The enzyme defect in this disorder is homogentisate oxidase
The disorder you are referring to is alkaptonuria, which is an autosomal recessive genetic disorder characterized by the inability to metabolize the amino acid tyrosine. In alkaptonuria, there is a deficiency in the enzyme homogentisate oxidase, which normally breaks down homogentisic acid (HGA), an intermediate in the metabolism of tyrosine and phenylalanine.
In individuals with alkaptonuria, HGA accumulates in the body and is excreted in urine, which can turn dark upon exposure to air, as mentioned in the previous question. The accumulation of HGA can also lead to the formation of deposits in various tissues, causing arthritis, heart valve problems, and other complications.
The deficiency of homogentisate oxidase results from mutations in the HGD gene, which provides instructions for making the homogentisate oxidase enzyme. The lack of functional homogentisate oxidase results in the inability to break down HGA, leading to its accumulation in the body and subsequent health problems.
Alkaptonuria is a rare disorder, and there is currently no cure for it. Treatment is mainly focused on managing symptoms and preventing complications.