Discovery and role of methylidene imidazolone, a highly electrophilic prosthetic group

The elimination of ammonia from a-amino acids is a chemically difficult process. While the non-acidic beta-proton has to be abstracted, the much more acidic ammonium protons must remain untouched to maintain the leaving group ability of this positively charged group. Histidine and phenylalanine ammonia-lyases (HAL and PAL) possess a catalytically essential electrophilic group which has been believed to be dehydroalanine for 30 years. Recently, the X-ray structure of HAL has been solved. The electron density was not consistent with dehydroalanine but showed the presence of methylidene imidazolone (MIO) instead. The high electrophilicity of this prosthetic group as well as the geometry at the active site support a previously proposed mechanism involving a Friedel-Crafts-type attack at the aromatic ring of the substrate. Further biochemical evidence for this unprecedented electrophile-assisted ammonia elimination is also presented. Although no Xray structure of PAL has been published as yet, spectrophotometrical evidence for the presence of MIO has been provided. Finally, a chemical model for the PAL reaction is described. (C) 2003 Elsevier Science B.V. All rights reserved.