Like other cytochrome P450 enzymes, 21-hydroxylase participates in the cytochrome P450 catalytic cycle and engages in one-electron transfer with NADPH-P450 reductase. Its structure includes an essential iron heme group centered within the protein, also common to all P450 enzymes. Variations of the 21-hydroxylase enzyme can be found in all vertebrates.[19] However, understanding of human 21-hydroxylase structure and function is of particular clinical value, as a failure of the enzyme to act appropriately results in congenital adrenal hyperplasia.