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The helical arrangement of nitrilase enzymes determines their substrate size

Nitrilases are commercially important biocatalysts found in diverse taxa that catalyse the hydrolysis of nitriles to amides and acids. In vitro they form rings, terminating spirals or long filaments that seem to be necessary for activity. In vivo, NIT1 assemblies have been observed in Arabidopsis. Substrate size is correlated with the helical twist of nitrilase filaments and can be altered by changing a single residue by site‐directed mutagenesis or by mixing with an excess of enzyme in a different helical conformation. We have determined the first close‐to‐atomic resolution structure of an active nitrilase by cryo‐EM and identified substrate‐specifying residues for site saturation directed evolution. In silico docking of the substrate, β‐L‐cyanoalanine, into the binding pocket led to the identification of a loop that limits the maximum length of a bound substrate and shifts position as a function of helical twist. We propose that the overall size of the binding pocket is specified by this loop, while binding pocket residues alter selectivity for different functional groups. We propose that this mechanism allows nitrilases to rapidly neofunctionalise according to cellular requirements.