SERINE-202 IS THE PUTATIVE PRECURSOR OF THE ACTIVE-SITE DEHYDROALANINE OF PHENYLALANINE AMMONIA-LYASE - SITE-DIRECTED MUTAGENESIS STUDIES ON THE ENZYME FROM PARSLEY (PETROSELINUM-CRISPUM L)

To investigate the possible role of serine as a precursor of dehydroalanine at the active site of phenylalanine ammonia lyase, two serines, conserved in all known PAL and histidase sequences, were changed to alanine by site-directed mutagenesis. The resulting mutant genes were subcloned into the expression vector pT7.7 and the gene products were assayed for PAL activity. Mutant PALMutS209A showed the same catalytic property as wild-type PAL, whereas mutant PALMutS202A was devoid of catalytic activity, indicating that serine-202 is the most likely precursor of the active site dehydroalanine.