Citrobacter freundiil-tyrosine phenol-lyase (TPL) was inactivated by a Pictet–Spengler reaction between the cofactor and a substrate, 3,4-dihyd-roxyphenyl-l-alanine (l-dopa), in proportion to an increase in the reaction temperature. Random mutagenesis of the tpl gene resulted in the genera-tion of a Thr15 to Ala mutant (T15A), which exhibited a two-fold improved activity towards l-DOPA as the substrate.