Eight histidine residues are catalytically essential in a membrane-associated iron enzyme, stearoyl-CoA desaturase, and are conserved in alkane hydroxylase and xylene monooxygenase

Analysis of the deduced amino acid sequences for the membrane desaturases from mammals, fungi, insects, higher plants, and cyanobacteria reveals three conserved primary sequence regions containing HX(3 or 4)H, HX(2 or 3)HH, and HX(2 or 3)HH and up to three hydrophobic regions, the length of which can span the membrane bilayer twice. The positioning of the conserved His-containing regions with respect to these potential membrane spanning domains suggests that a related protein fold for the membrane fatty acid desaturases and hydrocarbon hydroxylases. A functional analysis of the conserved His residues of Delta9 desaturase by site-directed mutageneis indicates that they are essential for the catalytic function of the enzyme and that they serve as ligands for the iron atom(s) in these enzymes.