Homogentisate 1,2-dioxygenase is an enzyme, which catalyzes the conversion of homogentisate to 4-maleylacetoacetate.
Homogentisate 1,2-dioxygenase or HGD is involved in the catabolism of aromatic rings, more specifically in the break down of the amino acids tyrosine and phenylalanine.
HGD appears in the metabolic pathway of tyrosine and phenylalanine degradation once the molecule homogentisate is produced. Homogentisate reacts with HGD to produce maleylacetoacetate, which then is further used in the metabolic pathway.
HGD requires the use of Fe2+ and O2 in order to cleave the aromatic ring of homogentisate.
Pathology
Homegentisate 1,2 dioxygenase is involved in a type of metabolic diseases, called alkaptonuria. This disorder is due to the inability of the body to deal with homogentisate, which when oxidized by the body will produce the compound known as the ochronotic pigment, which causes a black color, and has several negative effects.
This first of these effects is that the patient’s earwax will begin to turn black or red, depends on the patient’s diet, since the blood becomes oxidized and thus turns black due to excess of the ochronotic pigment. The other effect of the ochronotic pigment is that it can accumulate in the body’s connective tissue leading to degenerative arthritis, as the person grows older.
Alkaptonuria has another effect in that it can cause the urine to turn black as well if let to sit for long enough to become oxidized, though is this often a method for testing for the genetic defect. The metabolic disease is autosomal recessive, such that both parents must pass the gene on to their children in order for child to have the defect.