Characterization of Phenylalanine Ammonia Lyases from Lettuce (Lactuca sativa L.) as Robust Biocatalysts for the Production of d - and l -Amino Acids

Phenylalanine ammonia lyase (PAL) catalyzes the reversible conversion of l -phenylalanine into the corresponding trans -cinnamic acid, providing a route to optically pure α-amino acids. We explored the catalytic function of all five PALs encoded in the genome of lettuce ( Lactuca sativa L.) that are previously known to be involved in wound browning. All Ls PALs were active toward l -phenylalanine in the ammonia elimination reaction and displayed maximum activity at 55–60 °C and pH 9.0–9.5. However, four of them, Ls PAL1– Ls PAL4, showed significantly higher activity and thermal stability than Ls PAL5, as well as a broader substrate spectrum including some challenging substrates with steric demanding or electron-donating substituents. The best one Ls PAL3 was subjected to the kinetic resolution of a panel of 21 rac -phenylalanine derivatives, as well as the ammonia addition of 21 cinnamic acid derivatives. It showed excellent enantioselectivity in most cases and significantly better activity than previously described PALs for a number of challenging non-natural substrates, demonstrating its great potential in biocatalysis.