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PEDS Advance Access originally published online on May 16, 2005
Protein Engineering Design and Selection 2005 18(5):239-246; doi:10.1093/protein/gzi027
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org

Structure-guided saturation mutagenesis of N-acetylneuraminic acid lyase for the synthesis of sialic acid mimetics

G.J. Williams1,2,3, T. Woodhall1,4, A. Nelson1,4 and A. Berry1,2,5

1Astbury Centre for Structural Biology, 2School of Biochemistry and Microbiology and 4School of Chemistry, University of Leeds, Leeds LS2 9JT, UK 3Present address: School of Pharmacy, University of Wisconsin, 777 Highland Avenue, Madison, WI 53705, USA

5 To whom correspondence should be addressed. E-mail: a.berry{at}leeds.ac.uk


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
Analogues of N-acetylneuraminic acid (sialic acid, NANA, Neu5Ac), including 6-dipropylcarboxamides, have been found to be selective and potent inhibitors of influenza sialidases. Sialic acid analogues are, however, difficult to synthesize by traditional chemical methods and the enzyme N-acetylneuraminic acid lyase (NAL) has previously been used for the synthesis of a number of analogues. The activity of this enzyme towards 6-dipropylcarboxamides is, however, low. Here, we used structure-guided saturation mutagenesis to produce variants of NAL with improved activity and specificity towards 6-dipropylcarboxamides. Three residues were targeted for mutagenesis, Asp191, Glu192 and Ser208. Only substitution at position 192 produced significant improvements in activity towards the dipropylamide. One variant, E192N, showed a 49-fold improvement in catalytic efficiency towards the target analogue and a 690-fold shift in specificity from sialic acid towards the analogue. These engineering efforts provide a scaffold for the further tailoring of NAL for the synthesis of sialic acid mimetics.

Keywords: N-acetylneuraminic acid lyase/sialic acid mimetics/structure-guided saturation mutagenesis/synthesis


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
N-Acetylneuraminic acid (sialic acid, NANA, Neu5Ac, 3) is an essential component of complex carbohydrates, which play pivotal roles in recognition processes in a variety of important cellular recognition processes (Schauer, 1982Go; Varki, 1993Go, 1997Go; Schauer et al., 1995Go). These events are particularly well exemplified in host–pathogen and host–parasite interactions, where the oligosaccharide is often required for invasion, infectivity and survival of the invading organism in the host. Sialic acid analogues therefore represent attractive targets for novel chemotherapeutic agents against bacterial and viral infections (Koketsu et al., 1997Go). For example, influenza virus neuraminidase plays a crucial role in the life cycle of the virus and inhibitors of this enzyme such as zanamivir (GlaxoSmithKline's RelenzaTM) and osteltamivir (Roche's TamifluTM) may be used in the treatment of influenza (Woods et al., 1993Go; Kim et al., 1997Go). Recently, dihydropyrancarboxamides [such as 6 (see Fig. 1)] related to Zanamivir have been shown to be highly potent and selective inhibitors of influenza virus neuraminidases (Smith et al., 1996Go, 1998Go; Sollis et al., 1996Go; Taylor et al., 1998Go). However, the de novo chemical synthesis of sialic acid analogues is difficult and their production (Auge et al., 1984Go; Drueckhammer et al., 1991Go; Kong and von Itzstein, 1995Go, 1997Go; Kim, 1998Go) often relies on the use of the enzyme N-acetylneuraminic acid lyase (NAL) (otherwise called N-acetylneuraminic acid aldolase or sialic acid aldolase). This enzyme catalyses the reversible condensation of pyruvate (2, Figure 1) with N-acetylmannosamine (ManNAc, 1). Although the substrate specificity of the enzyme is relatively broad, couplings involving two-, three- and four-carbon aldehydes are generally unsuccessful (Wong and Whitesides, 1994Go) and the use of the enzyme in the synthesis of analogues is therefore limited.



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  		Fig. 1.. The rationale for the engineering of N-acetylneuraminic acid lyase (NAL). The wild-type NAL catalyses the reversible aldol condensation of pyruvate (2) with the open-chain form of N-acetylmannosamine (1). Cyclization then produces N-acetylneuraminic acid (Neu5Ac) (3) (the carbon atoms originating from pyruvate are marked with asterisks). Engineering of an enzyme was undertaken to condense the aldehyde (4) with pyruvate to yield the dipropylamides (5), which could be precursors of analogues of the sialidase inhibitor (6).

 
As part of our long-term aim of creating broad specificity variant aldolases for the synthesis of sialic acid mimetics, we report here the successful structure-guided redesign of the substrate specificity of NAL. We chose first to redesign the specificity towards amides such as 5, which have large, bulky substituents in place of the 6-glycerol moiety of sialic acid. The screening substrate 5a was chemically synthesized from isoascorbic acid for this purpose (Woodhall et al., 2005aGo,bGo). The known crystal structures of the N-acetylneuraminic acid lyases from Escherichia coli (Izard et al., 1994Go) and Haemophilus influenzae (Barbosa et al., 2000Go) were used to identify active site residues that interact with the polar 6-glycerol moiety of sialic acid. Saturation mutagenesis was then carried out at each identified position. The libraries of variants were then screened for the ability to cleave 5a in order to identify mutants with altered substrate specificity.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Acknowledgements
 References
 
Materials

The restriction enzymes EcoRI and HindIII, T4 DNA ligase, Pfu DNA polymerase, WizardTM Miniprep DNA purification kits and DNA markers were obtained from Promega (Southampton, UK), lactate dehydrogenase was from Boehringer Mannheim (Mannheim, Germany), nicotinamide adenine dinucleotide (reduced form) [NADH] lithium salt, N-acetylneuraminic acid (Type IV) and pyruvate were from Sigma Chemicals (Poole, UK), QIAquick and QIAspin DNA extraction kit from QIAgen (Crawley, UK) and Chelating Sepharose Fast Flow from Pharmacia (Milton Keynes, UK). Dialysis tubing was purchased from Medicell International (London, UK). All other chemicals were of analytical grade.

Bacterial strains

E.coli EP-MaxTM 10B Competent cells [{Delta}(mrr-hsdRMS-mrcBC) {Phi}80d lacZ{Delta}M15 {Delta}lacX74 deoR recA1 araD139 {Delta}(ara, leu)7697 galU galK rpsL nupG {lambda}–] were supplied by Bio-Rad (UK). The expression vector pKK223-3 was from Pharmacia.

Plasmid construction

The E.coli nanA gene was amplified by polymerase chain reaction (PCR) from genomic DNA and cloned into the expression vector pKK223-3 (Pharmacia) to yield the vector pKnanA. The gene was reamplified from pKnanA using primers NANA-HIS-FOR (5'-TCA GAG GAA TTC ATG GAA CAC CAT CAC CAT CAC CAT GCA ACG AAT TTA CGT-3') and pKK-REV (5'-GGC TGA AAA TCT TCT TCT-3') to add an N-terminal hexahistidine tag to the enzyme to ease purification. The PCR product from the second amplification was subcloned into pKK223-3 using EcoRI and HindIII restriction sites. The ligation mixture was transformed into E.coli EP-MaxTM 10B by electroporation. Plasmid was purified from a single colony and the DNA sequence of the gene was verified. This plasmid, designated pKnanA-His6, was used as the template for mutagenesis.

Saturation mutagenesis at positions Asp191, Glu192 and Ser208

Saturation mutagenesis at positions Asp191, Glu192 and Ser208 was carried out using the Megaprimer PCR method (Reikofski and Tao, 1992). The oligonucleotides used were D191X, 5'-TAT AAC GGT TAC NNK GAA ATC TTC GCC-3'; E192X, 5'-AAC GGT TAC GAA NNK ATC TTC GCC TCT-3'; and S208X, 5'-GGT GGT ATC GGC NNK ACC TAC AAC ATC-3'. In separate reactions, megaprimers were generated for each saturation library using the D191X, E192X and S208X oligos along with pKK-REV and using pKnanA-His6 as template. The PCR reaction was carried out in a 50 µl reaction mixture consisting of 100 pmol each primer, 0.5 µg of plasmid, 0.2 mM each dNTP and 2.5 units of Pfu DNA polymerase in the reaction buffer provided by the supplier. Amplification involved an initial denaturation step at 95°C for 5 min followed by cycling at 95°C for 1 min, 55°C for 1 min and 72°C for 2 min for 30 cycles, then a final extension for 2.5 min at 72°C. The megaprimers were purified from an agarose gel using the QIAspin kit and used as the forward primers in conjunction with pKK-FOR (5'-GGA TAA CAA TTT CAC ACA GG-3') in the next round of PCR, again with pKnanA-His6 as template. Full-length PCR products were purified from agarose gels and subcloned into pKK223-3 as described for the wild-type nanA gene above.

Library screening

Individual colonies were used to inoculate wells of a 96 deep-well plate containing 1.0 ml of 2TY medium with 50 µg/ml ampicillin and 0.1 mM IPTG. The plates were sealed and shaken at 250 r.p.m. at 37°C for 18 h. Following cell growth, 50 µl of each culture were transferred to a 96-well plate containing 50 µl glycerol in each well and stored at –20°C. The remainder was centrifuged at 2000 g and 4°C for 20 min and the supernatants were removed. Each cell pellet was resuspended in 0.5 ml of 50 mM Tris–HCl (pH 7.5) containing 1 mg/ml lysozyme. The plates were frozen and thawed at room temperature. Cell debris was collected by centrifugation at 2000 g and 4°C for 20 min and 50 µl of each cleared supernatant were transferred to a new 96-well plate for assay.

The cleavage of screening substrates was monitored by coupling the production of pyruvate to the oxidation of NADH using lactate dehydrogenase. A portion of the cleared supernatants was diluted 4-fold and 50 µl were transferred to a fresh 96-well microtitre plate. To each sample were added 250 µl of 50 mM Tris–HCl (pH 7.5) containing 0.33 mM NADH, 0.1 U of lactate dehydrogenase (LDH) and 0.5–3.0 mM substrate using a multi-channel pipette. The microplate was then placed in a FLUOstar Galaxy 96-well microplate spectrophotometer (BMG Labtechnologies, Aylesbury, UK) and shaken for 10 s (1 mm orbit). The absorbance of each well (after 0.1 s positioning delay) at 340 nm and 30°C was monitored every 60 s for 10 min and the rate of change of absorbance per minute was calculated using the FLUOstar Galaxy software. Selected clones were streaked on to 2TY–agar–ampicillin plates from the frozen glycerol stock plate and grown overnight. Cultures grown from single colonies were used for plasmid purification, DNA sequencing and protein expression.

Protein expression and purification

Glycerol stocks of E.coli EP-Max harbouring plasmids encoding mutant nanA genes were used to inoculate 3 ml of 2TY medium supplemented with ampicillin and grown overnight at 37°C with shaking. These cultures were then used to inoculate 5 l of 2TY medium supplemented with 50 µg/ml ampicillin and 0.1 mM IPTG and grown overnight at 37°C with shaking at 200 r.p.m. Cells were harvested in a Heraeus Contifuge 17RS continual action centrifuge at 19 000 g and then resuspended in ~50 ml of 20 mM potassium phosphate buffer (pH 7.5) containing 300 mM NaCl and 10 mM imidazole. The cells were lysed using a French Press at 1500 psi and 4°C. Cell debris was removed by centrifugation at 14 500 g for 1 h at 4°C. Protein was purified using Chelating Sepharose Fast Flow gel charged with Ni2+ using the centrifugation method according to the manufacturer's instructions. Protein was further purified using size-exclusion chromatography. Enzyme was dialysed against 50 mM Tris–HCl (pH 7.5) and applied to a Superdex 200 column previously equilibrated with the same buffer. Peak fractions were pooled, concentrated and stored at 4°C.

Enzyme assay

Sialic acid and analogue cleavage was determined by a standard coupled assay with LDH and NADH. The assay was performed in a 1 ml cuvette at 25°C containing 50 mM Tris–HCl (pH 7.5), 0.2 mM NADH, 0.5 U of LDH and a suitable aliquot of NAL (1–100 µg). The reaction was initiated by the addition of various concentrations of substrate. The decrease in absorbance at 340 nm was recorded as the measure of enzyme activity on a Uvikon 930 spectrophotometer. One unit of lyase activity is defined as the amount of enzyme which catalyses the oxidation of 1 µmol of NADH per minute in this system, using the molar extinction coefficient of NADH as 6220 M–1 cm–1. Kinetic parameters were estimated by non-linear regression analysis (Cleland, 1979).


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Acknowledgements
 References
 
Selection of residues for saturation mutagenesis

Our initial aim was to improve the specificity of NAL towards the production of the dipropylamides 5 (Figure 1), where the 6-glycerol moiety of sialic acid is replaced by a hydrophobic dipropylamide group. These two reactions therefore require different binding pockets in the active site of NAL. Specifically, there is a requirement to provide a favourable environment for the hydrophobic propyl groups whilst simultaneously providing specificity for the new amide group. Random mutagenesis across the entire nanA gene would provide one potential route to the identification of improved variants. However, this approach would require screening at least a few thousand colonies and would require large amounts of the target substrate. This fact, coupled with the availability of several crystal structures of NAL in complex with sialic acid analogues (Barbosa et al., 2000Go), prompted us to begin with a structure-guided approach. Structure-guided mutagenesis is a useful method for altering the substrate specificity of an enzyme when details of substrate/inhibitor–active site interactions are known (e.g. Wang et al., 2001Go; Hill et al., 2003Go). The crystal structure of the H.influenzae NAL in complex with three substrate analogues (Barbosa et al., 2000Go) allowed the identification of residues thought to be responsible for binding the 6-glycerol moiety of sialic acid (Figure 2A and B). The O8 and O9 hydroxyls of the 4-oxosialic acid inhibitor, and by implication the O8 and O9 hydroxyls of sialic acid, form a bidentate interaction with the carboxylate oxygens of Glu191 (H.influenzae numbering). In addition, O8 also forms a hydrogen bond with the NH of Asp190 and the hydroxyl group of Ser207 forms a hydrogen bond with O7. These three residues (Asp190, Glu191 and Ser207) are strictly conserved in the known NAL sequences from other organisms (Figure 2C).



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  		Fig. 2.. Structure and sequence alignment of NAL. (A) Molecular graphics representation of the active site of the Haemophilus influenzae NAL complexed with 4-oxosialic acid (1F7B.pdb) (Barbosa et al., 2000Go) showing the interaction of the glycerol moiety of the substrate with Asp191, Glu192 and Ser208. Figure produced using Molscript (Kraulis, 1991Go) and Povray. (B) Schematic illustration of the interactions of sialic acid with active site residues in NAL. (C) Multiple amino acid alignment of NAL from various microbial sources, showing the absolute conservation of Asp191, Glu192 and Ser208.

 
We therefore targeted these residues for random mutation to produce variants of NAL with altered specificity towards sialic acid mimetics with a dipropylamide in place of the 6-glycerol moiety. Moreover, it was anticipated that variants with improved specificity towards the dipropylamides 5 might also show activity towards other useful amides with alternative hydrophobic groups.

Library construction and screening

The equivalent residues in the E.coli NAL enzyme, Asp191, Glu192 and Ser208 (E.coli numbering used from here on), were selected for saturation mutagenesis by MegaPrimer PCR (see Materials and methods). Three mutant libraries, denoted ‘D191X’, ‘E192X’ and ‘S208X’, were constructed and protein libraries prepared by lysis of resuspended cell pellets harvested from 1 ml cultures of individually picked colonies from each saturation mutagenesis library. Analysis of randomly selected clones from each library by SDS–PAGE showed that 100% of the chosen colonies were expressing protein of similar size to wild-type NAL and that expression levels were not significantly altered (data not shown).

Our aim is to produce an enzyme capable of the synthesis of sialic acid mimetics such as 5. However it is easier to assay the enzyme reactions in the reverse, cleavage (retro-aldol) direction. In this way, successful substrates will yield pyruvate as product, which can in turn be assayed by its reduction with NADH using lactate dehydrogenase. Variants with increased activity in this assay should also show increased activity in the desired synthetic aldol condensation reaction owing to the principle of microscopic reversibility. Similarly, the activity of each variant towards the wild-type substrate, sialic acid, was also measured and the specificity profile of each library is shown in Figure 3. Data in Figure 3 are uncorrected for the level of expression of each variant, although most variants were found to express at approximately the same level as judged by SDS–PAGE analysis.



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  		Fig. 3.. Activities of saturation mutagenesis libraries. Individual bacterial colonies from the transformation of each saturation library were inoculated into the wells of 96 deep-well microtitre plates, grown and lysed in situ. Samples of lysate were then assayed for the cleavage of sialic acid (3) and the dipropylamide 5 each at a concentration of 1 mM. Activity was measured as the rate of decrease of absorbance at 340 nm in a coupled enzyme assay (see Materials and methods). Top panel, activities of colonies from the D191X library. Middle panel, activities of colonies from the E192X library. Bottom panel, activities of colonies from the S208X library. In each case, activity is expressed in change of absorbance at 340 nm per minute per 50 µl of crude cell lysate and is uncorrected for the level of expression of the enzyme or the amount of growth of the culture. The activity of the wild-type enzyme to both substrates is indicated as a solid circle.

 
In this screen of unpurified NAL in crude cell lysate and at the concentration of substrate used in the screen (1 mM), wild-type NAL accepts the dipropylamide 5a as substrate at only ~10% the rate of sialic acid, consistent with the structural differences between the target compound 5a and sialic acid. Most variants from the D191X library (Figure 3, top panel) were inactive towards both substrates, illustrating an important role of this position in stability and/or activity towards both sialic acid (3) and the analogue, 5a. With the S208X library (Figure 3, bottom panel), ~50% of the clones retained some, albeit diminished, activity towards sialic acid, while small improvements in activity towards the dipropylamide were observed (~3-fold). The most significant improvements in activity towards the target substrate (up to 9-fold improvements) were found in the E192X library (Figure 3, middle panel). More than half the clones tested showed ~5-fold increase in activity at the concentration of dipropylamide used. Moreover, these clones displayed almost no detectable activity towards the wild-type substrate, sialic acid. It is clear from these data that substitution at position 192 improves specificity towards the dipropylamide. After the initial screens to identify clones containing the desired activity (Figure 3), several of the most active clones from the E192X library, and also representatives from the D191X and S208X libraries, were regrown, the NAL enzymes were purified to homogeneity and a full steady-state kinetic analysis carried out in order to provide accurate kinetic data for the variants. The nanA genes from these chosen clones were also sequenced to identify the mutations present.

Activity and substrate specificity of wild-type and mutant NAL proteins

The wild-type N-terminally His6-tagged NAL, and also the chosen variants, were purified by metal chelation affinity chromatography and gel filtration to yield proteins at >95% homogeneity. The steady-state kinetic parameters for the cleavage of sialic acid 3 and the dipropylamide (5a) were then determined (Table I). The dipropylamide is a poor substrate for the wild-type enzyme, being accepted with a specificity constant (kcat/Km) ~9-fold lower than that for sialic acid. This ratio of activity agrees well with the ratio determined from assay of the crude cell lysates. The poor specificity constant for the dipropylamide results from both a 3.5-fold lower kcat and a 2.4-fold higher Km for 5a in comparison with sialic acid.


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  		Table I.. Steady-state kinetic parameters of wild-type and variant NAL enzymesa

 
Substitutions of Glu192

Clones from substitution at Glu192 produced the greatest improvements in activity with the dipropylamide 5a in the original screening procedure and these improvements were underscored by the steady-state kinetic parameters with pure enzyme (Table I).

All Glu192 variants examined displayed very different substrate specificity to the wild-type NAL. The variant that showed the largest improvement in kcat/Km for the dipropylamide was E192N. This substitution increased kcat for the dipropylamide 1.8-fold and decreased Km 27-fold, resulting in an ~50-fold increase in catalytic efficiency with the dipropylamide (as judged by the kcat/Km ratio) compared with wild-type NAL. Concomitantly, the E192N mutant is less efficient with sialic acid as substrate, with a 1.6-fold lower kcat and 8.6-fold higher Km for sialic acid. Hence E192N is 13.4-fold less efficient with sialic acid as substrate than wild-type NAL. As a consequence of these changes in catalytic turnover and Michaelis constant, the E192N variant displays a completely inverted specificity (640-fold) with respect to sialic acid and the dipropylamide, the variant E192N accepting the dipropylamide ~6-fold better than wild-type enzyme accepts the natural substrate, sialic acid.

Substitutions of valine and serine at position 192 resulted in similar, but slightly lower, increases in specificity towards the dipropylamide (Table I). Catalytic efficiency towards sialic acid is reduced with both E192V and E192S variants, mainly owing to increases in the Km values for this substrate. In addition to the higher Km value with sialic acid, variant E192V had a 2.2-fold lower kcat, whereas the kcat for the E192S variant was unchanged. With the dipropylamide as substrate, both the E192V and E192S variants showed >30-fold increases in catalytic efficiency; the result of small increases in the kcat of the reaction together with significant decreases in the value of Km for the dipropylamide.

Substitutions of Asp191

Many of the clones from the D191X library showed very low activity with sialic acid and only small improvements in activity with the dipropylamide and only one variant, D191P, was characterized further. Kinetic parameters for the cleavage of sialic acid by D191P could not be accurately determined owing to the very low activity of this mutant. This variant was, however, slightly more efficient with the dipropylamide (1.5-fold) as substrate compared with the wild-type NAL. This is a result of a marginally lower Km (1.9-fold) for the new substrate whereas the kcat for the reaction is not significantly altered. Asp191 is located in the loop between strand g and helix G. The substitution of a proline in this loop might be expected to increase the rigidity of the polypeptide backbone and lead to reductions in kcat for both substrates, as is observed.

Three double mutants (D191E/E192Q, D191E/E192T and D191E/E192M) identified by DNA sequencing of active clones from a screen of a library designed to probe the effect of random changes at position 192 while position 191 was substituted with a Glu were also isolated during the course of this work, but none showed any significant improvements of substrate specificity (Table S1, available as supplementary data at PEDS Online).

Substitutions of Ser208

In general, mutations at Ser208 caused decreases in activity with sialic acid as substrate with only small increases in activity for the dipropylamide. The variant with the highest activity towards the dipropylamide was found to contain a mutation of Ser208 to Gly. Steady-state kinetics of the purified S208G variant showed the same trends in specificity as had been found in the initial screen, but highlighted an inconsistency in the exact magnitude of the specificity change which might be due to the crude nature of the cell extracts used in the initial screen. The purified S208G variant showed a lower Km for the dipropylamide (~4-fold lower than the wild-type enzyme), coupled with a ~3-fold lower kcat for the reaction. At the same time, the S208G mutant was less efficient with the natural substrate with Km (sialic acid) increased by 2.4-fold compared with the wild-type whereas kcat was not significantly altered. Thus, S208G is only slightly improved with the dipropylamide in terms of catalytic efficiency (kcat/Km) and this mutant remains more efficient with the natural substrate, sialic acid.

Substrate specificity of E192N

The dipropylamide 5a was chosen as the initial screening substrate for these studies because of our hope that engineered active sites which could bind and utilize 5a might also successfully use a range of smaller tertiary amides as substrates. In order to investigate the specificity of the wild-type and E192N mutant NAL towards other 6-carboxamides with various hydrophobic substituents, a number of analogues of 5 were therefore tested as substrates in the coupled enzyme assay and the kcat/Km values determined by steady-state kinetics. A total of seven tertiary amides (including 5) were synthesized as ~4:1 mixtures of the C4-equatorial:C4-axial configurations (Woodhall et al., 2005aGo) and examined to probe the enzyme specificities. Figure 4 clearly shows that the substrate specificity of the E192N mutant is generally expanded towards tertiary amides in comparison with the wild-type NAL. The E192N mutant displays higher catalytic efficiencies than wild-type NAL with all the carboxamides tested. In fact, E192N is more efficient with five of these substrates than the wild-type enzyme is towards sialic acid. Catalytic efficiency increases with increasing hydrophobicity for the range of substrates tested. For example, the E192N mutant uses the piperidine derivative [–(CH2)5–] about eight times better than the less hydrophobic pyrrolidine derivative [–(CH2)2O(CH2)2–]. Similarly, E192N uses the diethylcarboxamide three times less efficiently than the dibutyl derivative. Interestingly, the E192N enzyme is more effective with the dibutyl analogue than with the dipropyl screening substrate 5a, indicating that the size of the engineered active site is not a prime determinant of specificity and suggesting that E192N may be able to tolerate other larger substituents.



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  		Fig. 4.. Substrate specificity of the wild-type NAL and mutant E192N towards various hydrophobic carboxamides. The catalytic efficiency (kcat/Km) of wild-type and E192N mutant NAL towards a range of substrate analogues was determined by measuring the initial reaction velocities in a coupled enzyme assay and fitting the data to the Michaelis–Menten equation.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Acknowledgements
 References
 
In an attempt to rationalize the effects of the reported mutations on the specificity changes found, we have considered both the structure of the wild-type NAL (Lawrence et al., 1997Go; Barbosa et al., 2000Go) and of related enzymes. NAL belongs to a sub-family of (ß/{alpha})8 enzymes which share a common catalytic step (formation of a Schiff base between a strictly conserved lysine residue and the C2 carbon of the common {alpha}-keto acid moiety of the substrate) but which catalyse different reactions in separate biochemical pathways. Members include dihydrodipicolinate synthase (DHDPS) (Blickling et al., 1997Go), trans-o-hydroxybenzylidenepyruvate hydratase-aldolase (HBPHA), D-5-keto-4-deoxyglucarate dehydratase (KDGDH), trans-2'-carboxybenzalpyruvate hydratase-adolase (CBPHA) (Iwabuchi and Harayama, 1998Go) and 2-keto-3-deoxygluconate aldolase (KDGA) (Buchanan et al., 1999Go). A previous comparison of the structures and sequences of this family of enzymes (Barbosa et al., 2000Go) suggested that Asp191, Glu192 and Ser208 form part of the ‘secondary’ set of amino acids, responsible for substrate specificity between the various family members. Asp191 makes the fewest interactions with substrate (inferred from the inhibitor-bound structures available), namely a single hydrogen bond to the O8 hydroxyl group. Asp191 is conserved in all NALs (Figure 2C) and is also conserved in DHDPS, where the equivalent residue, Asp118, is thought to bind the ammonium moiety of L-ASA, and in KDGA, where Asp181 was previously assumed to play a role in binding the hydroxyl group at the C6 position of the product. However, the recently solved crystal structure of KDGA (Theodossis et al., 2004Go) indicates that Asp181 is not directly involved in substrate binding, suggesting that this residue may play only a minor role in specificity. In agreement with this, mutations to Asp191 in this study generally resulted in enzymes with only minor increases in activity with the dipropylamide (Table I) coupled with low activity with the natural substrate, sialic acid. Similar decreases in the activity with the natural substrate have also been reported for mutations of the equivalent residue in the Clostridium perfringens NAL (Asp187) (Kruger et al., 2001Go).

Glu192 has a major effect on altering the substrate specificity of the E.coli NAL towards the dipropylamide (Table I). This agrees well with its postulated role as a specificity-determining residue within the NAL sub-family, where a glutamate is conserved at that position in all NALs (Figure 2C) but is varied in every other family member except HBHPA. Furthermore, it has been shown that the E188Q mutant of the C.perfringens NAL enhances the cleavage of an alternative substrate, 5,9-di-O-acetyl neuraminic acid, 1.3-fold relative to wild-type NAL (Kruger et al., 2001Go). Interestingly, it appears that specificity towards the dipropylamide can be increased by substitution of Glu192 with residues which are both polar (Asn and Ser) and non-polar (Val) (Table I). Although the aim of this study was not to delineate the effect of every substitution at position 192, it seems that charge plays an important role in determining specificity at position 192.

Ser208 in NAL is not conserved in any of the sub-family members, but is replaced by an alanine (Ala198) in KDGA. The crystal structure of KDGA (Theodossis et al., 2004Go), bound with either of the C4-epimeric substrates, 2-keto-3-deoxygluconate (KDG) or 2-keto-3-deoxygalactonate (KDGal), shows that Ala198 interacts with the C5-OH of the product via a water molecule when KDG is bound, but not when the diastereoisomer KDGal is bound. Hence Ala198 may play a role in controlling stereoselectivity in KDGA. By comparison, it may be suggested that Ser208 could play a similar role in NAL and evidence from protein simulations supports this hypothesis (Smith et al., 1999Go). However, the results presented here clearly show that alterations of Ser208 do not have a significant effect on the substrate specificity of the enzyme towards the dipropylamides 5.

Overall, our most improved variant, E192N, shows an impressive 50-fold increase in catalytic efficiency with the dipropylamide 5a. Such large increases in efficiency due to single point mutations are rare in protein engineering experiments where whole-gene random mutagenesis is carried out, although some exceptions have been reported. For example, using the E.coli mutator strain XL1 Red, after screening 150 000 colonies, a single amino acid variant of Aspergillus niger amine oxidase was obtained with a kcat 47-fold higher towards the target amine than the wild-type enzyme (Alexeeva et al., 2002Go). Another example includes the directed evolution of cytochrome P450 BM3 from Bacillus megaterium into a highly efficient catalyst for the conversion of a range of alkanes into alcohols (Glieder et al., 2002Go; Peters et al., 2003Go). Thus, whereas whole-gene random mutagenesis can yield improvements in activity towards unnatural substrates, saturation mutagenesis at selected residues known to contact the substrate can produce large increases in efficiency while requiring a search of considerably less sequence space. In this way, we have rapidly identified a single mutant of NAL with drastically altered specificity that can now be used as the scaffold for further directed evolution studies. We are currently using a combination of error-prone PCR, saturation mutagenesis and site-directed mutagenesis to ‘fine-tune’ NAL further towards stereoselective, thermostable biocatalysts for the synthesis of sialic acid mimetics.


   Acknowledgements
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
References
 
This work was supported by the Biotechnology and Biological Sciences Research Council, the Engineering and Physical Sciences Research Council and the Wellcome Trust and is a contribution from the Astbury Centre for Structural Molecular Biology.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
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Received April 11, 2005; accepted April 12, 2005.

Edited by Tony Wilkinson


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