Cotranslational incorporation of non-standard amino acids using cell-free protein synthesis

Over the last years protein engineering using non-standard amino acids has gained increasing attention. As a result, improved methods are now available, enabling the efficient and directed cotranslational incorporation of various non-standard amino acids to equip proteins with desired characteristics. In this context, the utilization of cell-free protein synthesis is particularly useful due to the direct accessibility of the translational machinery and synthesized proteins without having to maintain a vital cellular host. We review prominent methods for the incorporation of non-standard amino acids into proteins using cell-free protein synthesis. Furthermore, a list of non-standard amino acids that have been successfully incorporated into proteins in cell-free systems together with selected applications is provided.

Manipulation of enzyme properties by noncanonical amino acid incorporation

Since wild-type enzymes do not always have the properties needed for various applications, enzymes are often engineered to obtain desirable properties through protein engineering techniques. In the past decade, complementary to the widely used rational protein design and directed evolution techniques, noncanonical amino acid incorporation (NCAAI) has become a new and effective protein engineering technique. Recently, NCAAI has been used to improve intrinsic functions of proteins, such as enzymes and fluorescent proteins, beyond the capacities obtained with natural amino acids. Herein, recent progress on improving enzyme properties through NCAAI in vivo is reviewed and the challenges of current approaches and future directions are also discussed. To date, both NCAAI methods-residue- and site-specific incorporation-have been primarily used to improve the catalytic turnover number and substrate binding affinity of enzymes. Numerous strategies used to minimize structural perturbation and stability loss of a target enzyme upon NCAAI are also explored. Considering the generality of NCAAI incorporation, we expect its application could be expanded to improve other enzyme properties, such as substrate specificity and solvent resistance in the near future.