Materials-based strategies for multi-enzyme immobilization and co-localization: A review

Immobilized enzymes as biocatalysts have great potential both scientifically and industrially because of their technological and economic importance. Their highly efficient catalytic mechanisms and reusability have made them excellent candidates for green and sustainable applications. Previous studies have primarily focused on single enzyme immobilization. However, there are many situations where a single enzyme cannot completely catalyze reactions and multiple enzymes working together in a cascade are needed. It is very challenging to efficiently drive the multi-step reaction toward the desired direction, which is especially true when reactive intermediates are present. Nature overcomes this limitation through the use of multi-enzyme complexes (MECs) to promote the overall catalytic efficiency, which has inspired researchers to synthesize artificial MECs to controllably enhance the production of the desired compounds in multi-step reaction cascades in vitro. The most common approaches to synthesize artificial MECs are to use genetic engineering techniques to create fusion proteins or to co-localize multiple enzymes on suitable carriers. This review focuses on the latter with a particular emphasis on materials-based approaches to enzyme co-localization, which builds on techniques developed for single enzyme immobilization. The attachment techniques used in single enzyme immobilization are also effective in multiple enzyme co-localization, which has a direct impact on the overall enzyme orientation and activity. For carrier-based strategies, the platforms developed for single enzyme immobilization are also appropriate for attaching and co-localizing multiple enzymes. However, the involvement of multiple components in co-localization brings many challenges. The properties of different enzymes makes co-localization complicated when selecting attachment techniques and platforms to preserve enzymatic activity, because the structure and function of each component enzyme needs to be taken into consideration to preserve the overall enzyme activity. In addition, the relative position of the multiple enzymes in a confined space plays a significant role in the interactions between different enzymes, which makes spatial control important for co-localization. This review focuses on the potential of materials-based approaches for multiple enzyme co-localization for the design of sustainable multi-enzyme biocatalysts. A critical analysis of the attachment techniques and carriers platforms that have been used in enzyme immobilization and multi-enzyme co-localization in vitro is provided.