Porphobilinogen deaminase is a very important enzyme in the heme biosynthesis pathway. It has been isolated from a variety of sources since its discovery in the 1950's. Structural studies have shown that the enzyme is a monomeric protein composed of three equal sized domains. Each domain is a mixed alpha/beta structure whose hydrophobic core is made up from aliphatic side chains projecting from the alpha helices and beta strands. The catalytic cleft, located between domains one and two, contains a dipyrromethane cofactor and a number of invariant residues that are essential for enzyme activity. Mutations in the human porphobilinogen deaminase gene resulting in substitutions in any of these residues causes the inheritable and debilitating disease Acute Intermittent Porphyria.