Mechanistic Investigation of Phenylalanine Ammonia Lyase by Using N‐Methylated Phenylalanines

N‐Methyl‐L‐phenylalanine (5), N‐methyl‐4‐nitro‐L‐phenylalanine (6), and N,N‐dimethyl‐4‐nitro‐L‐phenylalanine (7⋅H+) were investigated as substrates or inhibitors of phenylalanine ammonia lyase from Petroselinum crispum. Whereas the former was a reluctant substrate (Km =6.6 mM, kcat =0.22 s−1), no reverse reaction could be detected by using methylamine and (E)‐cinnamate (2). The Km value for ammonia in the reverse reaction by using (E)‐cinnamate (2) was determined to be 4.4 and 2.6M at pH 8.8 and 10, respectively. The N‐methylated 4‐nitro‐L‐phenylalanines 6 and 7 showed only strong inhibitory effects (Ki =130 nM and 8 nM, resp.). These and former results are discussed in terms of the mechanism of action of phenyalalanine and histidine ammonia lyases.