Homogentisate 1,2-dioxygenase (homogentisic acid oxidase, homogentisate oxidase, homogentisicase) is an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

which catalyzes the conversion of

homogentisate Homogentisic acid (2,5-dihydroxyphenylacetic acid) is a phenolic acid usually found in ''Arbutus unedo'' (strawberry-tree) honey. It is also present in the bacterial plant pathogen ''Xanthomonas campestris'' pv. ''phaseoli'' as well as in the ye ...

4-maleylacetoacetate 4-Maleylacetoacetate (4-maleylacetoacetatic acid) is an intermediate in the metabolism of tyrosine. It is converted to fumarylacetoacetate by the enzyme 4-maleylacetoacetate cis-trans-isomerase. Gluthathione coenzymatically helps in conversion to ...

. Homogentisate 1,2-dioxygenase or HGD is involved in the catabolism of aromatic rings, more specifically in the breakdown of the amino acids

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the Gr ...

and

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the formula . It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amino a ...

. 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. Image:Homogentisic acid.svg,

Image:4-maleylacetoacetic acid.svg,

Enzyme active site

The active site of Homogentisate 1,2-dioxygenase was determined through the crystal structure, which was captured through the work of Titus et al. Through the crystal structure the active site was found to contain the following residues; His292, His335, His365, His371, and Glu341. Homogentisate binds in the active site to Glu341, His335, and His371 via the Fe²⁺ atom. The His292 binds to the hydroxyl group of the aromatic ring. His365 binds to Glu341 via hydrogen bonding to stabilize the amino acid side chains.

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 Ochronosis is a syndrome caused by the accumulation of homogentisic acid in connective tissues. The condition was named after the yellowish (ocher-like) discoloration of the tissue seen on microscopic examination. Macroscopically, though, the af ...

, 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 Osteoarthritis (OA) is a type of degenerative joint disease that results from breakdown of joint cartilage and underlying bone which affects 1 in 7 adults in the United States. It is believed to be the fourth leading cause of disability in the w ...

, 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 In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...

, such that both parents must pass the gene on to their children in order for child to have the defect.

Mechanism

Borowski et al. propose a mechanism for HGD in their article featured in the Journal of the American Chemical Society. They base their mechanism on results from hybrid DFT calculations with

B3LYP Hybrid functionals are a class of approximations to the exchange– correlation energy functional in density functional theory (DFT) that incorporate a portion of exact exchange from Hartree–Fock theory with the rest of the exchange–correl ...

functionals using the programs Gaussian03 and

Jaguar The jaguar (''Panthera onca'') is a large cat species and the only living member of the genus '' Panthera'' native to the Americas. With a body length of up to and a weight of up to , it is the largest cat species in the Americas and the th ...

. The opening of the aromatic ring in homogentisate is a multi-step process. In the first two steps Fe²⁺ coordinates to the carbonyl and ortho phenol oxygens. The iron atom is also coordinated to His335, His371, and Glu341. O₂ then binds to the iron atom, subsequently reacting with the aromatic ring to form a peroxo-bridged intermediate. In the next step, O₂ is cleaved with the formation of an epoxide. This epoxide intermediate allowing radical reactions to eventually open and oxidize the six-membered ring. File:Mech01.jpg, Steps 1-8 of the mechanism File:Mech02.jpg, Steps 9-11 of the mechanism

References

External links

GeneReviews/NCBI/NIH/UW entry on Alkaptonuria

OMIM entries on Alkaptonuria
* {{Amino acid metabolism enzymes EC 1.13.11