Submitted by Programa de Pós-graduação em Biotecnologia (mebiotec.ufba@gmail.com) on 2017-04-04T13:02:41Z No. of bitstreams: 1 Dissertação Final-Adriana E F Morato.pdf: 2704014 bytes, checksum: 20efa22afe3e61e735f04f4257d1f05a (MD5) / Approved for entry into archive by Delba Rosa (delba@ufba.br) on 2017-07-06T12:45:39Z (GMT) No. of bitstreams: 1 Dissertação Final-Adriana E F Morato.pdf: 2704014 bytes, checksum: 20efa22afe3e61e735f04f4257d1f05a (MD5) / Made available in DSpace on 2017-07-06T12:45:39Z (GMT). No. of bitstreams: 1 Dissertação Final-Adriana E F Morato.pdf: 2704014 bytes, checksum: 20efa22afe3e61e735f04f4257d1f05a (MD5) / CNPq / Atualmente, a produção de enzimas recombinantes é um dos mercados mais promissores para a biotecnologia. Processos catalisados por enzima são aplicados em diversos processos industriais por apresentarem alta eficiência catalítica, especificidade e seletividade, além do baixo consumo de energia, o que contribui positivamente com o Meio ambiente. A maioria dos biocatalisadores aplicados em processos industriais consiste em enzimas imobilizadas em resinas insolúveis. A imobilização aumenta a atividade catalítica, a estabilidade e também torna possível a aplicação da enzima em meio reacional contínuo assim como, sua reutilização no meio de reação. O interesse em torno do aprimoramento da técnica de imobilização e produção de biocatalisadores enzimáticos tem aumentado nos últimos anos, o que permite indicar o processo de purificação enzimática como o passo mais dispendioso na produção de enzimas. Este trabalho teve como objetivo desenvolver um processo adequado de purificação e imobilização de enzimas recombinantes em passo único, baseados na interação do his-tag com resinas específicas, que permita a produção de catalisadores enzimáticos aplicáveis à produção industrial com baixo custo. Para tanto, foram utilizadas três tipos de resinas da série Amberlite, as quais foram avaliadas quanto à sua interação com os íons Ni2+ ou Co2+, com proteínas inespecíficas e com a enzima lipase B de Pseudosyma antarctica transformada com cauda de hexahistidina. Paralelamente, análises da eficiência catalítica da lipase foram realizadas através da mensuração da atividade hidrolítica por meio de testes de degradação do pNPP e concentração da proteína pelo método colorimétrico de Bradford. No presente estudo, foi realizada uma prospecção tecnológica com o intuito de avaliar o desenvolvimento da tecnologia no mundo. Esse estudo indicou a necessidade de pesquisas no Brasil. Entre os suportes testados, a resina Amberlite IRN77 carregada com íons Ni2+ e pré-tratada com tampão Tris-HCl (pH 7.0) apresentou dados favoráveis quanto a ligação de íons metálicos e proteína inespecífica, sendo utilizada nas etapas de purificação e imobilização enzimática. A lipase PALB purificada na coluna apresentou uma boa atividade catalítica (522U/g) em meio aquoso, mesmo na presença de compostos desnaturantes. Com isso, um suporte de baixo custo foi empregado para o processo de purificação e imobilização enzimática. Este trabalho é um novo passo para o esclarecimento dos processos envolvidos na técnica de purificação e imobilização de enzimas em passo único, além de contribuir para a pesquisa no país por meio de uma patente que está sendo desenvolvida. / Currently, the production of recombinant enzymes is one of the most promising markets for biotechnology. Catalyzed processes by enzymes are applied in many industrial processes because they have high catalytic efficiency, specificity and selectivity, in addition to low power consumption, which contributes positively to the environment. Most of biocatalysts in industrial processes are applied in immobilized enzyme insoluble resins. The immobilization enhances the catalytic activity, stability and also makes possible the use of continuous enzyme reaction medium as well as reuse in the reaction medium. The interest around the improvement of production technique and immobilization of enzyme biocatalysts has increased in recent years, allowing indicate enzymatic purification process as the most expensive step in the production of enzymes. This study aimed to develop an appropriate process of purification and immobilization of recombinant enzymes in one step, based on the interaction of his-tag with specific resins, allowing the production of enzymatic catalysts applicable to industrial production at low cost. For this purpose, three types of resins Amberlite series, which were evaluated for their interaction with Ni2+ ions, were used or Co2+, with nonspecific proteins and their Pseudosyma antarctica lipase B transformed with hexahistidine tail. In parallel, analyses of the catalytic efficiency of lipase were performed by measuring the hydrolytic activity by testing the degradation of pNPP and protein concentration by the Bradford colorimetric method. In the present study, a technological exploration was performed in order to evaluate the development of technology in the world. This study indicated the need for research in Brazil. Among the tested media, the Amberlite IRN77 charged with Ni2+ ions and pre-treated with Tris-HCl buffer (pH 7.0) presented favorable data for binding of metal ions and nonspecific protein being used in purification and immobilization stage of enzyme. The purified lipase PALB column showed good catalytic activity (522U/g) in an aqueous medium, even in the presence of denaturing compounds. With this, a low-cost support was used for the process of purification and enzyme immobilization. This work is a further step towards the understanding of the processes involved in the art for purification and immobilization of enzymes in a one step, as well as contributing to research in the country through a patent that is being developed.

La quercétine (QCT) est un composé polyphénolique d'origine végétale connu pour ses activités antioxydantes et ses effets bénéfiques sur la santé. Sa solubilité, sa stabilité, sa biodisponibilité et ses activités biologiques peuvent être améliorées par une acylation sélective de ses groupements hydroxylés. Ce travail vise à étudier la possibilité d'une acétylation enzymatique de la QCT par la lipase B de Candida antarctica (CALB), la lipase la plus exploitée industriellement pour des estérifications régio- et énantiosélectives. Dans une perspective d'ingénierie rationnelle de l'enzyme, une démarche de modélisation moléculaire est mise en oeuvre pour mieux comprendre les interactions qui régissent le positionnement et l'orientation du substrat dans le site actif de la lipase. Dans une première partie expérimentale, l'absence d'activité d'acétylation de la CALB envers la QCT, en présence d'un excès d'acétate de vinyle, a été confirmé. Dans une seconde partie, cette inactivité de la CALB a été expliquée à l'aide de simulations de docking et de dynamique moléculaire. Elle résulte d'une orientation inappropriée du donneur d'acyle liée à la sérine catalytique et d'une proximité insuffisante des hydroxyles de la QCT vis-à-vis des résidus catalytiques. L'éloignement de la QCT de la triade catalytique est due à la rigidité de la molécule, l'étroitesse du site actif ainsi qu'à des interactions hydrophobes et électrostatiques entre le substrat et les résidus de la cavité. En revanche, cette approche de simulation moléculaire prédit un bon positionnement des deux substrats dans le site actif de la lipase de Pseudomonas cepacia (PCL), laquelle est capable d'acétyler la QCT. Dans une troisième partie, l'influence de mutations de deux résidus impliqués dans les liaisons de stabilisation hydrophobe de la QCT dans la CALB a été investiguée par simulation. La substitution d'isoleucines par des valines et des alanines conduit à une augmentation du volume de la poche catalytique et une mobilité accrue de la QCT. Mais ces mutations sont insuffisantes pour permettre un positionnement adéquat de l'acétate et de la QCT par rapport à la triade catalytique. La dernière partie focalise sur les interactions électrostatiques entre la QCT et le site actif de CALB. Les orientations du substrat dans la cavité suite à une méthylation ou une acétylation des groupements hydroxyles de la QCT sont précisées / Quercetin (QCT) is a plant-produced polyphenolic compound well-known for its antioxidant activities and beneficial health effects. Its solubility, stability, bioavailability and biological activities may be improved by a selective acylation of its hydroxyl groups. This work aims at studying the possibility of QCT enzymatic acetylation by Candida antarctica lipase B (CALB), the most industrially exploited lipase for regio- and enantioselective esterifications. In prospect of the rational enzyme design, a molecular modeling approach was implemented to understand the interactions that govern the substrate positioning and orientation in the lipase's active site. In a first experimental part, the absence of CALB acetylation activity towards quercetin in excess of vinyl acetate was confirmed. In a second part, this inactivity of CALB was explained by means of docking and molecular dynamics simulations. This results from an inappropriate positioning of the acyl donor linked to the catalytic serine and from an insufficient proximity of QCT hydroxyls vis-à-vis catalytic residues. The distance of QCT from the catalytic triad is due to its rigidity and to the narrow active site as well as to hydrophobic and electrostatic interactions between the substrate and the cavity residues. On the contrary, this molecular simulation approach predicts an appropriate positioning of both substrates in the active site of Pseudomonas cepacia lipase (PCL), which can perform QCT acetylation. In a third part, the impact of mutations of two residues implicated in the stabilization of QCT by hydrophobic interactions in CALB was investigated through simulations. The substitution of isoleucines by alanines and valines led to an increase in the catalytic pocket volume which intensified the mobility of QCT. However, these mutations are insufficient to allow an appropriate positioning of acetate and QCT in relation to the catalytic triad. The last part of this work focuses on the electrostatic interactions between QCT and CALB's active site. The substrate orientation in the cavity following methylation or acetylation of QCT's hydroxyl groups was clarified

Les peptides sont des molécules pouvant posséder une activité biologique intéressante (antibiotique, anti-oxydante, antivirale, anti-hypertensive…). Ce sont cependant des molécules difficiles à utiliser car elles possèdent un faible temps de demi-vie in vivo et sont peu bio-disponibles. Le greffage d’un acide gras permet de les protéger et d’accroître leur potentiel d’action. Cette réaction appelée acylation peut être catalysée par des enzymes. A l’heure actuelle, peu de recherches sont faites sur l’acylation de peptides par voie enzymatique et sur la recherche de nouveaux biocatalyseurs adaptés pour cette réaction. Les objectifs de cette thèse ont été, dans un premier temps, de comprendre les mécanismes de la sélectivité d’acylation de peptides de la lipase B de Candida antarctica par une approche de modélisation moléculaire combinant docking et dynamique moléculaire, couplée à une approche expérimentale. Cette étude a permis d’identifier des interactions enzyme-substrats impliquées dans la sélectivité enzymatique et a permis de construire un modèle expliquant la régio- et chimio-sélectivité de l’acylation peptidique catalysée par cette enzyme. Dans un deuxième temps, une étude préliminaire a été menée afin d’identifier de nouvelles enzymes de type acylases présentes dans des surnageants de culture de différentes espèces de Streptomyces. Ces enzymes sont capables de catalyser des réactions d’acylation de peptides en milieux aqueux. Une méthode de semi-purification a été établie et une étude comparative a été menée sur la sélectivité d’acylation de la lipase B de C. antarctica et celle de nouvelles enzymes de type aminoacylases présentes dans un extrait protéique de surnageant de culture de Streptomyces ambofaciens. Ces nouvelles enzymes présentent une spécificité différente de celle de la lipase B de C. antarctica, permettant notamment, une acylation des acides aminés sur leur fonction amine en position α. Une caractérisation partielle des activités amino-acylase du surnageant de culture de S. ambofaciens a été réalisée. Dans une troisième et dernière partie, une comparaison des séquences génétiques a été réalisée entre treptomyces mobaraensis et S. ambofaciens afin d’identifier les gènes codant pour les acylases découvertes chez S. ambofaciens. Des mutants de S. ambofaciens délétés pour ces gènes ont été construits et la fonctionnalité des enzymes codées par ces gènes a été vérifiée ; enfin, une expression hétérologue de l’ε-lysine acylase a été initiée / Peptides exhibit various beneficial effects such as antioxidant, anti-hypertensive, neuroprotective, antiviral or antimicrobial activities. However, their use can be limited by their short half-life and their low biological availability. One solution to overcome these drawbacks is the acylation of peptides with fatty acids. This reaction called acylation can be catalyzed using enzymes. To date, very few studies focus on enzymatic acylation of peptides and on finding new enzymes catalyzing this reaction. The objectives of this work were, in a first time, to understand the selectivity mechanisms of the lipase B of Candida antarctica for peptides acylation combining experimental and molecular modeling approaches. This study highlighted enzyme/substrate interactions involved in the enzymatic selectivity and a modelexplaining the chemo- and regio-selectivity of this enzyme for peptide acylation reactions was built. In a second time, a preliminary study was carried out in order to identify new aminoacylase enzymes produced in the culture supernatant of various species of Streptomyces. These enzymes are able to catalyze acylation of peptides in aqueous media. A partial purification method was set and a comparative study was performed on the selectivity of C. antarctica lipase Band that of the new aminoacylases discovered in the culture supernatant of Streptomyces ambofaciens ATCC 23877. These enzymes presented a selectivity different from C. antarctica lipase B allowing the acylation of the N-terminal amino group of amino acids or peptides. A partial description of the aminoacylase activity of the supernatant crude extract of S. ambofaciens was performed. In a third and final part, a comparison of sequences of aminoacylases from Streptomyces mobaraensis with the genome of S.s ambofaciens ATCC 23877 was performed in order to identify genetic sequences encoding the new discovered aminoacylases from S. ambofaciens ATCC 23877. Each identified gene was deleted to correlate it with the aminoacylase activity observed in the crude extract of S. ambofaciens. Lastly, a heterologous expression of the ε-lysine acylase was initiated

<p>Multifunctional enzymes have alternative functions or activities, known as “moonlighting” or “promiscuous”, which are often hidden behind a native enzyme activity and therefore only visible under special environmental conditions. In this thesis, the active-site of Pseudozyma (formerly Candida) antarctica lipase B was explored for a promiscuous conjugate addition activity. Pseudozyma antarctica lipase B is a lipase industrially used for hydrolysis or transacylation reactions. This enzyme contains a catalytic triad, Ser105-His224-Asp187, where a nucleophilic attack from Ser105 on carboxylic acid/ester substrates cause the formation of an acyl enzyme. For conjugate addition activity in Pseudozyma antarctica lipase B, replacement of Ser105 was assumed necessary to prevent competing hemiacetal formation. However, experiments revealed conjugate addition activity in both wild-type enzyme and the Ser105Ala variant. Enzyme-catalyzed conjugate additions were performed by adding sec-amine, thiols or 1,3-dicarbonyl compounds to various α,β-unsaturated carbonyl compounds in both water or organic solvent. The reactions followed Michaelis-Menten kinetics and the native ping pong bi bi reaction mechanism of Pseudozyma antarctica lipase B for hydrolysis/transacylation was rerouted to a novel ordered bi uni reaction mechanism for conjugate addition (Paper I, II, III). The lipase hydrolysis activity was suppressed more than 1000 times by the replacement of the nucleophilic Ser105 to Ala (Paper III).</p>

Recombinant Candida antarctica lipase B was successfullyproduced in the methylotropic yeast Pichia pastoris. Thespecific activities of Candida antarctica lipase B produced inPichia pastoris and commercial Candida antarctica lipase B fromNovozymes were the same. In shake-flask cultivations theexpression levels were about 25 mg L-1. Production levels couldbe increased to 1.5 g L-1, using a fermentor. A model tosimulate growth and oxygen consumption was described. The highcell density growth could be explained by the low maintenancecoefficient of Pichia pastoris. Enrichment of the aeration withoxygen increased the recombinant protein production. The lipasewas also produced as a fusion to a cellulose binding module.The cellulose binding module did not interfere with thespecific activity of the lipase. With this fusion proteincatalytic reactions can be performed in close proximity to acellulose surface. The binding module can also function as anaffinity tag for purification. Establishment of the Candidaantarctica lipase B production system allowed the engineeringof Candida antarctica lipase B variants. Four differentvariants were produced in order to investigate if electrostaticinteractions contributed to enantioselectivity. Theenantioselectivity of two halogenated secondary alcohols wasdoubled for the Ser47Ala variant. Thisimplied thatelectrostatic interactions are important forenantioselectivity. The Trp104His variant showed a decrease inenantioselectivity for all tested substrates. This was causedby an increase in the size of the stereoselectivity pocket.Symmetrical secondary alcohols of different size were used tomap the stereoselectivity pocket. A substituent as large as apropyl or isopropyl could be accommodated in the pocket of theTrp104His variant. In the wild-type lipase thestereoselectivity pocket was estimated to fit an ethyl group.The enzyme variants were subjected to a thermodynamic study, toelucidate changes in the enthalpic and entropic contributionsto enantioselectivity. The enthalpic and entropic contributionschanged for the different lipase variants and werecompensatory. The compensation was not perfect, allowing forchanges in enantioselectivity. In general one can conclude that rational design of newenzyme properties, in order to change the substrateselectivity, is feasible if based on a thorough model ofsubstrate enzyme interactions. <b>Key words:</b>Protein expression, Candida antarctica lipaseB, Pichia pastoris, sitedirected mutagenesis, fermentation,selectivity

The substrate selectivities for different serine hydrolases were utilized in various applications, presented in papers I-VI. The articles are discussed in the thesis in view of the kinetics of the enzyme catalysis involved. In paper I the enantioselectivities towards a range of secondary alcohols were reversed for Candida antarctica lipase B by site directed mutagenesis. The thermodynamic components of the enantioselectivity were determined for the mutated variant of the lipase. In papers II-III Candida antarctica lipase B was engineered for selective monoacylation using two different approaches. A variant of the lipase created for substrate assisted catalysis (paper II) and three different variants with mutations which decreased the volume of the active site (paper III) were evaluated. Enzyme kinetics for the different variants were measured and translated into activation energies for comparison of the approaches. In papers IV and V three different enzymes were used for rapid analysis of enantiomeric excess and conversion of O-acylated cyanohydrins synthesized by a defined protocol. Horse liver alcohol dehydrogenase, Candida antarctica lipase B and pig liver esterase were sequentially added to a solution containing the O-acylated cyanohydrin. Each enzyme caused a drop in absorbance from oxidation of NADH to NAD+. The product yield and enantiomeric excess was calculated from the relative differences in absorbance. In paper VI a method for C-terminal peptide sequencing was developed based on conventional Carboxypeptidase Y digestion combined with matrix assisted laser desorption/ionization mass spectrometry. An alternative nucleophile was used to obtain a stable peptide ladder and improve sequence coverage. / QC20100629

In a lipase investigation, Candida antarctica lipase B (CALB) are explored for enzyme catalytic promiscuity. Enzyme catalytic promiscuity is shown by enzymes catalyzing alternative catalytic transformations proceeding via different transition state structures than normal. CALB normally performs hydrolysis reactions by activating and coordinating carboxylic acid/ester substrates in an oxyanion hole prior to nucleophilic attack from an active-site serine resulting in acyl enzyme formation. The idea of utilizing the carbonyl activation oxyanion hole in the active-site of CALB to catalyze promiscuous reactions arose by combining catalytic and structural knowledge about the enzyme with chemical imagination. We choose to explore conjugate addition and direct epoxidation activities in CALB by combining molecular modeling and kinetic experiments. By quantum-chemical calculations, the investigated promiscuous reactions were shown to proceed via ordered reaction mechanisms that differ from the native ping pong bi bi reaction mechanism. The investigated promiscuous activities were shown to take place in the enzyme active-site by various kinetic experiments, but despite this, no enantioselectivity was displayed. The reason for this is unknown, but can be a result of a too voluminous active-site or the lack of covalent coordination of the substrates during enzyme-catalysis (Paper I-IV). Combining enzyme structural knowledge with chemical imagination may provide numerous novel enzyme activities to be discovered. In an ω-transaminase investigation, two (S)-selective ω-transaminases from Arthrobacter citreus (Ac-ωTA) and Chromobacterium violaceum (Cv-ωTA) are explored aiming to improve their catalytic properties. Structural knowledge of these enzymes was provided by homology modeling. A homology structure of Ac-ωTA was successfully applied for rational design resulting in enzyme variants with improved enantioselectivity. Additionally, a single-point mutation reversed the enantiopreference of the enzyme from (S) to (R), which was further shown to be substrate dependent (Paper V). A homology structure of Cv-ωTA guided the creation of an enzyme variant showing reduced isopropyl amine inhibition. / QC20100609

Biocatalysis has become a popular technique in organic synthesis due to high activity and selectivity of enzyme catalyzed reactions. Enantioselectivity is a particularly attractive enzyme property, which is utilized for the production of enantiopure substances. Determination of the temperature dependence of enzyme enantioselectivity allows for thermodynamic analyses that reveal the contribution of differential activation enthalpy, ΔR-SΔH‡, and entropy, ΔR-SΔS‡. In the present investigation the influence of substrate structure, variations on enzyme structure and of reaction media on the enantioselectivity of Candida Antarctica lipase B has been studied. The contribution of enthalpy, ΔR-SΔH‡, and entropy, TΔR-SΔS‡, to the differential free energy, ΔR-SΔG‡, of kinetic resolutions of sec-alcohols were of similar magnitude. Generally the two terms were counteracting, meaning that the enantiomer favored by enthalpy was disfavored by entropy. 3-Hexanol was an exception where the preferred enantiomer was favored both by enthalpy and by entropy. Resolution of 1-bromo-2-butanol revealed non-steric interactions to influence both ΔR-SΔH‡ and ΔR-SΔS‡. Molecular modeling of the spatial freedom of the enzyme-substrate transition state indicated correlation tothe transition state entropy. The acyl chain length was shown to affect enantioselectivity in transesterifications of a sec-alcohol. Point mutations in the active site were found to decrease or increase enantioselectivity. The changes were caused by partly compensatory changes in both ΔR-SΔH‡ and ΔR-SΔS‡. Studies on single and double mutation variants showed that the observed changes were not additive. Enantioselectivity was strongly affected by the reaction media. Transesterifications of a sec-alcohol catalyzed by Candida Antarctica lipase B was studied in eight liquidorganic solvents and supercritical carbon dioxide. A correlation of enantioselectivity and the molecular volume of the solvent was found. Differential activation enthalpy, ΔR-SΔH‡, and entropy, ΔR-SΔS‡, display a compensatory nature. However this compensation is not perfect, which allows for modifications of enantioselectivity. The components of the thermodynamic parameters are highly complex and interdependent but if their roles are elucidated rational design of enantioselective enzymatic processes may be possible. / QC 20100616

Telechelic polyesters have successfully been synthesized with lipase-catalyzed polymerization. The produced telechelics had a high degree of di­functionalization, high purity (requiring little or no workup) and controlled degree of polymerization. The syntheses were performed in one-pot one-step reaction systems. The use of protection/deprotection chemistry was not necessary, since the lipase selectivity was utilized in the syntheses. Two different types of lipase-catalyzed polymerizations were applied – ring-opening polymerization and polycondensation. In ring-opening polymerization telechelics were produced by a combination of initiation, α-functionalization, and linking through termination, w-func­tionalization. In polycondensation different types of end-cappers were used to synthesize telechelics. Several exampels of functional groups were used for end-functionalization - epoxide, methacrylate and tetraallyls. Enzyme kinetic schemes describing the different functionalization met­hods of polyesters are presented and discussed. Stoichiometry and different reaction conditions have been studied to understand the effects these functions have on the final structure of the synthesized telechelics. Polyesters are classified as biodegradable, and can also be synthesized from materials that can be extracted or fermented from renewable sources like plants. Lipase-catalysts have several beneficial attributes, like high selectivity, they are renewable and biodegradable, are non-toxic and metal-free and can operate under mild reaction conditions. The focus of this thesis has been on lipase-catalyzed syntheses and characterization of the produced telechelics, in addition some materials have been produced. Some uses of telechelics are surface modification, materials for block co-polymers, functional films and biomedical applications. / QC20100726

<p>Recombinant Candida antarctica lipase B was successfullyproduced in the methylotropic yeast Pichia pastoris. Thespecific activities of Candida antarctica lipase B produced inPichia pastoris and commercial Candida antarctica lipase B fromNovozymes were the same. In shake-flask cultivations theexpression levels were about 25 mg L-1. Production levels couldbe increased to 1.5 g L-1, using a fermentor. A model tosimulate growth and oxygen consumption was described. The highcell density growth could be explained by the low maintenancecoefficient of Pichia pastoris. Enrichment of the aeration withoxygen increased the recombinant protein production. The lipasewas also produced as a fusion to a cellulose binding module.The cellulose binding module did not interfere with thespecific activity of the lipase. With this fusion proteincatalytic reactions can be performed in close proximity to acellulose surface. The binding module can also function as anaffinity tag for purification. Establishment of the Candidaantarctica lipase B production system allowed the engineeringof Candida antarctica lipase B variants. Four differentvariants were produced in order to investigate if electrostaticinteractions contributed to enantioselectivity. Theenantioselectivity of two halogenated secondary alcohols wasdoubled for the Ser47Ala variant. Thisimplied thatelectrostatic interactions are important forenantioselectivity. The Trp104His variant showed a decrease inenantioselectivity for all tested substrates. This was causedby an increase in the size of the stereoselectivity pocket.Symmetrical secondary alcohols of different size were used tomap the stereoselectivity pocket. A substituent as large as apropyl or isopropyl could be accommodated in the pocket of theTrp104His variant. In the wild-type lipase thestereoselectivity pocket was estimated to fit an ethyl group.The enzyme variants were subjected to a thermodynamic study, toelucidate changes in the enthalpic and entropic contributionsto enantioselectivity. The enthalpic and entropic contributionschanged for the different lipase variants and werecompensatory. The compensation was not perfect, allowing forchanges in enantioselectivity.</p><p>In general one can conclude that rational design of newenzyme properties, in order to change the substrateselectivity, is feasible if based on a thorough model ofsubstrate enzyme interactions.</p><p><b>Key words:</b>Protein expression, Candida antarctica lipaseB, Pichia pastoris, sitedirected mutagenesis, fermentation,selectivity</p>