Histidine ammonia-lyase (HAL)

Histidine ammonia-lyase is a cytosolic enzyme that catalyzes the first step in the catabolism of the amino acid histidine, specifically the nonoxidative deamination of L-histidine to produce ammonia and trans-urocanic acid[1][3][5]. This reaction is essential for histidine degradation in humans and other organisms. HAL is a homotetramer with a unique autocatalytically formed MIO (3,5-dihydro-5-methylidene-4H-imidazol-4-one) cofactor at its active site, which is critical for its catalytic activity[2][4][5]. Mutations in the HAL gene result in histidinemia, an inborn error of metabolism characterized by elevated levels of histidine in blood and urine[1][7]. The enzyme is a member of the aromatic amino acid lyase family and structurally related to phenylalanine ammonia-lyase[3][5][6]. HAL plays a pivotal role in nitrogen metabolism and can serve as a biomarker for metabolic conditions affecting histidine degradation.

Other names

HistidaseHSTDHIShistidinaseL-histidine ammonia-lyase (urocanate-forming)