Description
Histidase (HAL) belongs to the PAL/Histidase family. Histidase deaminates histidine to urocanic acid, the first step in histidine degradation. It is closely related to the plant enzyme phenylalanine ammonia-lyase, but is absent in plants and viruses. HAL contains a unique cofactor called 4-methylidene-imidazole-5-one group (MIO), which is produced autocatalytically by a cyclization and dehydration of the three amino-acid residues alanine, serine and glycine, for its chain folding. Defects in HAL are the cause of histidinemia (HISTID). HISTID is characterized by decreased urocanic acid as well as increased histamine and histidine in body fluids