Novozym 435 (N435) is a commercially available immobilized lipase produced by Novozymes. It is based on immobilization via interfacial activation of lipase B from Candida antarctica on a resin, Lewatit VP OC 1600. This resin is a macroporous support formed by poly(methyl methacrylate) crosslinked with divinylbenzene. N435 is perhaps the most widely used commercial biocatalyst in both academy and industry. Here, we review some of the success stories of N435 (in chemistry, energy and lipid manipulation), but we focus on some of the problems that the use of this biocatalyst may generate. Some of these problems are just based on the mechanism of immobilization (interfacial activation) that may facilitate enzyme desorption under certain conditions. Other problems are specific to the support: mechanical fragility, moderate hydrophilicity that permits the accumulation of hydrophilic compounds (e.g., water or glycerin) and the most critical one, support dissolution in some organic media. Finally, some solutions (N435 coating with silicone, enzyme physical or chemical crosslinking, and use of alternative supports) are proposed. However, the N435 history, even with these problems, may continue in the coming future due to its very good properties if some simpler alternative biocatalysts are not developed.

This review discusses the possible roles of polyethylenimine (PEI) in the design of improved immobilized biocatalysts from diverse perspectives. This includes their use to activate supports and immobilize enzymes via ion exchange, as well as to improve immobilized enzymes by coating with PEI. PEI is a polymer containing primary, secondary and tertiary amino groups, having a strong anion exchange capacity under a broad range of conditions, and the capability to chemically react with different moieties on either an enzyme or a support. Also, as a multifunctional polymer, it has been modified stepwise to introduce different functionalities into the same polymer. This polymer (in combination with other anionic ones) permits the generation of “saline” environments around enzyme molecules, improving enzyme stability in the presence of hydrophobic compounds. The use of PEI as a physical glue useful to crosslink enzyme subunits in multimeric enzymes, monomeric enzymes immobilized via physical interactions or production of enzyme multilayers will be specially emphasized as new open avenues for enzyme coimmobilization. The coimmobilization of enzymes and cofactors using PEI may become one of the future developments allowed through an adequate use of this polymer and new pathways towards the design of enzyme combi-catalysts for their use in cascade reactions. Some unexplored but suggested uses derived from the properties of PEI are also proposed in the review, like the use of the buffering power of this multifunctional polymer to avoid pH gradients inside biocatalyst particles. Thus, although PEI has been a largely popular polymer in biocatalyst design, it looks like a long and in some cases almost unexplored road lies ahead.