE3) (Novagen) competent cells were employed for the overexpression of cloned genes. The conditions for cultivation of recombinant cells, media and overexpression were in accordance with previously-published methods (Wen et al., 2005, Biochemistry, 44, 9197-9205), and plasmid pET26b(+) was used for the cloning and expression of recombinant glucanase genes.
The chromosomal DNA of T. maritima was isolated from cultured cell using QIAGEN Genomic-tip 20/G (QIAGEN). The DNA was used as the template for PCR amplification of the open reading frame of TmLam gene (without the presence of N-terminal signal sequence) by a pair of specific primers TmNcoI and TmNotI. See Table 1. The amplified DNA fragments were digested with NcoI and Nod and then subcloned into pET26b(+) vector to form pTmLam.
The TFsW203F plasmid (pTFsW203F) previously created (Tsai et al., 2011, Biochem. Bioph. Res. Comm., 407, 593-598) was used to create hybrid enzymes with the CBM and catalytic domains of TmLam. The primers for the construction of hybrid glucanase genes are shown in Table 1.
aVector-specific primers
The DNA fragments of N-terminal CBM4-1 (TmB1) and C-terminal CBM4-2 (TmB2) from the TmLam gene were obtained by PCR amplification with specific primer pairs, 5A and TmB1(−)/NcoI and TmB2(+)/EcoRI and 3B, respectively. The amplified DNA products were digested with appropriate restriction enzymes and ligated into plasmids pTFsW203F, before being pre-digested with the same restriction enzymes to obtain new chimera plasmids pTmB1-TFsW203F and pTFsW203F-TmB2. Furthermore, the amplified TmB1 DNA fragments and plasmid pTFsW203F-TmB2 were all pre-digested with NcoI and then ligated together to create another gene construct, pTmB1-TFsW203F-TmB2. The chimera plasmids containing the catalytic domain of TmLam, pTmLamCD or the hybrid enzyme genes, pTFsW203F-TmLamCD and pTmLamCD-TFsW203F, in other fibrolytic enzymes, were created and obtained using specific pairs of primers: TmB1(+)/NcoI and TmB2(−)/HindIII, TmB1/NcoI and TmB2(−)/NcoI as well as TmB1(+)/EcoRI and TmB2(−)/NotI, respectively.
All of the created gene constructs were verified by the automated sequencing method through the use of the ABI Prism BigDye Terminator Cycle Sequencing Ready Reaction kit (Applied Biosystems). The confirmed plasmid genes were transformed into E. coli BL21(DE3) host cells for protein overexpression.
E. coli BL21(DE3) cells harboring the appropriate plasmids were grown in two liters of LB medium containing kanamycin at 33° C. until the OD600 reached 0.8-1.0. Protein expression was then induced by the addition of IPTG (0.6 mM) and the culture was allowed to shake at 28-30° C. for 16 hours. The supernatant, containing expressed proteins, was harvested from the culture medium, and PMSF (1 mM) and leupeptin (1 μg/ml) were added to avoid protein degradation. Subsequently, the supernatant was concentrated using a Pellicon™-2 Mini Cassette Holder with a Biomax™ 10K filter assembled on a Labscale™ TFF system (Millipore). The concentrate was dialyzed against sodium phosphate buffer (50 mM, pH 8.0) containing imidazole (10 mM) and NaCl (0.3 M) at 4° C., and the resulting solution was applied to a 1.5×20 cm column containing pre-equilibrated HIS-select™ nickel affinity gel (Sigma). The column was then washed with sodium phosphate buffer (50 mM, pH 8.0) containing imidazole (20 mM) and NaCl (0.3 M), followed with a five-times column volume of imidazole gradient (20-250 mM) in the same buffer, for protein elution. The fractions were analyzed by 12% SDS-PAGE, and the target proteins were pooled together for dialysis against sodium phosphate buffer (50 mM, pH 7.0) at 4° C. Purified proteins were stored in the presence of glycerol (10%) at −20° C., and protein concentration was determined by the Bradford method (Bio-Rad), using BSA (Sigma) as the standard.
The enzymatic activities of the purified parental or hybrid glucanases were measured by determining the rate of reducing sugar production from the hydrolysis of substrates, lichenan and laminarin (Sigma). The reduction of sugars was quantified by the use of 3,5-dinitrosalicylic acid (DNS) reagent (Wood and Bhat, 1988, Methods in Enzymology, Vol. 160, Academic Press, pp. 87-112) with glucose as the standard. A standard enzyme activity assay was performed in a 0.3 ml reaction mixture, as described previously (Cheng et al., 2002, Biochemistry, 41, 8759-8766). One unit of enzyme activity was defined as the amount of enzyme required to produce 1 μmol of reducing sugar (glucose equivalent) per minute, and the specific activity was expressed in μmol of glucose per minute per nmol of protein. Various amounts of purified enzymes were used in each kinetic assay reaction, depending on the enzymatic activity. The kinetic data was analyzed using either ENZFITTER (BIOSOFT).
The substrate-binding capability of the single domain and hybrid glucanases was evaluated using affinity SDS-PAGE. Unheated protein samples in a lysis buffer, in which the enzymes were not denatured and remained active, were separated on a 12% SDS gel with or without the presence of 0.1% substrate lichenan, along with a set of molecular weight (MW) standards (PageRuler™ Prestained Protein Ladder 10-170-kDa, Fermentas) on the same gel. After electrophoresis, proteins were visualized by staining with Coomassie brilliant blue R-250. Rf0 and Rf were defined as the ratio of the migration distance moved by each protein sample to the migration distance of the dye front on gel without and with substrate, respectively.
Purified parental and hybrid glucanases were incubated individually for ten minutes at temperatures within the range of 30° C. and 80° C., at intervals of 5° C. Residual enzyme activity was determined immediately after heat treatment in sodium citrate buffer (50 mM, pH 6.0) at 45° C. using lichenan as the substrate (Cheng et al., 2002, Biochemistry, 41, 8759-8766).
The fluorescence emission spectra of TFsW203F and TFsW203F-TmLamCD were taken on an AMICO-Bowman Series 2 spectrometer (Spectronic Instruments) with a 10×10 mm quartz cuvette incubated at 20° C., 50° C. and 75° C., respectively. Emission spectra were recorded from 310 nm to 430 nm by excitation at 295 nm, with a 4-nm monochromator bandpass. A final protein concentration of 30 μg/ml in sodium phosphate buffer (50 mM, pH 7.0) was used for all assays. Glucanase samples denatured with urea (8 M), or urea-denatured and then renatured by dialysis against sodium phosphate buffer (50 mM, pH 7.0) at 4° C. for 24 hours, were also analyzed for their fluorescence emission spectra at 25° C. Each measurement was carried out in triplicate.
CD spectrometric studies on the TFsW203F and TFsW203F-TmLamCD proteins were carried out on a Jasco J715 CD spectrometer with a 10-mm cell at a range of temperatures from 30° C. to 90° C. Spectra were collected from 200 nm to 260 nm in 0.1 nm increments. Each spectrum was blank-collected and smoothed using a software package provided with the instruments.
Purified TFsW203F and TFsW203F-TmLamCD enzymes (30 μg/ml) were pretreated at 90° C. for ten minutes and then transferred to room temperature (25° C.). Recovery of enzymatic activity and protein re-folding of the heat-treated proteins within a ten-minute time frame at room temperature were measured using standard enzyme activity assay and fluorescence spectrometry respectively.
In order to facilitate the interpretation of the kinetic data of TFsW203F and hybrid enzymes from a structural point of view, we created a protein model complexed with oligosaccharides using the 3D structures of TmB2 (1GUI_A), truncated Fsβ-glucanase, TFs (1ZM1_A) (Tsai et al., 2005, J. Mol. Biol., 354, 642-651), TFsW203F (3H0O_A) (Tsai et al., 2011, Biochem. Bioph. Res. Comm., 407, 593-598) and TmLamCD (3AZZ_A) (Jeng et al., 2011, J. Biol. Chem., 286, 45030-45040), along with a model of TmB1 newly created for this study, the structure of which has not yet been resolved. The structural model of TmB1 was generated by using the HHpred (Soding et al., 2005, Nucleic Acids Res., 33, W244-W248), a website for homology detection and structure prediction by HMM-HMM comparison, based on the top five templates with the highest scores (PDB accession code: 3K4Z_A, 1CX1_A, 3P6B_A, 1GU3_A and 1 GUI_A). The secondary structure matching (SSM) algorithm, with default settings for multiple 3D alignment in the PDBeFold server (Krissinel and Henrick, 2004, Acta Crystallogr. D Biol. Crystallogr., 60, 2256-2268), was used to superimpose all protein structures. After this the proteins were docked with β-1,3-cellohexose (from 1GUI) or β-1,3-1,4-celloheptaose (modeled), which were generated and energy-minimized by Coot (Emsley and Cowtan, 2004, Acta Crystallogr. D Biol. Crystallogr., 60, 2126-2132) and REFMAC5 (Murshudov et al., 2011, Acta Crystallogr. D Biol. Crystallogr., 67, 355-367.) from the CCP4 program suite (Collaborative Computational Project, Number 4, 1994; Winn et al., 2011, Acta Crystallogr. D Biol. Crystallogr., 67, 235-242), with the β-1,3-1,4-cellotriose in 1ZM1 as the template. The structural figures were then produced using PyMOL (DeLano Scientific; world wide web at pymol.org).
The N-terminal CBM domain TmB1, catalytic domain TmLamCD, and C-terminal CBM domain TmB2 of TmLam (see SEQ ID NO:1) were amplified using PCR, and then ligated to the TFsW203F enzyme in different combinations. See
The substrate binding ability and behavior of the non-catalytic modules TmB1 and TmB2 from heterogeneous enzyme (TmLam) in the constructs of hybrid lichenases were evaluated by visualizing the relative mobility retardation of individual proteins in 12% SDS gel with or without the presence of 0.1% lichenan, along with a set of MW standards on the same gel. The relative mobility distances (Rf values) of the MW standards were used as the negative control of substrate retardation effect and as the reference for demonstrating the electrophoretic quality of the same set of protein samples separated in two independent gels with or without substrate. See
aRf0 and Rf were defined as the ratio of the migration distance moved by each protein sample to the migration distance of the dye front in gel without and with substrate, respectively.
bAccording to the Zverlov et al. method (2001), the retardation factor Kr was calculated using the relative migration distance Rf0 (without substrate) and Rf (with substrate): Kr = Rf × (Rf0 − Rf)−1. No retardation (Kr > 10); observable retardation (Kr 6.0 − 0.1); strong retardation (Kr < 0.1).
Notably, both TmB1-TFsW203F-TmB2 and TmB1-TFsW203F enzymes showed significant discrepancies between Rf0 and Rf values, and as a result, Kr values (1 and 2) were within the range representing retardation (6.0-0.1). Although little changes of both Rf0 and Rf values and high Kr value (>10) were observed for TFsW203F-TmB2 protein, a group of smeared tailing bands distributed at higher MW range than the original protein band of TFsW203-TmB2 in lichenan-containing gel, suggesting retardation occurred. These results indicate that CBMs from thermophilic T. maritima could function normally for substrate binding when fused with TFsW203F from mesophilic F. succinogenes.
The kinetic properties of purified parental TFsW203F, TmLamCD and their hybrid glucanases were determined under their respective optimal pH and temperature, with lichenan and/or laminarin as the substrate. The values for the Michaelis constant (KM), turnover number (kcat), and catalytic efficiency (kcat/KM) are presented in Table 3.
To compare the specific activities among the enzymes with differing molecular mass in this study, the specific activity is expressed as unit per nmol (U/nmol) of the protein. When lichenan was used as the substrate, the specific activity, kcat, KM and kcat/KM of TFsW203F were determined as 626±38 U/nmol, 10100 s−1, and 5.3±0.6 mg/ml, and 1908 s−1(mg/ml)−1, respectively. Comparing the kinetic data of the truncated mutant TFsW203F with the truncated wild-type PCR-TF-glucanase from the same organism Fibrobacter succinogenes (Wen et al., 2005, Biochemistry, 44, 9197-9205), TFsW203F showed a 1.4-fold increase in catalytic efficiency [1908 vs. 1358 s−1 (mg/ml)−1]. TFsW203F was thus used as the parental enzyme to create various hybrid enzymes in this study.
The kinetic data in Table 3 show that the hybrid enzymes, TmB1-TFsW203F, TFsW203F-TmB2 and TmB1-TFsW203F-TmB2, each had either a slight increased or very similar specific activity and turnover number (˜1.1-fold) as compared to that of TFsW203F, while a slight decrease (1.2-fold) was found in TFsW203F-TmLamCD. The KM for lichenan increased 2.0-fold in TmB1-TFsW203F and 2.6-fold in TFsW203F-TmLamCD, and decreased slightly (1.3-fold) in TmB1-TFsW203F-TmB2. Given this, the catalytic efficiency of the hybrid enzymes increased by 1.5-fold in TmB1-TFsW203F-TmB2, but decreased, by 1.8-fold and 3.1-fold respectively, in TmB1-TFsW203F and TFsW203F-TmLamCD relative to that of TFsW203F (see Table 3). Notably, TmLamCD also revealed activity and affinity toward lichenan, with the specific activity, kcat, KM and kcat/KM determined as 2.3±0.1 U/nmol, 38 s−1, 8.3±0.9 mg/ml and 5 s−1(mg/ml)−1, respectively.
aThe kinetic study was performed with lichenan used as a substrate.
bData from Wen et al. 2005.
cThe kinetic study was performed with laminarin used as a substrate.
dThe kinetic study performed with laminarin used as a substrate showed no detectable activity.
eThe enzymatic reaction was performed at the respective optimal temperatures and pHs as indicated.
f N.D.: Activity not detected.
gOne unit of enzyme activity was defined as the amount of enzyme required to produce 1 μmol of reducing sugar per minute.
When laminarin was used as the substrate in the activity assays, TFsW203F showed no activity, even with a protein concentration four hundred times higher than that where lichenan was used as the substrate. The specific activity, kcat, KM and kcat/KM of TmLamCD were 6.5±0.3 U/nmol, 109 s−1, 1.3±0.2 mg/ml, and 82 s−1(mg/ml)−1 respectively. Although the TFsW203F enzyme did not show any activity in the presence of laminarin, when it was fused to TmLamCD to form the hybrid TFsW203F-TmLamCD enzyme, it displayed superior specific activity and kcat (3.6-fold increase) to TmLamCD, with a value of 23.3±0.0 U/nmol and 389 s−1, though the kcat/KM of the hybrid enzyme remained similar to that of TmLamCD [78 vs. 82 s−1(mg/ml)−1], owing to an increase in KM value (5.0±0.0 mg/ml). See Table 3.
The effects of temperature and pH on the enzymatic activity of the purified parental and hybrid glucanases were also examined. The optimal temperature for TFsW203F, TFsW203F-TmLamCD, TmB1-TFsW203F, TFsW203F-TmB2 and TmB1-TFsW203F-TmB2 was between 45° C. and 50° C., and an optimal pH of 6.0 or 7.0 was observed. The optimal temperature of TmLamCD was approximately 95° C. All of the enzymes exhibited similar pH response profiles in terms of their activities when the individual enzymes were pre-incubated at room temperature for one hour in buffers with pH values ranging from pH 3.0 to 9.0. All of the enzymes showed <20% residual activity after pH 3.0 pre-treatment, and little or no difference (85-100% activity) was found in the tested enzymes pre-incubated at pH 4.0-9.0 (data not shown).
To investigate the influence of the newly-introduced protein domain(s) on the thermal stability of TFsW203F, the proteins were incubated individually for ten minutes at temperatures between the range of 30° C. and 80° C. in 5° C. steps, and the residual enzyme activities measured using lichenan as the substrate. As shown in
Because TFsW203F and TFsW203F-TmLamCD exhibited the most differing temperature sensitivity at 50° C. out of the enzymes compared, we therefore examined their secondary structures at temperatures between 30° C. and 90° C. at 10° C. intervals, using CD spectroscopy. The CD spectral profile of TFsW203F protein revealed no difference at 30° C. and 40° C., but protein unfolding and a substantial loss of structural integrity appeared when temperatures were elevated to 50° C. or 70° C., and protein denaturated at temperatures between 80° C. and 90° C., with random coiled spectra occurring. See
Furthermore, fluorescence spectrometry was employed to investigate the structural integrity of TFsW203F and TFsW203F-TmLamCD glucanases under native (25° C.), heat-treated (50° C. and 75° C.), 8 M urea-denatured, and denatured/renatured conditions. At 25° C. and 50° C., the emission spectra of both glucanases showed similar profiles, with a maximum emission peak of 336 nm. See
As shown in
To evaluate the recovery efficiency of enzymatic activity after high-temperature treatment (90° C., ten minutes), the activities of TFsW203F and hybrid TFsW203F-TmLamCD recovered at room temperature at different time intervals were examined, in parallel to monitoring the fluorescence emission spectra of the enzymes. As is shown in
By modeling, β-1,3-1,4-celloheptaose (representing the original substrate lichenan) fitted neatly into the catalytic cleft of TFsW203F, but laminarihexose (representative of laminarin) did not fit well, and indeed collided with the residues at the active site. The three sequential β-1,3-glucose moieties in the modeled laminarihexose did, however, fit neatly into the active site of TFsW203F. Because the TFsW203F-TmLamCD showed an improvement (a 3.6-fold increase) in specific activity against laminarin as compared to TmLamCD alone, and because we have recently demonstrated that laminaritriose is the major product of TmLamCD toward the substrate laminarin (Jeng et al., 2011, J. Biol. Chem., 286, 45030-45040), the TFsW203F domain in TFsW203F-TmLamCD may play a role in capturing the product laminaritriose, which results in the facilitation and enhancement of the catalytic activity of TFsW203F-TmLamCD toward laminarin. However, a slight decrease (2.6˜3.7-fold) in binding affinity with lichenan and laminarin was also observed in the hybrid enzyme as compared with the TFsW203F and TmLamCD domain alone. This suggest that there might be some subtle steric hindrance from other domains in the hybrid glucanases. On the other hand, in the modeled structures of TmLamCD complexed with β-1,3-1,4-celloheptaose and laminarihexose, the oligosaccharides all fitted nicely into the catalytic cleft of the laminarinase, which in combination with the kinetic data indicated that TmLamCD is capable of hydrolyzing both types of β-glucan substrate.
On the basis of the structural modeling, both TmB1 and TmB2 formed a concave catalytic-like open cleft like that of TFsW203F, which may contribute to holding the lichenan chain in place, and in turn to facilitating the efficient hydrolysis of lichenan in TFsW203F. Furthermore, in comparison to the TmB1 and TmB2 structures modeled, the narrow gate formed by two tryptophans in the binding cleft of TmB2 (about 7.6-8.2 Å) is wider than the carbohydrate binding cleft of TmB1 (about 6.3-7.3 Å) for polysaccharide binding, which may explain why TmB1-TFsW203F had a lower lichenan-binding affinity than TFsW203F-TmB2. The co-existence of both CBMs in TFsW203F do, however, compensate somewhat for the weaker substrate binding, with a consequent slight decrease in KM value, and a corresponding improvement in the overall catalytic efficiency of TmB1-TFsW203F-TmB2 as compared to the single domain enzyme TFsW203F [2834 vs. 1908 s−1(mg/ml)−1].
All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.
From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims.
This application claims priority to U.S. Provisional Application No. 61/789,804, filed on Mar. 15, 2013, the content of which is hereby incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61789804 | Mar 2013 | US |
