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Cross-Linked Enzyme Aggregates (CLEA) in Enzyme Improvement – a Review

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Cross-Linked Enzyme Aggregates (CLEA) in Enzyme Improvement – a Review Biocatalysis 2015; 1: 166–177 Review Open Access Susana Velasco-Lozano*, Fernando López-Gallego, Juan C. Mateos-Díaz, Ernesto Favela-Torres Cross-linked enzyme aggregates (CLEA) in enzyme improvement – a review DOI 10.1515/boca-2015-0012 industries [1-3]. However, to be profitable at the industrial Received September 17, 2015; Accepted November 25, 2015 level, industrial enzymes must allow easy handling Abstract: Structural and functional catalytic and operation procedures, stability and reuse. For this characteristics of cross-linked enzyme aggregates purpose, enzyme immobilization has emerged as a tool (CLEA) are reviewed. Firstly, advantages of enzyme to produce robust industrial biocatalysts; and it has been immobilization and existing types of immobilization employed over the last forty years for the successful are described. Then, a wide description of the factors utilization of enzymes in industrial processes [4]. that modify CLEA activity, selectivity and stability is Immobilization is defined as the confinement of presented. Nowadays CLEA offers an economic, simple enzymes in a defined space, retaining their catalytic and easy tool to reuse biocatalysts, improving their activity and allowing their repeated and continuous catalytic properties and stability. This immobilization use [5]. Furthermore, immobilization of enzymes must methodology has been widely and satisfactorily tested meet other criteria to obtain robust biocatalysts. Any with a great variety of enzymes and has demonstrated immobilized enzyme should comprise non-catalytic its potential as a future tool to optimize biocatalytic requirements (shape, size, thickness, length, etc.) processes. designed to aid separation and reuse; as well as, catalytic requirements (activity, selectivity, stability, pH and Keywords: cross-linking, enzyme, immobilization, temperature optimum, etc.) designed to convert the target stabilization compounds [6]. The immobilization of enzymes affects their conformation, rigidity and aggregation state, modifying their reactivity [7]. Thus, enzyme immobilization has 1 Introduction been also utilized to modulate enzyme selectivity [8-10]. The immobilization of enzymes offers the following Enzymes play an important role in relevant advantages: 1) The possibility of reuse, 2) The increase in biotechnological processes in energy, food and chemical stability, such as resistance to high temperatures, extreme pHs, high substrate concentration, polar solvents, mechanical shear, among others, 3) Increased volumetric *Corresponding author: Susana Velasco-Lozano, Heterogeneous activity, and 4) Increase and modulation of selectivity, Biocatalysis group, CIC Biomagune, Parque Tecnológico de San Sebastián, Edificio Empresarial “C”, Paseo Miramón 182, 20009, such as regio-, acyl-, chemo- and enantioselectivity [11-14]. Donostia-San Sebastián Guipúzcoa, Spain, E-mail: svelasco@ Methods with physical or chemical interactions are cicbiomagune.es used for enzyme immobilization. When physical methods Susana Velasco-Lozano, Ernesto Favela-Torres, Departamento de are used, the enzyme does not undergo structural changes Biotecnología, Universidad Autónoma Metropolitana-Iztapalapa. Av. as it is physically attached or trapped by the support San Rafael Atlixco #186, Col. Vicentina 09340, D.F. México Fernando López-Gallego, Heterogeneous Biocatalysis group, CIC structure. The main advantage being that the immobilized Biomagune, Parque Tecnológico de San Sebastián, Edificio Empre- enzyme, trapped or encapsulated, nearly maintains its sarial “C”, Paseo Miramón 182, 20009, Donostia-San Sebastián native state; allowing its reuse and, in most cases, an Guipúzcoa, Spain improvement in the enzyme stability [15-17]. However, the Fernando López-Gallego, Ikerbasque, Basque Foundation for Sci- main disadvantage is that the union between the support ence, 48011, Bilbao, Spain and the enzyme is very weak; causing enzyme leakage Juan C. Mateos-Díaz, Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco, A.C., Unidad de Biotec- while the biocatalyst is operating in aqueous medium. nología Industrial, Guadalajara, México Adsorption, entrapment, and encapsulation are the © 2016 Susana Velasco-Lozano et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Cross-linked enzyme aggregates (CLEA) in enzyme improvement – a review 167 more representative types of physical immobilization. In successfully used through the last fifteen years over a contrast, chemical immobilization may alter the protein wide range of enzymes, and has proved to be an easy, fast, structure because enzymes are attached on the surface efficient and suitable approach in enzyme optimization; and/or inside the support by chemical bonds which may therefore, in this mini-review we summarize a battery of modify its shape, rigidity and aggregation state [18]. examples where enzymes properties has been improved Covalent bonding to substrates and self-immobilization by this technology. are within these types of immobilization. Immobilization of enzymes by covalent attachment to a support is a method in which proteins are irreversibly 2 Carrier-free immobilization types, linked. To achieve immobilization, the reactive amino advantages and disadvantages. groups of amino acids such as lysine, typically present in the surface of proteins, are mainly used. With these amino The carrier-free immobilized enzymes do not need acids, the covalent attachment of enzymes to a reactive an extra inactive mass, the support. They are usually group of the support leads to a highly stable covalent produced by direct cross-linking of dissolved enzymes bond. Epoxides are commonly used as reactive groups (CLE), crystallized enzymes (CLEC), and aggregated on supports to react with the amino groups of lysine enzymes (CLEA). within the enzyme [19]. Covalent attachment usually The first report of such structures was described by provides the strongest bond between the enzyme and Quiocho and Richards [25], who found that insoluble the support compared to other types of immobilization, proteins linkage could be obtained by using bifunctional being able to minimize the leakage of enzyme from the cross-linking chemical groups, such as glutarladehyde. support [18]. Besides allowing its reuse, the majority of These linked proteins known as cross-linked enzymes supports provide the enzyme with a protective barrier (CLE) retained their catalytic activity. When the link is [14]. Nonetheless, in many cases, the supports used have based on purified enzymes, linked proteins are known as high prices raising the cost of production of the thereby cross-linked enzyme crystals (CLEC) [26]. However, at the immobilized biocatalysts [20]; on the other hand, carrier- time the work of Quiocho and Richards was published, the free immobilization or self-immobilization is possible efforts in immobilization research were focused towards since it achieves linking of two enzyme molecules through immobilization by adsorption, entrapment and chemical bifunctional cross- linking agents or simply cross-linkers bonding to supports, due to the low activity and stability without the use of a support. The bifunctional compound retention that CLE exhibited. These structures began to be most commonly used for this purpose is glutaraldehyde studied more intensively in the 1990’s [11,27]. (GA) [21], which is a small dialdehyde molecule. Some The CLE are formed with proteins in solution that are advantages of self-immobilization are greater volumetric linked together by a cross-linking agent having sizes from activity per biocatalyst mass, increased specific activity, 1 to 100 μm. Compared to the native enzymes, this type of easier production, reduced production costs, higher biocatalyst improves their thermal stability, but requires purity, less interferences and/or contaminations by the the care of various factors such as pH, the amount of cross- support and lower mass transfer limitations [6,22]. Carrier- linking agent, ionic strength and temperature. However, free immobilized enzymes are advantageous catalysts in CLE present disadvantages such as low native activity processes requiring high yields and productivities, where retention (usually less than 50%), poor reproducibility, enzymes cannot be stabilized through the conventional and low mechanical stability [6]. These inconveniences solid supports [23]. have been partially overcome by entrapping CLE in a gel Although there are many immobilization strategies, matrix or membranes [28-30]. none of them overcome all enzymes’ limitations in all Along with CLE, studies were directed to the production kinds of processes. The selection of specific strategies of CLEC, that are suitable for biotransformations in non- of immobilization will depend on many factors such aqueous media, due to its high stability under harsh as limitations inherent to the enzyme and the process conditions [25,31]. CLEC are crystalline proteins, bonded in which it will be used [24]. Thus, the selection of the together with a cross-linking agent. They achieved best enzyme immobilization technique will depend on improved mechanical strength and high stability against its application demands as well as the balance between temperature, pH, organic solvents and proteolysis cost-production and the potential uses of the immobilized [22,32,33]. For instance, lipase-CLEC have been developed biocatalyst on its direct application. In this context, cross- in
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