enzymology 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 ...

, a kynurenine 3-monooxygenase () 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 ...

that

catalyzes Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

the

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

: -kynurenine +

NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NAD ...

+ H⁺ + O₂ 3-hydroxy--kynurenine + NADP⁺ + H₂O Kynurenine 3-monooxygenase is the expression product of the

KMO (gene) Kynurenine 3-monooxygenase is an enzyme that in humans is encoded by the ''KMO'' gene. Kynurenine 3-monooxygenase (KMO; EC 1.14.13.9) is an NADPH-dependent flavin monooxygenase that catalyzes the hydroxylation of the L-tryptophan metabolite L-kyn ...

. The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...

of this enzyme class is -kynurenine, NADPH:oxygen oxidoreductase (3-hydroxylating). Other names in common use include kynurenine 3-hydroxylase, kynurenine hydroxylase, and -kynurenine-3-hydroxylase. It participates in

tryptophan metabolism Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic b ...

through the kynurenine catabolic pathway. This enzyme belongs to the family of

oxidoreductase In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule, the reductant, also called the electron donor, to another, the oxidant, also called the electron acceptor. This group of enzymes usually ut ...

s, to be specific, those acting on paired donors, with O₂ as the oxidant. Kynurenine 3-monooxygenase catalyzes the insertion of molecular oxygen into the aromatic ring of kynurenine to produce 3-hydroxy--kynurenine. It employs one cofactor,

FAD A fad or trend is any form of collective behavior that develops within a culture, a generation or social group in which a group of people enthusiastically follow an impulse for a short period. Fads are objects or behaviors that achieve short- ...

. Kynurenine 3-monooxygenase serves as an important branch point in the kynurenine pathway and, as a result, is an attractive drug target for

immunological Immunology is a branch of medicineImmunology for Medical Students, Roderick Nairn, Matthew Helbert, Mosby, 2007 and biology that covers the medical study of immune systems in humans, animals, plants and sapient species. In such we can see there ...

neurodegenerative A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...

, and neuroinflammatory diseases. Currently, most research on the kynurenine 3-monooxygenase enzyme has been focused primarily on

rat Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' ( pack rats), ''Bandicota'' (bandicoot ...

models and in

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...

, both of which have been demonstrated to have high sequence

homology Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor * Sequence homology, biological homology between DNA, RNA, or protein sequences *Homologous chrom ...

with the human kynurenine 3-monooxygenase protein. Studies have shown the beneficial effects of enzyme inhibition in these eukaryotic kynurenine 3-monooxygenase active sites, thus making this enzyme an attractive target for human drug design.

Structure

Kynurenine 3-monooxygenase is a

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ...

containing asymmetric subunits and has one FAD-binding domain as its prosthetic group. Kynurenine 3-monooxygenase contains a linker region involved in substrate binding following a second strand of an antiparallel

β-sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gen ...

, a six-stranded antiparallel β-sheet domain, and an

α-helix The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues e ...

at the carboxy-terminal. The

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

C-terminus acts as the mitochondrial anchoring domain and participates in enzymatic activity.

Active site

While no scientific literature reports a crystal image of a kynurenine 3-monooxygenase complex with -kynurenine, structural studies of the enzyme in yeast co-crystallized with UPF 648 reveal how the FAD cofactor and substrate are bound in the active site. Chemical similarities between UPF 648 and -kynurenine suggest that the substrate binds adjacent to the ''Re''-face of the flavoprotein. A loop containing the residues Pro₃₂₁–Gln₃₂₅ is believed to be the oxygen-binding site above the re-side of the FAD prosthetic group. Each monomer contains a conserved hydrophobic pocket (residues Leu₂₂₁, Met₂₃₀, Ile₂₃₂, Leu₂₃₄, Phe₂₄₆, Pro₃₂₁, Phe₃₂₂) positioned around the substrate’s aromatic benzene moiety. A conserved Gln₃₂₅ polar residue is also involved in hydrogen bonding on the -kynurenine carbonyl group, as well as on the hydrogen on the FAD N3 atom. Arg₈₃ and Tyr₉₇ also form polar contacts with the carboxylate in the amino acid moiety on the substrate.

Mechanism

Kynurenine-3-monooxygenase catalyzes the

hydroxylation In chemistry, hydroxylation can refer to: *(i) most commonly, hydroxylation describes a chemical process that introduces a hydroxyl group () into an organic compound. *(ii) the ''degree of hydroxylation'' refers to the number of OH groups in a ...

of -kynurenine to 3-hydroxy--kynurenine with concomitant interconversion of NADPH to NADP⁺. The reaction mechanism is not entirely known, but is believed to follow mechanisms related to the flavin-dependent monooxygenases. After -kynurenine binds, NADPH reduces FAD and leaves as NADP⁺. Oxygen then binds and creates an -kynurenine-FAD-hydroperoxide intermediate. This intermediate is the electrophilic source for the hydroxylation reaction, yielding a primary

ketimine In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bon ...

form of the product and the C4a-hydroxy-FAD. Tautomerization yields 3-hydroxy--kynurenine in complex with the enzyme (E Fl HOH-P). Dissociation of 3-hydroxy--kynurenine and H₂O leads to the free enzyme (E Fl_ox).

Biological function

Kynurenine 3-monooxygenase catalyzes the conversion of -kynurenine to 3-hydroxy--kynurenine, an important bioactive metabolite in the

kynurenine pathway 400px, The kynurenine pathway The kynurenine pathway is a metabolic pathway leading to the production of nicotinamide adenine dinucleotide (NAD+). Metabolites involved in the kynurenine pathway include tryptophan, kynurenine, kynurenic acid, xa ...

. The kynurenine pathway is responsible for over 95% of

tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α- carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...

oxidative degradation. -Kynurenine is an important branch point of this metabolic pathway, being converted into the

neurotoxin Neurotoxins are toxins that are destructive to nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insultsSpencer 2000 that can adversely affect function in both developing and mature ner ...

3-hydroxy--kynurenine via kynurenine 3-monooxygenase, the neuroprotectant

kynurenic acid Kynurenic acid (KYNA or KYN) is a product of the normal metabolism of amino acid -tryptophan. It has been shown that kynurenic acid possesses neuroactive activity. It acts as an antiexcitotoxic and anticonvulsant, most likely through acting as an ...

through kynurenine amino transferases, or

anthranilic acid Anthranilic acid is an aromatic acid with the formula C6H4(NH2)(CO2H) and has a sweetish taste. The molecule consists of a benzene ring, ''ortho''-substituted with a carboxylic acid and an amine. As a result of containing both acidic and basic f ...

kynureninase Kynureninase or L-Kynurenine hydrolase (KYNU) () is a PLP dependent enzyme that catalyses the cleavage of kynurenine (Kyn) into anthranilic acid (Ant). It can also act on 3-hydroxykynurenine (to produce 3-hydroxyanthranilate) and some other (3 ...

. Kynurenine 3-monooxygenase regulates the downstream production of

quinolinic acid Quinolinic acid (abbreviated QUIN or QA), also known as pyridine-2,3-dicarboxylic acid, is a dicarboxylic acid with a pyridine backbone. It is a colorless solid. It is the biosynthetic precursor to niacin. Quinolinic acid is a downstream produ ...

, which can generate reactive free radicals and activates the

NMDA ''N''-methyl--aspartic acid or ''N''-methyl--aspartate (NMDA) is an amino acid derivative that acts as a specific agonist at the NMDA receptor mimicking the action of glutamate, the neurotransmitter which normally acts at that receptor. Unlike ...

subtype of glutamate receptors, producing excitotoxic lesions in the central nervous system of mammals. Quinolinic acid is also the bioprecursor of NAD⁺. Inhibition of kynurenine 3-monooxygenase leads to an increase of kynurenic acid in the kynurenine pathway. This metabolite functions as an

antagonist An antagonist is a character in a story who is presented as the chief foe of the protagonist. Etymology The English word antagonist comes from the Greek ἀνταγωνιστής – ''antagonistēs'', "opponent, competitor, villain, enemy, riv ...

of the α7 nicotinic acetylcholine receptor and as an

at the glycine site of the NMDA receptor. As a result, regulation at the kynurenine 3-monooxygenase enzyme determines the neurotoxic and neuroprotective potential of the kynurenine pathway.

Disease relevance

Kynurenine 3-monooxygenase is an attractive drug target for several neurodegenerative and neuroinflammatory diseases, especially

Huntington's Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unst ...

Alzheimer's Alzheimer's disease (AD) is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As t ...

, and

Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...

. Administration of potent enzyme inhibitors has demonstrated promising pharmacological results. Specifically, genetic elimination of the kynurenine 3-monooxygenase enzyme has been shown to suppress toxicity of the

huntingtin Huntingtin (Htt) is the protein coded for in humans by the ''HTT'' gene, also known as the ''IT15'' ("interesting transcript 15") gene. Mutated ''HTT'' is the cause of Huntington's disease (HD), and has been investigated for this role and also for ...

protein in yeast and ''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...

'' models of Huntington's disease. Kynurenine 3-monooxygenase deficiency, which can be caused by genetic polymorphisms,

cytokines Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrin ...

, or both, leads to an accumulation of

kynurenine -Kynurenine is a metabolite of the amino acid -tryptophan used in the production of niacin. Kynurenine is synthesized by the enzyme tryptophan dioxygenase, which is made primarily but not exclusively in the liver, and indoleamine 2,3-dioxygenase ...

and to a shift within the tryptophan

metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reac ...

towards kynurenic acid and

. Recent research suggests that hyperphysiologic concentrations of

in kynurenine 3-monooxygenase-deficient patients results in a shift towards kynurenic acid production, believed to be related to cognitive deficits in predictive pursuit and visuospatial working memory. Kynurenine-3-monooxygenase deficiency is associated with disorders of the brain (e.g.

schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social withdra ...

tic disorders Tic disorders are defined in the ''Diagnostic and Statistical Manual of Mental Disorders'' (DSM) based on type (motor or phonic) and duration of tics (sudden, rapid, nonrhythmic movements). Tic disorders are defined similarly by the World Health O ...

) and of the liver.

References

* * * {{Portal bar, Biology, border=no EC 1.14.13 NADPH-dependent enzymes Flavoproteins Enzymes of known structure