Summary: Sir2 family
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Sirtuin Edit Wikipedia article
Crystallographic structure of yeast sir2 (rainbow colored cartoon, N-terminus = blue, C-terminus = red) complexed with ADP (space-filling model, carbon = white, oxygen = red, nitrogen = blue, phosphorus = orange) and a histone H4 peptide (magenta) containing an acylated lysine residue (displayed as spheres).
Sirtuin or Sir2 proteins are a class of proteins that possess either mono-ADP-ribosyltransferase, or deacylase activity, including deacetylase, desuccinylase, demalonylase, demyristoylase and depalmitoylase activity. Sirtuins regulate important biological pathways in bacteria, archaea and eukaryotes. The name Sir2 comes from the yeast gene 'silent mating-type information regulation 2', the gene responsible for cellular regulation in yeast.
Sirtuins have been implicated in influencing a wide range of cellular processes like aging, transcription, apoptosis, inflammation and stress resistance, as well as energy efficiency and alertness during low-calorie situations. Sirtuins can also control circadian clocks and mitochondrial biogenesis.
Yeast Sir2 and some, but not all, sirtuins are protein deacetylases. Unlike other known protein deacetylases, which simply hydrolyze acetyl-lysine residues, the sirtuin-mediated deacetylation reaction couples lysine deacetylation to NAD hydrolysis. This hydrolysis yields O-acetyl-ADP-ribose, the deacetylated substrate and nicotinamide, itself an inhibitor of sirtuin activity. The dependence of sirtuins on NAD links their enzymatic activity directly to the energy status of the cell via the cellular NAD:NADH ratio, the absolute levels of NAD, NADH or nicotinamide or a combination of these variables.
Whereas bacteria and archaea encode either one or two sirtuins, eukaryotes encode several sirtuins in their genomes. In yeast, roundworms, and fruitflies, sir2 is the name of the sirtuin-type protein. This research started in 1991 by Leonard Guarente of MIT. Mammals possess seven sirtuins (SIRT1-7) that occupy different subcellular compartments such as the nucleus (SIRT1, -2, -6, -7), cytoplasm (SIRT1 and SIRT2) and the mitochondria (SIRT3, -4 and -5).
The first sirtuin was identified in yeast (a lower eukaryote) and named sir2. In more complex mammals, there are seven known enzymes that act in cellular regulation, as sir2 does in yeast. These genes are designated as belonging to different classes, depending on their amino acid sequence structure. Several Gram positive prokaryotes as well as the Gram negative hyperthermophilic bacterium Thermotoga maritima possess sirtuins that are intermediate in sequence between classes. These are placed in class U.
|I||a||Sir2 or Sir2p,
Hst1 or Hst1p
|b||Hst2 or Hst2p||Sirt2||SIRT2||cytoplasm||deacetylase||cell cycle,
|c||Hst3 or Hst3p,
Hst4 or Hst4p
|III||Sirt5||SIRT5||mitochondria||demalonylase, desuccinylase and deacetylase||ammonia detoxification|
|IV||a||Sirt6||SIRT6||nucleus||Demyristoylase, depalmitoylase, ADP-ribosyl
transferase and deacetylase
Sirtuin activity is inhibited by nicotinamide, which binds to a specific receptor site, so it is thought that drugs that interfere with this binding should increase sirtuin activity. Development of new agents that would specifically block the nicotinamide-binding site could provide an avenue for development of newer agents to treat degenerative diseases such as cancer, diabetes, atherosclerosis, and gout.
Sirtuins have been proposed as a chemotherapeutic target for type II diabetes mellitus.
Preliminary studies with resveratrol, a possible SIRT1 activator, have led some scientists to speculate that resveratrol may extend lifespan. Further experiments conducted by Rafael de Cabo et al. showed that resveratrol-mimicking drugs such as SRT1720 could extend the lifespan of obese mice by 44%. Comparable molecules are now undergoing clinical trials in humans.
Cell culture research into the behaviour of the human sirtuin SIRT1 shows that it behaves like the yeast sirtuin Sir2: SIRT2 assists in the repair of DNA and regulates genes that undergo altered expression with age. Adding resveratrol to the diet of mice inhibit gene expression profiles associated with muscle aging and age-related cardiac dysfunction.
A study performed on transgenic mice overexpressing SIRT6, showed an increased lifespan of about 15% in males. The transgenic males displayed lower serum levels of insulin-like growth factor 1 (IGF1) and changes in its metabolism, which may have contributed to the increased lifespan.
- Biological immortality
- Caloric restriction
- Trichostatin A
- Histone deacetylases or HDACs
- "Structural basis for nicotinamide cleavage and ADP-ribose transfer by NAD(+)-dependent Sir2 histone/protein deacetylases". Proc. Natl. Acad. Sci. U.S.A. 101 (23): 8563–8. doi:10.1073/pnas.0401057101. PMC 423234. PMID 15150415.; Zhao K, Harshaw R, Chai X, Marmorstein R (June 2004).
- North BJ, Verdin E (2004). "Sirtuins: Sir2-related NAD-dependent protein deacetylases". Genome Biol. 5 (5): 224. doi:10.1186/gb-2004-5-5-224. PMC 416462. PMID 15128440.
- Yamamoto H, Schoonjans K, Auwerx J (August 2007). "Sirtuin functions in health and disease". Mol. Endocrinol. 21 (8): 1745–55. doi:10.1210/me.2007-0079. PMID 17456799.
- Du J, Zhou Y, Su X, Yu JJ, Khan S, Jiang H, Kim J, Woo J, Kim JH, Choi BH, He B, Chen W, Zhang S, Cerione RA, Auwerx J, Hao Q, Lin H. (2011). "Sirt5 is a NAD-dependent protein lysine demalonylase and desuccinylase.". Science 334 (6057): 806–809. doi:10.1126/science.1207861. PMC 3217313. PMID 22076378.
- Jiang H, Khan S, Wang Y, Charron G, He B, Sebastian C, Du J, Kim R, Ge E, Mostoslavsky R, Hang HC, Hao Q, Lin H. (2013). "SIRT6 regulates TNF-α secretion through hydrolysis of long-chain fatty acyl lysine.". Nature 496 (7443): 110–113. doi:10.1038/nature12038. PMC 3635073. PMID 23552949.
- EntrezGene 23410
- Preyat N, Leo O. (2013). "Sirtuin deacylases: a molecular link between metabolism and immunity.". J. Leuk. Biol. 93 (5): 669–680. doi:10.1189/jlb.1112557. PMID 23325925.
- Satoh A, Brace CS, Ben-Josef G, West T, Wozniak DF, Holtzman DM, Herzog ED, Imai S. (2010). "SIRT1 Promotes the Central Adaptive Response to Diet Restriction through Activation of the Dorsomedial and Lateral Nuclei of the Hypothalamus.". Journal of Neuroscience 30 (30): 10220–32. doi:10.1523/JNEUROSCI.1385-10.2010. PMC 2922851. PMID 20668205.
- Blander G, Guarente L (2004). "The Sir2 family of protein deacetylases". Annu. Rev. Biochem. 73 (1): 417–35. doi:10.1146/annurev.biochem.73.011303.073651. PMID 15189148.
- Wade N (2006-11-08). "The quest for a way around aging". Health & Science. International Herald Tribune. Retrieved 2008-11-30.
- "MIT researchers uncover new information about anti-aging gene". Massachusetts Institute of Technology, News Office. 2000-02-16. Retrieved 2008-11-30.
- Frye R (2000). "Phylogenetic classification of prokaryotic and eukaryotic Sir2-like proteins". Biochem Biophys Res Commun 273 (2): 793–8. doi:10.1006/bbrc.2000.3000. PMID 10873683.
- Dryden S, Nahhas F, Nowak J, Goustin A, Tainsky M (2003). "Role for human SIRT2 NAD-dependent deacetylase activity in control of mitotic exit in the cell cycle". Mol Cell Biol 23 (9): 3173–85. doi:10.1128/MCB.23.9.3173-3185.2003. PMC 153197. PMID 12697818.
- Zhao K, Chai X, Marmorstein R (March 2004). "Structure and substrate binding properties of cobB, a Sir2 homolog protein deacetylase from Escherichia coli". J. Mol. Biol. 337 (3): 731–41. doi:10.1016/j.jmb.2004.01.060. PMID 15019790.
- Schwer B, Verdin E (February 2008). "Conserved metabolic regulatory functions of sirtuins". Cell Metab. 7 (2): 104–12. doi:10.1016/j.cmet.2007.11.006. PMID 18249170.
- North B, Verdin E (2004). "Sirtuins: Sir2-related NAD-dependent protein deacetylases". Genome Biol 5 (5): 224. doi:10.1186/gb-2004-5-5-224. PMC 416462. PMID 15128440.
- Avalos JL, Bever KM, Wolberger C (March 2005). "Mechanism of sirtuin inhibition by nicotinamide: altering the NAD(+) cosubstrate specificity of a Sir2 enzyme". Mol. Cell 17 (6): 855–68. doi:10.1016/j.molcel.2005.02.022. PMID 15780941.
- Adams JD Jr, Klaidman LK (2008). "Sirtuins, Nicotinamide and Aging: A Critical Review" (PDF). Letters in Drug Design & Discovery 4 (1): 44–48. doi:10.2174/157018007778992892.
- Taylor DM, Maxwell MM, Luthi-Carter R, Kazantsev AG (September 2008). "Biological and Potential Therapeutic Roles of Sirtuin Deacetylases". Cell. Mol. Life Sci. 65 (24): 4000–18. doi:10.1007/s00018-008-8357-y. PMID 18820996.
- Milne JC, Lambert PD, Schenk S, Carney DP, Smith JJ, Gagne DJ, Jin L, Boss O, Perni RB, Vu CB, Bemis JE, Xie R, Disch JS, Ng PY, Nunes JJ, Lynch AV, Yang H, Galonek H, Israelian K, Choy W, Iffland A, Lavu S, Medvedik O, Sinclair DA, Olefsky JM, Jirousek MR, Elliott PJ, Westphal CH (November 2007). "Small molecule activators of SIRT1 as therapeutics for the treatment of type 2 diabetes". Nature 450 (7170): 712–6. doi:10.1038/nature06261. PMC 2753457. PMID 18046409.
- Wade N (2008-06-04). "New Hints Seen That Red Wine May Slow Aging". NYTimes.com. Retrieved 2008-11-30.
- Wade N (2011-08-18). "Longer Lives for Obese Mice, With Hope for Humans of All Sizes". NYTimes.com. Retrieved 2012-05-13.
- Oberdoerffer P, Michan S, McVay M, Mostoslavsky R, Vann J, Park SK, Hartlerode A, Stegmuller J, Hafner A, Loerch P, Wright SM, Mills KD, Bonni A, Yankner BA, Scully R, Prolla TA, Alt FW, Sinclair DA (November 2008). "SIRT1 redistribution on chromatin promotes genomic stability but alters gene expression during aging". Cell 135 (5): 907–18. doi:10.1016/j.cell.2008.10.025. PMC 2853975. PMID 19041753.
- Barger JL, Kayo T, Vann JM, Arias EB, Wang J, Hacker TA, Wang Y, Raederstorff D, Morrow JD, Leeuwenburgh C, Allison DB, Saupe KW, Cartee GD, Weindruch R, Prolla TA (2008). Tomé, Daniel, ed. "A low dose of dietary resveratrol partially mimics caloric restriction and retards aging parameters in mice". PLoS ONE 3 (6): e2264. doi:10.1371/journal.pone.0002264. PMC 2386967. PMID 18523577.
- Kanfi, Yariv; Naiman, Shoshana; Amir, Gail; Peshti, Victoria; Zinman, Guy; Nahum, Liat; Bar-Joseph, Ziv; Cohen, Haim Y. (2012). "The sirtuin SIRT6 regulates lifespan in male mice". Nature 483 (7388): 218–21. doi:10.1038/nature10815. ISSN 0028-0836. PMID 22367546.
- Sirtuins at the US National Library of Medicine Medical Subject Headings (MeSH)
- Rice J (2008-11-26). "How Cells Age: Parallels between mice and yeast uncover a potentially universal aging mechanism". Technology Review. Massachusetts Institute of Technology. Retrieved 2008-12-20.
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.
Sir2 family Provide feedback
This region is characteristic of Silent information regulator 2 (Sir2) proteins, or sirtuins. These are protein deacetylases that depend on nicotine adenine dinucleotide (NAD). They are found in many subcellular locations, including the nucleus, cytoplasm and mitochondria. Eukaryotic forms play in important role in the regulation of transcriptional repression. Moreover, they are involved in microtubule organisation and DNA damage repair processes .i
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR003000
The sirtuin (also known as Sir2) family is broadly conserved from bacteria to human. Yeast Sir2 (silent mating-type information regulation 2), the founding member, was first isolated as part of the SIR complex required for maintaining a modified chromatin structure at telomeres. Sir2 functions in transcriptional silencing, cell cycle progression, and chromosome stability [PUBMED:7498786]. Although most sirtuins in eukaryotic cells are located in the nucleus, others are cytoplasmic or mitochondrial.
This family is divided into five classes (I-IV and U) on the basis of a phylogenetic analysis of 60 sirtuins from a wide array of organisms [PUBMED:10873683]. Class I and class IV are further divided into three and two subgroups, respectively. The U-class sirtuins are found only in Gram-positive bacteria [PUBMED:10873683]. The S. cerevisiae genome encodes five sirtuins, Sir2 and four additional proteins termed 'homologues of sir two' (Hst1p-Hst4p) [PUBMED:7498786]. The human genome encodes seven sirtuins, with representatives from classes I-IV [PUBMED:10873683,PUBMED:15128440].
Sirtuins are responsible for a newly classified chemical reaction, NAD-dependent protein deacetylation. The final products of the reaction are the deacetylated peptide and an acetyl ADP-ribose [PUBMED:11747420]. In nuclear sirtuins this deacetylation reaction is mainly directed against histones acetylated lysines [PUBMED:11722841].
Sirtuins typically consist of two optional and highly variable N- and C- terminal domain (50-300 aa) and a conserved catalytic core domain (~250 aa). Mutagenesis experiments suggest that the N- and C-terminal regions help direct catalytic core domain to different targets [PUBMED:11722841, PUBMED:10381378].
The 3D-structure of an archaeal sirtuin in complex with NAD reveals that the protein consists of a large domain having a Rossmann fold and a small domain containing a three-stranded zinc ribbon motif. NAD is bound in a pocket between the two domains [PUBMED:11336676].
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Molecular function||NAD+ binding (GO:0070403)|
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.
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The members of this family adopt a Rossmann fold, similar to CLAN:CL0063. However, the members of this family are distinguished in that the FAD/NAD cofactor is bound in the opposite direction. In this arrangement, the adenosine moiety is found bound at the second half of the fold. In addition, the conserved GxGxxG motif found in classical NADP binding Rossmann folds is absent. Finally, another distinguishing characteristic is the formation of an internal hydrogen bond in the FAD molecule .
The clan contains the following 9 members:CO_dh DS DUF4917 ETF_alpha PNTB SIR2 SIR2_2 TPP_enzyme_M TPP_enzyme_M_2
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:||19|
|Number in full:||4093|
|Average length of the domain:||169.60 aa|
|Average identity of full alignment:||28 %|
|Average coverage of the sequence by the domain:||51.44 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 11927849 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||14|
|Download:||download the raw HMM for this family|
Weight segments by...
Change the size of the sunburst
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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.
There are 5 interactions for this family. More...
We determine these interactions using iPfam, which considers the interactions between residues in three-dimensional protein structures and maps those interactions back to Pfam families. You can find more information about the iPfam algorithm in the journal article that accompanies the website.
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 SIR2 domain has been found. There are 211 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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