L-ornithine N5 monooxygenase (EC 1.14.13.195 or EC 1.14.13.196) 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 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 ...

one of the following

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 ...

L-ornithine + NADPH + O₂ $\rightleftharpoons$ N(5)-hydroxy-L-ornithine + NADP⁺ + H₂O L-ornithine + NAD(P)H + O₂ $\rightleftharpoons$ N(5)-hydroxy-L-ornithine + NAD(P)⁺ + H₂O

The three

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...

s of this enzyme are

L-ornithine Ornithine is a non-proteinogenic amino acid that plays a role in the urea cycle. Ornithine is abnormally accumulated in the body in ornithine transcarbamylase deficiency. The radical is ornithyl. Role in urea cycle L-Ornithine is one of the produc ...

(

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

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- ...

( cofactor), and

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 ...

or NAD(P)H (cofactor and electron donor).

Enzyme classification

L-ornithine N5 monooxygenase is classified under two EC numbers - EC1.14.13.195 and EC 1.14.13.196. The first number, 1, identifies the enzyme as an oxidoreductase. The subsequent 14 refers to the fact that this enzyme acts "on paired donors, with incorporation or reduction of molecular oxygen". The 13 identifies this enzyme as using NADH or NAD(P)H as one donor, while incorporating one atom of oxygen onto the other. This is why there are two EC numbers for this enzyme - one ends with 195 referring to NADPH as the donor, while the 196 refers to NAD(P)H as the donor.

Structure

L-ornithine N5 monooxygenase adopbts an oxidoreductase Rossmann fold tertiary structure that binds

and NADP cofactors. Crystallographic structures have been solved for this class of enzymes from ''

Aspergillus fumigatus ''Aspergillus fumigatus'' is a species of fungus in the genus ''Aspergillus'', and is one of the most common ''Aspergillus'' species to cause disease in individuals with an immunodeficiency. ''Aspergillus fumigatus'', a saprotroph widespread in ...

''. These structures reflect structural changes which take place when the enzyme binds combinations of

s, including ornithine and NADP. Additional structures have also been solved for strain Af293. These structures reflect different

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

and ligation states. The following table briefly describes these crystal structures: In ''A. fumigatus'', the enzyme is named ''Af'' SidA for siderophore biosynthesis protein A. It has three domains for ornithine (

), FAD ( cofactor), and NAD(P)H (cofactor and electron donor). The enzyme is a

homotetramer A tetrameric protein is a protein with a protein quaternary structure, quaternary structure of four subunits (tetrameric). Homotetramers have four identical Protein subunit, subunits (such as glutathione S-transferase), and heterotetramers are M ...

. N-hydroxylating flavin-containing

monooxygenase Monooxygenases are enzymes that incorporate one hydroxyl group (−OH) into substrates in many metabolic pathways. In this reaction, the two atoms of dioxygen are reduced to one hydroxyl group and one H2O molecule by the concomitant oxidation o ...

(NMO) enzymes such as this target the nucleophilic terminal amine groups of primary aliphatic amines such as

. The enzyme operates via a multistep oxidative mechanism which has a C4a-hydroperoxyflavin intermediate. SidA stabilizes this intermediate and keeps NADP+ bound throughout the remainder of the catalytic cycle because it is necessary for intermediate stabilization. The nicotinamide-ribose moiety and H-bonding between the main chain and residues Lys107, Asn293, and Ser469 position the L-ornithine alpha carbon such that its side chain amino group can be hydroxylated by the C4a-(hydro)peroxyflavin. Unlike many other NMOs, ''A. fumigatus'' SidA strictly acts on ornithine. Interactions with arginine increase interactivity between the reduced flavin and oxygen. The

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...

is located within a cleft at the interface between the three domains on each subunit. SidA has a resting state
6X0H
in which neither L-ornithine nor NAD(P)H is bound. This resting state has an "out" active site caused by large rotations of the FAD isoalloxazine and a 10-Å movement of the Tyrosine loop. Either flavin reduction or NAD(P)H binding drives the active site to the "in" conformation
6X0I
. SidA demonstrates typical

kinetics Kinetics ( grc, κίνησις, , kinesis, ''movement'' or ''to move'') may refer to: Science and medicine * Kinetics (physics), the study of motion and its causes ** Rigid body kinetics, the study of the motion of rigid bodies * Chemical ki ...

when saturated with L-ornithine. Inhibition is caused by high concentrations of NADPH and NADH. There is an 8-fold increase in

catalytic efficiency In the field of biochemistry, the specificity constant (also called kinetic efficiency or k_/K_), is a measure of how efficiently an enzyme converts substrates into products. A comparison of specificity constants can also be used as a measure of t ...

for NADPH compared to NADH. NADP+ is a competitive inhibitor with respect to NADPH.

Function

This enzyme is widely distributed, especially among

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

s, being found in

Fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

Metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

Protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...

a, Viridiplantae,

Choanoflagellate The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates, having a funnel shaped collar of interconne ...

s, and Icththyosporeans. Among

Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

, it is found in

Kutzneria ''Kutzneria'' is a genus of bacteria in the phylum Actinomycetota. This genus was named after Hans-Jürgen Kutzner, a German microbiologist.Brock Biology of Microorganisms; Madigan (et al.); Pearson Education Inc., 2003; Appendix 2, pg. A-12. ...

sp. 744, and an ornithine hydroxylase from ''Pseudomonas'' ''aeruginosa'' has a similar structure and 41%

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

similarity to that of '' A. nidulans''. In addition to being found in the non-pathogenic fungi such as ''

Aspergillus nidulans ''Aspergillus nidulans'' (also called ''Emericella nidulans'' when referring to its sexual form, or teleomorph) is one of many species of filamentous fungi in the phylum Ascomycota. It has been an important research organism for studying eukaryot ...

'', it is also found in many fungal pathogens such as ''

'', ''

Botrytis cinerea ''Botrytis cinerea'' is a necrotrophic fungus that affects many plant species, although its most notable hosts may be wine grapes. In viticulture, it is commonly known as "botrytis bunch rot"; in horticulture, it is usually called "grey mould" or ...

'', '' Fusarium oxysporum'', ''

Magnaporthe oryzae ''Magnaporthe'' is a genus of ascomycete fungi. Several of the species are cereal pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that ca ...

'', '' Sclerotinia sclerotiorum'', '' Spizellomyces punctatus'', and ''

Ustilago maydis Corn smut is a plant disease caused by the pathogenic fungus ''Ustilago maydis'' that causes smut on maize and teosinte. The fungus forms galls on all above-ground parts of corn species. It is edible, and is known in Mexico as the delicacy ''h ...

''. In ''A. fumigatus'', it is classified as a flavoprotein because FAD is a cofactor. It catalyzes the FAD and NADPH-dependent

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 L-ornithine in biosynthesis of the

ferrichrome Ferrichrome is a cyclic hexa-peptide that forms a complex with iron atoms. It is a siderophore composed of three glycine and three modified ornithine residues with hydroxamate groups N(OH)C(=O)C- The 6 oxygen atoms from the three hydroxamate gr ...

siderophores triacetylfusarinine and desferriferricrocin. It is produced primarily under iron-limited conditions. Siderophores are also important for virulence. In Kutzneria sp. 744, this enzyme is involved in the biosynthesis of piperazate, which contributes to the biosynthesis of kutzneride

antifungal An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as crypto ...

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

References

{{Portal bar, Biology, border=no EC 1.14.13 Enzymes of known structure