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Orexin Edit Wikipedia article
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|orexin (hypocretin) neuropeptide precursor|
Solution phase NMR structure of orexin B based on the PDB coordinates .
|Alt. symbols||PPOX, OX|
|Locus||Chr. 17 q21|
Orexin, also called hypocretin, is a neuropeptide that regulates arousal, wakefulness, and appetite. The most common form of narcolepsy, in which the sufferer briefly loses muscle tone (cataplexy), is caused by a lack of orexin in the brain due to destruction of the cells that produce it.
There are approximately 70,000 orexin producing neurons in the human brain that project from the lateral hypothalamus to neurons and brain regions that modulate wakefulness. However, the axons from these neurons extend throughout the entire brain and spinal cord, where there are also receptors for orexin.
Orexin was discovered in 1998 almost simultaneously by two independent groups of rat-brain researchers. One group named it orexin, from orexis, meaning "appetite" in Greek; the other group named it hypocretin, because it is produced in the hypothalamus and bears a weak resemblance to secretin, another peptide. The use of both terms is now a practical necessity, as hypocretin is used to refer to the genetic products and orexin is used to refer to the protein products.
There are two types of orexin: orexin-A and -B (hypocretin-1 and -2). They are excitatory neuropeptides with approximately 50% sequence identity, produced by cleavage of a single precursor protein. Orexin-A is 33 amino acid residues long and has two intrachain disulfide bonds; orexin-B is a linear 28 amino acid residue peptide. Although these peptides are produced by a very small population of cells in the lateral and posterior hypothalamus, they send projections throughout the brain. The orexin peptides bind to the two G-protein coupled orexin receptors, OX1 and OX2, with orexin-A binding to both OX1 and OX2 with approximately equal affinity while orexin-B binds mainly to OX2 and is 5 times less potent as OX1.
The orexins are strongly conserved peptides, found in all major classes of vertebrates.
The orexin system was initially suggested to be primarily involved in the stimulation of food intake, based on the finding that central administration of orexin-A and -B increased food intake. In addition, it stimulates wakefulness, regulates energy expenditure, and modulates visceral function.
Brown fat activation
Obesity in orexin knockout mice is a result of inability of brown preadipocytes to differentiate into brown adipose tissue (BAT), which in turn reduces BAT thermogenesis. BAT differentiation can be restored in these knockout mice through injections of orexin. Deficiency in orexin has also been linked to narcolepsy, a sleep disorder. Furthermore, narcoleptic people are more likely to be obese. Hence obesity in narcoleptic patients may be due to orexin deficiency leading to impaired thermogenesis and energy expenditure.
Orexin seems to promote wakefulness. Recent studies indicate that a major role of the orexin system is to integrate metabolic, circadian and sleep debt influences to determine whether an animal should be asleep or awake and active. Orexin neurons strongly excite various brain nuclei with important roles in wakefulness including the dopamine, norepinephrine, histamine and acetylcholine systems and appear to play an important role in stabilizing wakefulness and sleep.
The discovery that an orexin receptor mutation causes the sleep disorder canine narcolepsy in Doberman Pinschers subsequently indicated a major role for this system in sleep regulation. Genetic knockout mice lacking the gene for orexin were also reported to exhibit narcolepsy. Transitioning frequently and rapidly between sleep and wakefulness, these mice display many of the symptoms of narcolepsy. Researchers are using this animal model of narcolepsy to study the disease. Narcolepsy results in excessive daytime sleepiness, inability to consolidate wakefulness in the day (and sleep at night), and cataplexy, which is the loss of muscle tone in response to strong, usually positive, emotions. Dogs that lack a functional receptor for orexin have narcolepsy, while animals and people lacking the orexin neuropeptide itself also have narcolepsy.
Central administration of orexin-A strongly promotes wakefulness, increases body temperature and locomotion, and elicits a strong increase in energy expenditure. Sleep deprivation also increases orexin-A transmission. The orexin system may thus be more important in the regulation of energy expenditure than food intake. In fact, orexin-deficient narcoleptic patients have increased obesity rather than decreased BMI, as would be expected if orexin were primarily an appetite stimulating peptide. Another indication that deficits of orexin cause narcolepsy is that depriving monkeys of sleep for 30–36 hours and then injecting them with the neurochemical alleviates the cognitive deficiencies normally seen with such amount of sleep loss.
In humans, narcolepsy is associated with a specific variant of the human leukocyte antigen (HLA) complex. Furthermore, genome-wide analysis shows that, in addition to the HLA variant, narcoleptic humans also exhibit a specific genetic mutation in the T-cell receptor alpha locus. In conjunction, these genetic anomalies cause the immune system to attack and kill the critical orexin neurons. Hence the absence of orexin-producing neurons in narcoleptic humans may be the result of an autoimmune disorder.
Orexin increases the craving for food, and correlates with the function of the substances that promote its production. Orexin is also shown to increase meal size by suppressing inhibitory postingestive feedback. However, some studies suggest that the stimulatory effects of orexin on feeding may be due to general arousal without necessarily increasing overall food intake.
Leptin is a hormone produced by fat cells and acts as a long-term internal measure of energy state. Ghrelin is a short-term factor secreted by the stomach just before an expected meal, and strongly promotes food intake.
Orexin-producing cells have recently been shown to be inhibited by leptin (through the leptin receptor pathway), but are activated by ghrelin and hypoglycemia (glucose inhibits orexin production). Orexin, as of 2007, is claimed to be a very important link between metabolism and sleep regulation. Such a relationship has been long suspected, based on the observation that long-term sleep deprivation in rodents dramatically increases food intake and energy metabolism, i.e., catabolism, with lethal consequences on a long-term basis. Sleep deprivation then leads to a lack of energy. In order to make up for this lack of energy, many people use high-carbohydrate and high-fat foods that ultimately can lead to poor health and weight gain. Other dietary nutrients, amino acids, also can activate orexin neurons, and they can suppress the glucose response of orexin neurons at physiological concentration, causing the energy balance that orexin maintains to be thrown off its normal cycle.
Merck reported at the Sleep 2012 conference that insomniacs taking an orexin blocker, suvorexant, fell asleep faster and slept an hour longer. Suvorexant was tested for three months on over a thousand patients in a phase III trial. Suvorexant has undergone three phase III trials and was approved on August 13, 2014 by the US Food and Drug Administration (FDA) after being denied approval the year before. It is available as "Belsomra".
Preliminary research has been conducted that shows potential for orexin blockers in the treatment of alcoholism. Lab rats given drugs which targeted the orexin system lost interest in alcohol despite being given free access in experiments.
A study has reported that transplantation of orexin neurons into the pontine reticular formation in rats is feasible, indicating the development of alternative therapeutic strategies in addition to pharmacological interventions to treat narcolepsy.
Because orexin-A receptors have been shown to regulate relapse to cocaine seeking, a new study investigated its relation to nicotine by studying rats. By blocking the orexin-A receptor with low doses of the selective antagonist SB-334,867, nicotine self-administration decreased and also the motivation to seek and obtain the drug. The study showed that blocking of receptors in the insula decreased self-administration, but not blocking of receptors in the adjacent somatosensory cortex. The greatest decrease in self-administration was found when blocking all orexin-A receptors in the brain as a whole. A rationale for this study was the fact that the insula has been implicated in regulating feelings of craving. The insula contains orexin-A receptors. It has been reported that smokers who sustained damage to the insula lost the desire to smoke.
Orexin-A (OXA) has been recently demonstrated to have a direct effect on an aspect of lipid metabolism. OXA stimulates glucose uptake in 3T3-L1 adipocytes and that increased energy uptake is stored as lipids (triacylglycerol). OXA thus increases lipogenesis. It also inhibits lipolysis and stimulates the secretion of adiponectin. These effects are thought to be mostly conferred via the PI3K pathway because this pathway inhibitor (LY294002) completely blocks OXA effects in adipocytes. The link between OXA and the lipid metabolism is new and currently under more research.
High levels of orexin-A have been associated with happiness in human subjects, while low levels have been associated with sadness. The finding suggests that boosting levels of orexin-A could elevate mood in humans, being thus a possible future treatment for disorders like depression.
History and nomenclature
In 1996, Gautvik, de Lecea, and colleagues reported the discovery of several genes in the rat brain, including one they dubbed "clone 35." Their work showed that clone 35 expression was limited to the lateral hypothalamus.
Masashi Yanagisawa and colleagues at the University of Texas Southwestern Medical Center at Dallas, coined the term orexin to reflect the orexigenic (appetite-stimulating) activity of these peptides. In their 1998 paper (with authorship attributed to Sakurai and colleagues) describing these neuropeptides, they also reported discovery of two orexin receptors, dubbed OX1R and OX2R.
In 1998, Luis de Lecea, Thomas Kilduff, and colleagues also reported discovery of these same peptides, dubbing them hypocretins to indicate that they are synthesized in the hypothalamus and to reflect their structural similarity to the hormone secretin (i.e., hypothalamic secretin). This is the same group that first identified clone 35 two years earlier. De Lecea and colleagues were originally in search of novel genes expressed in the hypothalamus. To do this, they extracted selective DNA found in the lateral hypothalamus. They cloned this DNA and studied it under electron microscopy. Neurotransmitters found in this area were oddly similar to the gut hormone, secretin, so de Lecea decided to name the two forms of peptides hypocretin-1 and hypocretin-2. These cells were first thought to reside and work only within the lateral hypothalamus area, but immunocytochemistry tactics revealed the various projections this area truly had to other parts of the brain. A majority of these projections reached the limbic system and structures associated with it (including the amygdala, septum, and basal forebrain area).
The history of the discovery of the orexin system is reflected in its nomenclature. "Hypocretin" refers to the gene or genetic products and "orexin" refers to the protein, reflecting the differing approaches that resulted in its discovery. The use of both terms is also a practical necessity because "HCRT" is the standard gene symbol in databases like GenBank and "OX" is used to refer to the pharmacology of the peptide system by the International Union of Basic and Clinical Pharmacology.
Orexin neuron neurotransmitters
Orexinergic neurons have been shown to be sensitive to inputs from Group III metabotropic glutamate receptors, cannabinoid receptor 1 and CB1–OX1 receptor heterodimers, adenosine A1 receptors, muscarinic M3 receptors, serotonin 5-HT1A receptors, neuropeptide Y receptors, cholecystokinin A receptors, and catecholamines, as well as to ghrelin, leptin, and glucose. Orexinergic neurons themselves regulate release of acetylcholine, serotonin and noradrenaline.
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- Stanford Center for Narcolepsy FAQ (retrieved 27-Mar-2012)
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Direct CB1-HcrtR1 interaction was first proposed in 2003 (Hilairet et al., 2003). Indeed, a 100-fold increase in the potency of hypocretin-1 to activate the ERK signaling was observed when CB1 and HcrtR1 were co-expressed ... In this study, a higher potency of hypocretin-1 to regulate CB1-HcrtR1 heteromer compared with the HcrtR1-HcrtR1 homomer was reported (Ward et al., 2011b). These data provide unambiguous identification of CB1-HcrtR1 heteromerization, which has a substantial functional impact. ... The existence of a cross-talk between the hypocretinergic and endocannabinoid systems is strongly supported by their partially overlapping anatomical distribution and common role in several physiological and pathological processes. However, little is known about the mechanisms underlying this interaction.
• Figure 1: Schematic of brain CB1 expression and orexinergic neurons expressing OX1 or OX2
• Figure 2: Synaptic signaling mechanisms in cannabinoid and orexin systems
• Figure 3: Schematic of brain pathways involved in food intake
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OX1–CB1 dimerization was suggested to strongly potentiate orexin receptor signaling, but a likely explanation for the signal potentiation is, instead, offered by the ability of OX1 receptor signaling to produce 2-arachidonoyl glycerol, a CB1 receptor ligand, and a subsequent co-signaling of the receptors (Haj-Dahmane and Shen, 2005; Turunen et al., 2012; Jäntti et al., 2013). However, this does not preclude dimerization.
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Orexin receptor subtypes readily formed homo- and hetero(di)mers, as suggested by significant BRET signals. CB1 receptors formed homodimers, and they also heterodimerized with both orexin receptors. ... In conclusion, orexin receptors have a significant propensity to make homo- and heterodi-/oligomeric complexes. However, it is unclear whether this affects their signaling. As orexin receptors efficiently signal via endocannabinoid production to CB1 receptors, dimerization could be an effective way of forming signal complexes with optimal cannabinoid concentrations available for cannabinoid receptors.
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- Bernard R, Lydic R, Baghdoyan HA (Oct 2003). "Hypocretin-1 causes G protein activation and increases ACh release in rat pons". The European Journal of Neuroscience. 18 (7): 1775–85. doi:10.1046/j.1460-9568.2003.02905.x. PMID 14622212.
- Frederick-Duus D, Guyton MF, Fadel J (Nov 2007). "Food-elicited increases in cortical acetylcholine release require orexin transmission". Neuroscience. 149 (3): 499–507. doi:10.1016/j.neuroscience.2007.07.061. PMID 17928158.
- Soffin EM, Gill CH, Brough SJ, Jerman JC, Davies CH (Jun 2004). "Pharmacological characterisation of the orexin receptor subtype mediating postsynaptic excitation in the rat dorsal raphe nucleus". Neuropharmacology. 46 (8): 1168–76. doi:10.1016/j.neuropharm.2004.02.014. PMID 15111023.
- orexins at the US National Library of Medicine Medical Subject Headings (MeSH)
- Compare Different Sleep Aids, National Sleep Foundation
- Orexin receptor antagonists: A new class of sleeping pill, National Sleep Foundation
This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.
Prepro-orexin Provide feedback
No Pfam abstract.
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR001704
Orexins (also known as hypocretins) are neuropeptides that are specifically localised to the hypothalamus. They are thought to interact with autonomic, neurendocrine and neuroregulatory systems, and play an important role in the regulation of feeding behaviour [PUBMED:9892705, PUBMED:9419374]. When applied to hypothalamic neurones, these peptides are neuroexcitatory, which action is probably mediated by their binding to a new family of G-protein-coupled receptors (orexin receptors 1 and 2), which were previously orphan [PUBMED:9491897].
To date, two orexins have been characterised (orexin-A and -B), both encoded by a single mRNA transcript (prepro-orexin): orexin-A is a 33-residue peptide with two intramolecular disulphide bonds in the N-terminal region; and orexin-B is a linear 28-residue peptide. These peptides have 46% identity at the amino acid sequence level, and show some similarity to the glucagon/vasoactive intestinal polypeptide/secretin peptide family.
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Biological process||feeding behavior (GO:0007631)|
|neuropeptide signaling pathway (GO:0007218)|
Below is a listing of the unique domain organisations or architectures in which this domain is found. More...
The graphic that is shown by default represents the longest sequence with a given architecture. Each row contains the following information:
- the number of sequences which exhibit this architecture
a textual description of the architecture, e.g. Gla, EGF x 2, Trypsin.
This example describes an architecture with one
Gladomain, followed by two consecutive
EGFdomains, and finally a single
- a link to the page in the Pfam site showing information about the sequence that the graphic describes
- the UniProt description of the protein sequence
- the number of residues in the sequence
- the Pfam graphic itself.
Note that you can see the family page for a particular domain by clicking on the graphic. You can also choose to see all sequences which have a given architecture by clicking on the Show link in each row.
Finally, because some families can be found in a very large number of architectures, we load only the first fifty architectures by default. If you want to see more architectures, click the button at the bottom of the page to load the next set.
Loading domain graphics...
We store a range of different sequence alignments for families. As well as the seed alignment from which the family is built, we provide the full alignment, generated by searching the sequence database (reference proteomes) using the family HMM. We also generate alignments using four representative proteomes (RP) sets, the UniProtKB sequence database, the NCBI sequence database, and our metagenomics sequence database. More...
There are various ways to view or download the sequence alignments that we store. We provide several sequence viewers and a plain-text Stockholm-format file for download.
We make a range of alignments for each Pfam-A family:
- the curated alignment from which the HMM for the family is built
- the alignment generated by searching the sequence database using the HMM
- Representative Proteomes (RPs) at 15%, 35%, 55% and 75% co-membership thresholds
- alignment generated by searching the UniProtKB sequence database using the family HMM
- alignment generated by searching the NCBI sequence database using the family HMM
- alignment generated by searching the metagenomics sequence database using the family HMM
You can see the alignments as HTML or in three different sequence viewers:
- a Java applet developed at the University of Dundee. You will need Java installed before running jalview
- an HTML page showing the whole alignment.Please note: full Pfam alignments can be very large. These HTML views are extremely large and often cause problems for browsers. Please use either jalview or the Pfam viewer if you have trouble viewing the HTML version
- an HTML-based representation of the alignment, coloured according to the posterior-probability (PP) values from the HMM. As for the standard HTML view, heatmap alignments can also be very large and slow to render.
You can download (or view in your browser) a text representation of a Pfam alignment in various formats:
You can also change the order in which sequences are listed in the alignment, change how insertions are represented, alter the characters that are used to represent gaps in sequences and, finally, choose whether to download the alignment or to view it in your browser directly.
You may find that large alignments cause problems for the viewers and the reformatting tool, so we also provide all alignments in Stockholm format. You can download either the plain text alignment, or a gzipped version of it.
We make a range of alignments for each Pfam-A family. You can see a description of each above. You can view these alignments in various ways but please note that some types of alignment are never generated while others may not be available for all families, most commonly because the alignments are too large to handle.
1Cannot generate PP/Heatmap alignments for seeds; no PP data available
Key: available, not generated, — not available.
Format an alignment
We make all of our alignments available in Stockholm format. You can download them here as raw, plain text files or as gzip-compressed files.
You can also download a FASTA format file containing the full-length sequences for all sequences in the full alignment.
HMM logos is one way of visualising profile HMMs. Logos provide a quick overview of the properties of an HMM in a graphical form. You can see a more detailed description of HMM logos and find out how you can interpret them here. More...
If you find these logos useful in your own work, please consider citing the following article:
This page displays the phylogenetic tree for this family's seed alignment. We use FastTree to calculate neighbour join trees with a local bootstrap based on 100 resamples (shown next to the tree nodes). FastTree calculates approximately-maximum-likelihood phylogenetic trees from our seed alignment.
Note: You can also download the data file for the tree.
Curation and family details
This section shows the detailed information about the Pfam family. You can see the definitions of many of the terms in this section in the glossary and a fuller explanation of the scoring system that we use in the scores section of the help pages.
|Author:||Mian N, Bateman A|
|Number in seed:||4|
|Number in full:||50|
|Average length of the domain:||114.70 aa|
|Average identity of full alignment:||59 %|
|Average coverage of the sequence by the domain:||75.70 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 17690987 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||13|
|Download:||download the raw HMM for this family|
Weight segments by...
Change the size of the sunburst
selected sequences to HMM
a FASTA-format file
- 0 sequences
- 0 species
This visualisation provides a simple graphical representation of the distribution of this family across species. You can find the original interactive tree in the More....
This chart is a modified "sunburst" visualisation of the species tree for this family. It shows each node in the tree as a separate arc, arranged radially with the superkingdoms at the centre and the species arrayed around the outermost ring.
How the sunburst is generated
The tree is built by considering the taxonomic lineage of each sequence that has a match to this family. For each node in the resulting tree, we draw an arc in the sunburst. The radius of the arc, its distance from the root node at the centre of the sunburst, shows the taxonomic level ("superkingdom", "kingdom", etc). The length of the arc represents either the number of sequences represented at a given level, or the number of species that are found beneath the node in the tree. The weighting scheme can be changed using the sunburst controls.
In order to reduce the complexity of the representation, we reduce the number of taxonomic levels that we show. We consider only the following eight major taxonomic levels:
Colouring and labels
Segments of the tree are coloured approximately according to their superkingdom. For example, archeal branches are coloured with shades of orange, eukaryotes in shades of purple, etc. The colour assignments are shown under the sunburst controls. Where space allows, the name of the taxonomic level will be written on the arc itself.
As you move your mouse across the sunburst, the current node will be highlighted. In the top section of the controls panel we show a summary of the lineage of the currently highlighed node. If you pause over an arc, a tooltip will be shown, giving the name of the taxonomic level in the title and a summary of the number of sequences and species below that node in the tree.
Anomalies in the taxonomy tree
There are some situations that the sunburst tree cannot easily handle and for which we have work-arounds in place.
Missing taxonomic levels
Some species in the taxonomic tree may not have one or more of the main eight levels that we display. For example, Bos taurus is not assigned an order in the NCBI taxonomic tree. In such cases we mark the omitted level with, for example, "No order", in both the tooltip and the lineage summary.
Unmapped species names
The tree is built by looking at each sequence in the full alignment for the family. We take the name of the species given by UniProt and try to map that to the full taxonomic tree from NCBI. In some cases, the name chosen by UniProt does not map to any node in the NCBI tree, perhaps because the chosen name is listed as a synonym or a misspelling in the NCBI taxonomy.
So that these nodes are not simply omitted from the sunburst tree, we group them together in a separate branch (or segment of the sunburst tree). Since we cannot determine the lineage for these unmapped species, we show all levels between the superkingdom and the species as "uncategorised".
Since we reduce the species tree to only the eight main taxonomic levels, sequences that are mapped to the sub-species level in the tree would not normally be shown. Rather than leave out these species, we map them instead to their parent species. So, for example, for sequences belonging to one of the Vibrio cholerae sub-species in the NCBI taxonomy, we show them instead as belonging to the species Vibrio cholerae.
Too many species/sequences
For large species trees, you may see blank regions in the outer layers of the sunburst. These occur when there are large numbers of arcs to be drawn in a small space. If an arc is less than approximately one pixel wide, it will not be drawn and the space will be left blank. You may still be able to get some information about the species in that region by moving your mouse across the area, but since each arc will be very small, it will be difficult to accurately locate a particular species.
The tree shows the occurrence of this domain across different species. More...
We show the species tree in one of two ways. For smaller trees we try to show an interactive representation, which allows you to select specific nodes in the tree and view them as an alignment or as a set of Pfam domain graphics.
Unfortunately we have found that there are problems viewing the interactive tree when the it becomes larger than a certain limit. Furthermore, we have found that Internet Explorer can become unresponsive when viewing some trees, regardless of their size. We therefore show a text representation of the species tree when the size is above a certain limit or if you are using Internet Explorer to view the site.
If you are using IE you can still load the interactive tree by clicking the "Generate interactive tree" button, but please be aware of the potential problems that the interactive species tree can cause.
For all of the domain matches in a full alignment, we count the number that are found on all sequences in the alignment. This total is shown in the purple box.
We also count the number of unique sequences on which each domain is found, which is shown in green. Note that a domain may appear multiple times on the same sequence, leading to the difference between these two numbers.
Finally, we group sequences from the same organism according to the NCBI code that is assigned by UniProt, allowing us to count the number of distinct sequences on which the domain is found. This value is shown in the pink boxes.
We use the NCBI species tree to group organisms according to their taxonomy and this forms the structure of the displayed tree. Note that in some cases the trees are too large (have too many nodes) to allow us to build an interactive tree, but in most cases you can still view the tree in a plain text, non-interactive representation. Those species which are represented in the seed alignment for this domain are highlighted.
You can use the tree controls to manipulate how the interactive tree is displayed:
- show/hide the summary boxes
- highlight species that are represented in the seed alignment
- expand/collapse the tree or expand it to a given depth
- select a sub-tree or a set of species within the tree and view them graphically or as an alignment
- save a plain text representation of the tree
Please note: for large trees this can take some time. While the tree is loading, you can safely switch away from this tab but if you browse away from the family page entirely, the tree will not be loaded.
For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Orexin domain has been found. There are 4 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.
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