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HGD gene

homogentisate 1,2-dioxygenase

Normal Function

The HGD gene provides instructions for making an enzyme called homogentisate oxidase, which is active chiefly in the liver and kidneys. This enzyme participates in a step-wise process that breaks down two protein building blocks (amino acids), phenylalanine and tyrosine, when they are no longer needed or are present in excess. These two amino acids also play a role in making certain hormones, pigments, and brain chemicals called neurotransmitters.

Homogentisate oxidase is responsible for a specific step in the breakdown of phenylalanine and tyrosine. Previous steps convert the two amino acids into a molecule called homogentisic acid. Homogentisate oxidase adds two oxygen atoms to homogentisic acid, converting it to another molecule called maleylacetoacetate. Other enzymes break down maleylacetoacetate into smaller molecules that are later used for energy or to make other products that can be used by the body.

Health Conditions Related to Genetic Changes


More than 65 mutations in the HGD gene have been identified in people with alkaptonuria. Most of these mutations change single amino acids used to build the homogentisate oxidase enzyme. For example, a substitution of the amino acid valine for the amino acid methionine at protein position 368 (also written as Met368Val) is the most common HGD mutation in European populations.

Mutations in the HGD gene inactivate homogentisate oxidase by changing its structure. Without a functional version of this enzyme, phenylalanine and tyrosine are not broken down properly and homogentisic acid builds up in the body. Excess homogentisic acid and related compounds are deposited in connective tissues such as cartilage and skin, which causes them to darken. Over time, a buildup of this substance in the joints leads to arthritis. Homogentisic acid is also excreted in urine, making the urine turn dark when exposed to air.

More About This Health Condition

Other Names for This Gene

  • AKU
  • HGO
  • homogentisate 1,2-dioxygenase (homogentisate oxidase)
  • Homogentisic acid oxidase
  • homogentisicase

Additional Information & Resources

Tests Listed in the Genetic Testing Registry

Scientific Articles on PubMed

Catalog of Genes and Diseases from OMIM

Gene and Variant Databases


  • Amaya AA, Brzezinski KT, Farrington N, Moran GR. Kinetic analysis of human homogentisate 1,2-dioxygenase. Arch Biochem Biophys. 2004 Jan 1;421(1):135-42. doi: 10.1016/ Citation on PubMed
  • Goicoechea De Jorge E, Lorda I, Gallardo ME, Perez B, Perez De Ferran C, Mendoza H, Rodriguez De Cordoba S. Alkaptonuria in the Dominican Republic: identification of the founder AKU mutation and further evidence of mutation hot spots in the HGO gene. J Med Genet. 2002 Jul;39(7):E40. doi: 10.1136/jmg.39.7.e40. No abstract available. Citation on PubMed or Free article on PubMed Central
  • Introne WJ, Perry M, Chen M. Alkaptonuria. 2003 May 9 [updated 2021 Jun 10]. In: Adam MP, Feldman J, Mirzaa GM, Pagon RA, Wallace SE, Bean LJH, Gripp KW, Amemiya A, editors. GeneReviews(R) [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2024. Available from Citation on PubMed
  • Rodriguez JM, Timm DE, Titus GP, Beltran-Valero De Bernabe D, Criado O, Mueller HA, Rodriguez De Cordoba S, Penalva MA. Structural and functional analysis of mutations in alkaptonuria. Hum Mol Genet. 2000 Sep 22;9(15):2341-50. doi: 10.1093/oxfordjournals.hmg.a018927. Citation on PubMed
  • Srsen S, Muller CR, Fregin A, Srsnova K. Alkaptonuria in Slovakia: thirty-two years of research on phenotype and genotype. Mol Genet Metab. 2002 Apr;75(4):353-9. doi: 10.1016/S1096-7192(02)00002-1. Citation on PubMed
  • Zatkova A, Chmelikova A, Polakova H, Ferakova E, Kadasi L. Rapid detection methods for five HGO gene mutations causing alkaptonuria. Clin Genet. 2003 Feb;63(2):145-9. doi: 10.1034/j.1399-0004.2003.00027.x. Citation on PubMed

The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.