Molecular Access Engineering for Microbial Biocatalysis: from Enzyme Tunnels to Microbial Cell Factories

Microbial biocatalysis spans biological platforms ranging from purified enzymes and multienzyme assemblies to electroenzymatic systems, whole-cell biocatalysts, and genetically programmable microbial cell factories. Yet despite their biological diversity, functional biocatalytic performance is often influenced by both catalytic activity and molecular accessibility. This recurring constraint highlights molecular accessibility as a common engineering consideration across microbial biocatalysis. Accordingly, this Review presents the concept of molecular access engineering as the rational control of molecular accessibility to improve functional biocatalytic performance across biological scales. The concept encompasses three complementary strategies governing molecular entry, intermediate transfer, and molecular exchange throughout biological systems. Examples from gas-converting enzymes, multienzyme assemblies, electroenzymatic systems, catalytic cascades, whole-cell biocatalysts, and microbial cell factories illustrate how engineering molecular accessibility can improve catalytic robustness, pathway efficiency, biological compatibility, and systems-level productivity. These examples therefore suggest that molecular access engineering has the potential to broaden the design space for microbial biocatalysis by complementing catalytic engineering.