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

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

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, ''Xanthomonas campestris'' p ...

to 4-maleylacetoacetate. 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 conditionally essential amino acid with a polar side group. The word "tyrosine" is ...

and

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

. 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, 4-maleylacetoacetate

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 Alkaptonuria is a rare inherited genetic disease which is caused by a mutation in the ''HGD'' gene for the enzyme homogentisate 1,2-dioxygenase (); if a person inherits an abnormal copy from both parents (it is a dominance relationship, recessive c ...

. 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 In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the Phenotype, effect of a different variant of the same gene on Homologous chromosome, the other copy of the chromosome. The firs ...

, 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 The Department for Transport (DfT) is a ministerial department of the Government of the United Kingdom. It is responsible for the English transport network and a limited number of transport matters in Scotland, Wales, and Northern Ireland t ...

calculations with B3LYP functionals using the programs Gaussian03 and

Jaguar The jaguar (''Panthera onca'') is a large felidae, cat species and the only extant taxon, living member of the genus ''Panthera'' that is native to the Americas. With a body length of up to and a weight of up to , it is the biggest cat spe ...

. 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 In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ...

. 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