What is the enzyme in alkaptonuria?

In alkaptonuria, the enzyme that is deficient or absent is called homogentisate 1,2-dioxygenase (HGD). Homogentisate 1,2-dioxygenase is responsible for breaking down an amino acid called phenylalanine and a related compound called tyrosine. Without functional HGD, these compounds are not properly metabolized, leading to the accumulation of a substance called homogentisic acid in the body. The buildup of homogentisic acid results in various symptoms and complications associated with alkaptonuria.