LAUSR

The M379A mutant of Citrobacter freundii tyrosine phenol‐lyase (TPL) has been prepared. M379A TPL is a robust catalyst to prepare a number of tyrosines substituted at the 3‐position with bulky groups that cannot be made with wild type TPL. The three dimensional structures of M379A TPL complexed with L‐methionine and 3‐bromo‐DL‐phenylalanine have been determined by X‐ray crystallography. Methionine is bound as a quinonoid complex in a closed active site in 3 of 4 chains of homotetrameric M379A TPL. M379A TPL reacts with L‐methionine about 8‐fold slower than wild type TPL. The temperature dependence shows that the slower reaction is due to less positive activation entropy. The structure of the M379A TPL complex of 3‐bromo‐DL‐phenylalanine has a quinonoid complex in two subunits, with an open active site conformation. The effects of the M379A mutation on TPL suggest that the mutant enzyme has altered the conformational dynamics of the active site.