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Trypsin Edit Wikipedia article
|SCOPe||1c2g / SUPFAM|
Trypsin (EC 22.214.171.124) is a serine protease from the PA clan superfamily, found in the digestive system of many vertebrates, where it hydrolyzes proteins. Trypsin is formed in the small intestine when its proenzyme form, the trypsinogen produced by the pancreas, is activated. Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine. It is used for numerous biotechnological processes. The process is commonly referred to as trypsin proteolysis or trypsinisation, and proteins that have been digested/treated with trypsin are said to have been trypsinized. Trypsin was discovered in 1876 by Wilhelm KÃ¼hne and was named from the Ancient Greek word for rubbing since it was first isolated by rubbing the pancreas with glycerin.
In the duodenum, trypsin catalyzes the hydrolysis of peptide bonds, breaking down proteins into smaller peptides. The peptide products are then further hydrolyzed into amino acids via other proteases, rendering them available for absorption into the blood stream. Tryptic digestion is a necessary step in protein absorption, as proteins are generally too large to be absorbed through the lining of the small intestine.
Trypsin is produced as the inactive zymogen trypsinogen in the pancreas. When the pancreas is stimulated by cholecystokinin, it is then secreted into the first part of the small intestine (the duodenum) via the pancreatic duct. Once in the small intestine, the enzyme enteropeptidase activates trypsinogen into trypsin by proteolytic cleavage.
The enzymatic mechanism is similar to that of other serine proteases. These enzymes contain a catalytic triad consisting of histidine-57, aspartate-102, and serine-195. This catalytic triad was formerly called a charge relay system, implying the abstraction of protons from serine to histidine and from histidine to aspartate, but owing to evidence provided by NMR that the resultant alkoxide form of serine would have a much stronger pull on the proton than does the imidazole ring of histidine, current thinking holds instead that serine and histidine each have effectively equal share of the proton, forming short low-barrier hydrogen bonds therewith. By these means, the nucleophilicity of the active site serine is increased, facilitating its attack on the amide carbon during proteolysis. The enzymatic reaction that trypsin catalyzes is thermodynamically favorable, but requires significant activation energy (it is "kinetically unfavorable"). In addition, trypsin contains an "oxyanion hole" formed by the backbone amide hydrogen atoms of Gly-193 and Ser-195, which through hydrogen bonding stabilize the negative charge which accumulates on the amide oxygen after nucleophilic attack on the planar amide carbon by the serine oxygen causes that carbon to assume a tetrahedral geometry. Such stabilisation of this tetrahedral intermediate helps to reduce the energy barrier of its formation and is concomitant with a lowering of the free energy of the transition state. Preferential binding of the transition state is a key feature of enzyme chemistry.
The aspartate residue (Asp 189) located in the catalytic pocket (S1) of trypsin is responsible for attracting and stabilizing positively charged lysine and/or arginine, and is, thus, responsible for the specificity of the enzyme. This means that trypsin predominantly cleaves proteins at the carboxyl side (or "C-terminal side") of the amino acids lysine and arginine except when either is bound to a C-terminal proline, although large-scale mass spectrometry data suggest cleavage occurs even with proline. Trypsin is considered an endopeptidase, i.e., the cleavage occurs within the polypeptide chain rather than at the terminal amino acids located at the ends of polypeptides.
Human trypsin has an optimal operating temperature of about 37 Â°C. In contrast, the Atlantic cod has several types of trypsins for the poikilotherm fish to survive at different body temperatures. Cod trypsins include trypsin I with an activity range of 4 to 65 Â°C (40 to 150 Â°F) and maximal activity at 55 Â°C (130 Â°F), as well as trypsin Y with a range of 2 to 30 Â°C (36 to 86 Â°F) and a maximal activity at 21 Â°C (70 Â°F).
As a protein, trypsin has various molecular weights depending on the source. For example, a molecular weight of 23.3 kDa is reported for trypsin from bovine and porcine sources.
The activity of trypsin is not affected by the enzyme inhibitor tosyl phenylalanyl chloromethyl ketone, TPCK, which deactivates chymotrypsin. This is important because, in some applications, like mass spectrometry, the specificity of cleavage is important.
Trypsin should be stored at very cold temperatures (between −20 and −80 Â°C) to prevent autolysis, which may also be impeded by storage of trypsin at pH 3 or by using trypsin modified by reductive methylation. When the pH is adjusted back to pH 8, activity returns.
These human genes encode proteins with trypsin enzymatic activity:
Other isoforms of trypsin may also be found in other organisms.
Activation of trypsin from proteolytic cleavage of trypsinogen in the pancreas can lead to a series of events that cause pancreatic self-digestion, resulting in pancreatitis. One consequence of the autosomal recessive disease cystic fibrosis is a deficiency in transport of trypsin and other digestive enzymes from the pancreas. This leads to the disorder termed meconium ileus, which involves intestinal obstruction (ileus) due to overly thick meconium, which is normally broken down by trypsin and other proteases, then passed in feces.
Trypsin is available in high quantity in pancreases, and can be purified rather easily. Hence, it has been used widely in various biotechnological processes.
In a tissue culture lab, trypsin is used to resuspend cells adherent to the cell culture dish wall during the process of harvesting cells. Some cell types adhere to the sides and bottom of a dish when cultivated in vitro. Trypsin is used to cleave proteins holding the cultured cells to the dish, so that the cells can be removed from the plates.
Trypsin can also be used to dissociate dissected cells (for example, prior to cell fixing and sorting).
Trypsin can be used to break down casein in breast milk. If trypsin is added to a solution of milk powder, the breakdown of casein causes the milk to become translucent. The rate of reaction can be measured by using the amount of time needed for the milk to turn translucent.
Trypsin is commonly used in biological research during proteomics experiments to digest proteins into peptides for mass spectrometry analysis, e.g. in-gel digestion. Trypsin is particularly suited for this, since it has a very well defined specificity, as it hydrolyzes only the peptide bonds in which the carbonyl group is contributed either by an arginine or lysine residue.
Trypsin can also be used to dissolve blood clots in its microbial form and treat inflammation in its pancreatic form.
In veterinary medicine, typsin is an ingredient in wound spray products, such as Debrisol, to dissolve dead tissue and pus in wounds in horses, cattle, dogs, and cats.
Commercial protease preparations usually consist of a mixture of various protease enzymes that often includes trypsin. These preparations are widely used in food processing:
- as a baking enzyme to improve the workability of dough
- in the extraction of seasonings and flavorings from vegetable or animal proteins and in the manufacture of sauces
- to control aroma formation in cheese and milk products
- to improve the texture of fish products
- to tenderize meat
- during cold stabilization of beer
- in the production of hypoallergenic food where proteases break down specific allergenic proteins into nonallergenic peptides, for example, proteases are used to produce hypoallergenic baby food from cow's milk, thereby diminishing the risk of babies developing milk allergies.
To prevent the action of active trypsin in the pancreas, which can be highly damaging, inhibitors such as BPTI and SPINK1 in the pancreas and Î±1-antitrypsin in the serum are present as part of the defense against its inappropriate activation. Any trypsin prematurely formed from the inactive trypsinogen is then bound by the inhibitor. The protein-protein interaction between trypsin and its inhibitors is one of the tightest bound, and trypsin is bound by some of its pancreatic inhibitors nearly irreversibly. In contrast with nearly all known protein assemblies, some complexes of trypsin bound by its inhibitors do not readily dissociate after treatment with 8M urea.
- "Trypsin specificity as elucidated by LIE calculations, X-ray structures, and association constant measurements". Protein Science. 13 (4): 1056â€“70. doi:10.1110/ps.03498604. PMC 2280040. PMID 15044735. ; Leiros HK, Brandsdal BO, Andersen OA, Os V, Leiros I, Helland R, Otlewski J, Willassen NP, SmalÃ¥s AO (April 2004).
- Rawlings ND, Barrett AJ (1994). Families of serine peptidases. Methods in Enzymology. 244. pp. 19â€“61. doi:10.1016/0076-6879(94)44004-2. ISBN 978-0-12-182145-6. PMID 7845208.
- The German physiologist Wilhelm KÃ¼hne (1837-1900) discovered trypsin in 1876. See: KÃ¼hne W (1877). "Ãœber das Trypsin (Enzym des Pankreas)". Verhandlungen des naturhistorisch-medicinischen Vereins zu Heidelberg. new series. 1 (3): 194â€“198.
- Engelking LR (2015-01-01). Textbook of Veterinary Physiological Chemistry (Third ed.). Boston: Academic Press. pp. 39â€“44. ISBN 9780123919090.
- "Verhandlungen des Naturhistorisch-medizinischen Vereins zu Heidelberg". archive.org. Retrieved 2017-04-24.
- "Digestion of Proteins". intranet.tdmu.edu.ua. Retrieved 2017-04-24.
- PolgÃ¡r L (October 2005). "The catalytic triad of serine peptidases". Cellular and Molecular Life Sciences. 62 (19â€“20): 2161â€“72. doi:10.1007/s00018-005-5160-x. PMID 16003488.
- Voet D, Voet JG (2011). Biochemistry (4th ed.). Hoboken, NJ: John Wiley & Sons. ISBN 9780470570951. OCLC 690489261.
- "Sequencing Grade Modified Trypsin" (PDF). www.promega.com. 2007-04-01. Retrieved 2009-02-08.
- Rodriguez J, Gupta N, Smith RD, Pevzner PA (January 2008). "Does trypsin cut before proline?" (PDF). Journal of Proteome Research. 7 (1): 300â€“5. CiteSeerX 10.1.1.163.4052. doi:10.1021/pr0705035. PMID 18067249.
- GudmundsdÃ³ttir A, PÃ¡lsdÃ³ttir HM (2005). "Atlantic cod trypsins: from basic research to practical applications". Marine Biotechnology. 7 (2): 77â€“88. doi:10.1007/s10126-004-0061-9. PMID 15759084.
- Noone PG, Zhou Z, Silverman LM, Jowell PS, Knowles MR, Cohn JA (December 2001). "Cystic fibrosis gene mutations and pancreatitis risk: relation to epithelial ion transport and trypsin inhibitor gene mutations". Gastroenterology. 121 (6): 1310â€“9. doi:10.1053/gast.2001.29673. PMID 11729110.
- "Trypsin-EDTA (0.25%)". Stem Cell Technologies. Retrieved 2012-02-23.
- "Debrisol". drugs.com.
- "Protease - GMO Database". GMO Compass. European Union. 2010-07-10. Archived from the original on 2015-02-24. Retrieved 2012-01-01.
- Voet D, Voet JG (1995). Biochemistry (2nd ed.). John Wiley & Sons. pp. 396â€“400. ISBN 978-0-471-58651-7.
- Levilliers N, PÃ©ron M, Arrio B, Pudles J (October 1970). "On the mechanism of action of proteolytic inhibitors. IV. Effect of 8 M urea on the stability of trypsin in trypsin-inhibitor complexes". Archives of Biochemistry and Biophysics. 140 (2): 474â€“83. doi:10.1016/0003-9861(70)90091-3. PMID 5528741.
- Brosens JJ, Salker MS, Teklenburg G, Nautiyal J, Salter S, Lucas ES, Steel JH, Christian M, Chan YW, Boomsma CM, Moore JD, Hartshorne GM, Sucurovic S, Mulac-Jericevic B, Heijnen CJ, Quenby S, Groot Koerkamp MJ, Holstege FCP, Shmygol A, Macklon NS (6 February 2014). "Uterine Selection of Human Embryos at Implantation". Scientific Reports. 4. doi:10.1038/srep03894. PMC 3915549. PMID 24503642. Article number 3894. Retrieved 15 March 2019. Article on the role of trypsin in the implantation of human embryos.
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.
Trypsin Provide feedback
No Pfam abstract.
Sprang S, Standing T, Fletterick RJ, Stroud RM, Finer-Moore J, Xuong NH, Hamlin R, Rutter WJ, Craik CS; , Science 1987;237:905-909.: The Three Dimensional Structure of Asnl02 Trypsin: Role of Aspl02 in Serine Protease Catalysis PUBMED:3112942 EPMC:3112942
Internal database links
|SCOOP:||DUF1986 DUF316 Peptidase_S29 Peptidase_S32 Peptidase_S46 Peptidase_S7 Trypsin_2|
|Similarity to PfamA using HHSearch:||DUF316 DUF1986 Trypsin_2|
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR001254
This entry represents the active-site-containing domain found in the trypsin family members. The catalytic activity of the serine proteases from the trypsin family is provided by a charge relay system involving an aspartic acid residue hydrogen-bonded to a histidine, which itself is hydrogen-bonded to a serine. The sequences in the vicinity of the active site serine and histidine residues are well conserved in this family of proteases [PUBMED:3136396]. A partial list of proteases known to belong to the trypsin family is shown below.
- Blood coagulation factors VII, IX, X, XI and XII, thrombin, plasminogen, and protein C.
- Cathepsin G.
- Complement components C1r, C1s, C2, and complement factors B, D and I.
- Complement-activating component of RA-reactive factor.
- Cytotoxic cell proteases (granzymes A to H).
- Duodenase I.
- Elastases 1, 2, 3A, 3B (protease E), leukocyte (medullasin).
- Enterokinase (EC 126.96.36.199) (enteropeptidase).
- Hepatocyte growth factor activator.
- Glandular (tissue) kallikreins (including EGF-binding protein types A, B, and C, NGF-gamma chain, gamma-renin, prostate specific antigen (PSA) and tonin).
- Plasma kallikrein.
- Mast cell proteases (MCP) 1 (chymase) to 8.
- Myeloblastin (proteinase 3) (Wegener's autoantigen).
- Plasminogen activators (urokinase-type, and tissue-type).
- Trypsins I, II, III, and IV.
All the above proteins belong to family S1 in the classification of peptidases [PUBMED:7845208] and originate from eukaryotic species. It should be noted that bacterial proteases that belong to family S2A are similar enough in the regions of the active site residues that they can be picked up by the same patterns. These proteases are listed below.
- Achromobacter lyticus protease I.
- Lysobacter alpha-lytic protease.
- Streptogrisin A and B (Streptomyces proteases A and B).
- Streptomyces griseus glutamyl endopeptidase II.
- Streptomyces fradiae proteases 1 and 2.
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Molecular function||serine-type endopeptidase activity (GO:0004252)|
|Biological process||proteolysis (GO:0006508)|
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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This clan contains a diverse set of peptidases with the trypsin fold.
The clan contains the following 26 members:DUF1986 DUF31 DUF316 Peptidase_C107 Peptidase_C24 Peptidase_C3 Peptidase_C30 Peptidase_C37 Peptidase_C3G Peptidase_C4 Peptidase_C62 Peptidase_S29 Peptidase_S3 Peptidase_S30 Peptidase_S31 Peptidase_S32 Peptidase_S39 Peptidase_S46 Peptidase_S55 Peptidase_S6 Peptidase_S64 Peptidase_S7 Peptidase_S76 Pico_P2A Trypsin Trypsin_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.
|Seed source:||SCOP and Prosite|
|Author:||Lutfiyya LL, Sonnhammer ELL|
|Number in seed:||70|
|Number in full:||55824|
|Average length of the domain:||206.30 aa|
|Average identity of full alignment:||25 %|
|Average coverage of the sequence by the domain:||55.68 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 47079205 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||27|
|Download:||download the raw HMM for this family|
Weight segments by...
Change the size of the sunburst
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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 67 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 Trypsin domain has been found. There are 2959 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 sequence.
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