catalysts Editorial Special Issue “Biocatalysts: Design and Application” Cesar Mateo 1, * and Jose M. Palomo 2, *   Citation: Mateo, C.; Palomo, J.M. Special Issue “Biocatalysts: Design and Application”. Catalysts 2021, 11, 778. https://doi.org/10.3390/ catal11070778 Received: 13 June 2021 Accepted: 18 June 2021 Published: 27 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Group of Chemical Processes Catalyzed by Enzymes, Department of Biocatalysis, Institute of Catalysis (ICP-CSIC), Cantoblanco, Campus UAM, 28049 Madrid, Spain 2 Group of Chemical Biology and Biocatalysis, Department of Biocatalysis, Institute of Catalysis (ICP-CSIC), Cantoblanco, Campus UAM, 28049 Madrid, Spain * Correspondence: ce.mateo@icp.csic.es (C.M.); josempalomo@icp.csic.es (J.M.P.) The use of biocatalysts in chemical reactions is of great interest because reactions can be carried out under very mild and green conditions. Within biocatalysis there are different areas that are key when using this type of process for the synthesis of products of interest. In the first place is the selection of the best type of biocatalyst to carry out the reaction so that both enzymes and whole cells can be used to catalyze the reaction of interest. In addition, once the biocatalyst has been selected, there would be the study of the design and preparation of both enzymatic and cell catalysts that are more active and robust so that in the case of enzymes they are immobilized and therefore heterogeneous catalysts and in the case of whole cells that are modified so that the compound of interest is produced in an optimal way, overexpressing the enzymes with the activities of interest and minimizing the activities that may lead to the consumption of the product of interest or that catalyze unwanted parallel reactions. The last step would be the optimization of the reaction conditions to optimize the production of the compound of interest. For this, it is important to choose the best activity conditions for the biocatalysts but combined with the choice of the parameters that may intervene in the thermodynamics of the reaction. In this issue different types of biocatalysts and methodologies have been published. Thus, different papers using enzymes as catalysts have been published. Within the use of enzymes as biocatalysts, lipases, and esterases are one of the most widely used enzymes, which is why different papers have been published both regarding their use and the optimization of their production: Dong et al. have immobilized a new esterases from Bacillus altitudinis encoded by est BAS gene onto an Epoxy Resin. In this way, they have achieved catalysts with a high enzymatic load and more thermostable that has used to synthesize chloramphenicol palmi- tate by regioselective modification at the primary hydroxyl group, obtaining yields of 94.7% in 24 h [1]. Toro et al. Co-immobilized different lipases from Thermomyces lanuginosus (TLL), Candida antarctica (CALB) and Rhizomocur miehei (RML) on supports as Lewatit ® VPOC1600 (LW) and Purolite ® ECR1604 (PU), to produce new Combi-lipases (CL) systems to produce fatty acid ethyl esters (EE) which are the main component of ethylic biodiesel [2]. Li et al. immobilized and characterized a system in which the enzyme Phospholipase A1 (PLA1) has been used as the biocatalyst to produce high value L-α-glycerylphosphorylcholine (L-α-GPC) through hydrolysis of phosphatidylcholine (PC). For this, they used a simple co- precipitation method to encapsulate PLA1 in a metal–surfactant nanocomposite (MSNC), then modified it using alkalescent 2-Methylimidazole (2-Melm) to promote catalytic effi- ciency in biphasic systems by creating microenvironments [3]. Other study with an immobilized sterase was published by Meng et al. [4]. In this study, lipase-immobilized mesoporous silica particles (LMSPs) are employed as both Pickering stabilizers and biocatalysts. A series of alkyl silanes with the different carbon length are used to modify LMSPs to obtain suitable wettability and enlarge the interfacial area of Pickering emulsion improving the yields and allowing the reusability. Catalysts 2021, 11, 778. https://doi.org/10.3390/catal11070778 https://www.mdpi.com/journal/catalysts