Homogentisate 1,2-dioxygenase
Encyclopedia
Homogentisate 1,2-dioxygenase (homogentisic acid oxidase, homogentisicase) is an enzyme
, which catalyzes the conversion of homogentisate 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
and phenylalanine
. 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.
. 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, such that both parents must pass the gene on to their children in order for
child to have the defect.
calculations with B3LYP functionals using the programs Gaussian03
and Jaguar
. 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.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
, which catalyzes the conversion of homogentisate to 4-maleylacetoacetate
4-Maleylacetoacetate
4-Maleylacetoacetate is an intermediate in the metabolism of tyrosine.-See also:* Homogentisate 1,2-dioxygenase...
. 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
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
and phenylalanine
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
. 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.
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 alkaptonuriaAlkaptonuria
Alkaptonuria is a rare inherited genetic disorder of phenylalanine and tyrosine metabolism. This is an autosomal recessive condition that is due to a defect in the enzyme homogentisate 1,2-dioxygenase , which participates in the degradation of tyrosine...
. 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, 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 Chemistry. They base their mechanism on results from hybrid DFTDensity functional theory
Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...
calculations with B3LYP functionals using the programs Gaussian03
GAUSSIAN
Gaussian is a computational chemistry software program initially released in 1970 by John Pople and his research group at Carnegie-Mellon University as Gaussian 70. It has been continuously updated since then...
and Jaguar
JAGUAR
JAGUAR is an ab initio quantum chemistry package for both gas and solution phase calculations, with strength in treating metal-containing systems...
. 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
Epoxide
An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...
. This epoxide intermediate allowing radical reactions to eventually open and oxidize the six-membered ring.