Mechanisms of inhibition of phenylalanine ammonia-lyase by phenol inhibitors and phenol/glycine synergistic inhibitors

Phenylalanine ammonia-lyase (PAL) catalyzes the β-elimination of ammonia from l-phenylalanine to trans-cinnamic acid. A study of inhibition of PAL by phenol, ortho-cresol, and meta-cresol gave mixed inhibition; para-cresol is not an inhibitor. The calculated values of Ki and αKi are phenol, Ki=2.1±0.5 mM and αKi=3.45±0.95 mM; ortho-cresol, Ki=0.8±0.2 mM and αKi=3.4±0.2 mM; meta-cresol, Ki=2.85±0.15 mM and αKi=18.5±1.5 mM. The synergistic inhibition of the same inhibitors with glycine showed a lack of inhibition with the para-cresol/glycine pair, while mixed inhibition was observed with the ortho-cresol/glycine pair (Ki=0.038±0.008 mM, αKi=0.13±0.04 mM) and phenol/glycine pair (Ki=0.014±0.003 mM, αKi=0.058±0.01 M). The meta-cresol/glycine pair gave competitive inhibition (Ki=0.36±0.076 mM). The strong synergistic inhibition observed implies that the inhibitors bind at the active site: in fact, the inhibitors used imitate the structure of the substrate. The order of synergistic inhibition is the same for the sites related to Ki and αKi. These results are in agreement with the inhibitors entering two active sites located in two different subunits.