Characterization of Phenylalanine Ammonia Lyases from Lettuce ( 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 -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 ( L.) that are previously known to be involved in wound browning. All 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, PAL1-PAL4, showed significantly higher activity and thermal stability than PAL5, as well as a broader substrate spectrum including some challenging substrates with steric demanding or electron-donating substituents. The best one PAL3 was subjected to the kinetic resolution of a panel of 21 -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.