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253  structures 3260  species 0  interactions 7381  sequences 108  architectures

Family: Alk_phosphatase (PF00245)

Summary: Alkaline phosphatase

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Alkaline phosphatase Edit Wikipedia article

Alkaline phosphatase
Ribbon diagram (rainbow-color, N-terminus = blue, C-terminus = red) of the dimeric structure of bacterial alkaline phosphatase.[1]
EC number3.1.3.1
CAS number9001-78-9
IntEnzIntEnz view
ExPASyNiceZyme view
MetaCycmetabolic pathway
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
Alkaline phosphatase
PDB 1alk EBI.jpg
Structure of alkaline phosphatase.[1]

Alkaline phosphatase (ALP, ALKP, ALPase, Alk Phos) (EC, or basic phosphatase,[2] is a homodimeric protein enzyme of 86 kilodaltons. Each monomer contains five cysteine residues, two zinc atoms and one magnesium atom crucial to its catalytic function, and it is optimally active at alkaline pH environments.[3][4]

ALP has the physiological role of dephosphorylating compounds. The enzyme is found across a multitude of organisms, prokaryotes and eukaryotes alike, with the same general function but in different structural forms suitable to the environment they function in. Alkaline phosphatase is found in the periplasmic space of E. coli bacteria. This enzyme is heat stable and has its maximum activity at high pH. In humans, it is found in many forms depending on its origin within the body â€“ it plays an integral role in metabolism within the liver and development within the skeleton. Due to its widespread prevalence in these areas, its concentration in the bloodstream is used by diagnosticians as a biomarker in helping determine diagnoses such as hepatitis or osteomalacia.[5]

The level of alkaline phosphatase in the blood is checked through the ALP test, which is often part of routine blood tests. The levels of this enzyme in the blood depend on factors such as age, sex, blood type.[6] Blood levels of alkaline phosphatase also increase by two to four times during pregnancy. This is a result of additional alkaline phosphatase produced by the placenta.[7] Additionally, abnormal levels of alkaline phosphatase in the blood could indicate issues relating to the liver, gall bladder or bones. Kidney tumors and infections as well as malnutrition have also shown abnormal level of alkaline phosphatase in blood.[8] Alkaline phosphatase levels in a cell can be measured through a process called "The scoring method". This is a technique used where a sample of the enzyme is extracted from the inside of blood cells and is analyzed and compared for varying enzyme activity. A blood smear is usually taken and undergoes differential centrifugation to isolate leukocytes and staining to categorize each leukocyte into specific "leukocyte alkaline phosphatase indices". This marker is designed to distinguish leukocytes and determine different enzyme activity from each sample's extent of staining.[9]


In Gram-negative bacteria, such as Escherichia coli (E. coli), alkaline phosphatase is located in the periplasmic space, external to the inner cell membrane and within the peptidoglycan portion of the cell wall. Since the periplasmic gap is more prone to environmental variation than the inner cell, alkaline phosphatase is suitably resistant to inactivation, denaturation, or degradation. This characteristic of the enzyme is uncommon to many other proteins.[10]

The precise structure and function of the four isozymes (Int in E.coli) are solely geared to supply a source of inorganic phosphate when the environment lacks this metabolite. The four enzymes are dependent upon the location of the tissue expression. The four sites of tissue expression are the Intestinal AlP, Placental ALP, Germ Cell ALP and Liver/Bone/Kidney ALP.[11] The inorganic phosphates produced by these isozymes are then bound to carrier proteins which deliver the inorganic phosphates to a specific high-affinity transport system, known as the Pst system, which transports phosphate across the cytoplasmic membrane.[12]

While the outer membrane of E. coli contains porins that are permeable to phosphorylated compounds, the inner membrane does not. Then, an issue arises in how to transport such compounds across the inner membrane and into the cytosol. Surely, with the strong anionic charge of phosphate groups along with the remainder of the compound they are very much immiscible in the nonpolar region of the bilayer. The solution arises in cleaving the phosphate group away from the compound via ALP. In effect, along with the concomitant compound the phosphate was bound to, this enzyme yields pure inorganic phosphate which can be ultimately targeted by the phosphate-specific transport system (Pst system)[13] for translocation into the cytosol.[14] As such, the main purpose of dephosphorylation by alkaline phosphatase is to increase the rate of diffusion of the molecules into the cells and inhibit them from diffusing out.[15]

Alkaline phosphatase is a zinc-containing dimeric enzyme with the MW: 86,000 Da, each subunit containing 429 amino acids with four cysteine residues linking the two subunits.[16] Alkaline phosphatase contains four Zn ions and two Mg ions, with Zn occupying active sites A and B, and Mg occupying site C, so the fully active native alkaline phosphatase is referred to as (ZnAZnBMgC)2 enzyme. The mechanism of action of alkaline phosphatase involves the geometric coordination of the substrate between the Zn ions in the active sites, whereas the Mg site doesn't appear to be close enough to directly partake in the hydrolysis mechanism, however, it may contribute to the shape of the electrostatic potential around the active center.[16] Alkaline phosphatase has a Km of 8.4 x 10−4.[17]

Alkaline phosphatase in E. coli is uncommonly soluble and active within elevated temperature conditions such as 80 Â°C. Due to the kinetic energy induced by this temperature the weak hydrogen bonds and hydrophobic interactions of common proteins become degraded and therefore coalesce and precipitate. However, upon dimerization of ALP the bonds maintaining its secondary and tertiary structures are effectively buried such that they are not affected as much at this temperature. Furthermore, even at more elevated temperatures such as 90 Â°C ALP has the uncommon characteristic of reverse denaturation. Due to this, while ALP ultimately denatures at about 90 Â°C it has the added ability to accurately reform its bonds and return to its original structure and function once cooled back down.[10]

Alkaline phosphatase in E. coli is located in the periplasmic space and can thus be released using techniques that weaken the cell wall and release the protein. Due to the location of the enzyme, and the protein layout of the enzyme, the enzyme is in solution with a smaller amount of proteins than there are in another portion of the cell. [18] The proteins' heat stability can also be taken advantage of when isolating this enzyme (through heat denaturation). In addition, alkaline phosphatase can be assayed using p-Nitrophenyl phosphate. A reaction where alkaline phosphatase dephosphorylates the non-specific substrate, p-Nitrophenyl phosphate in order to produce p-Nitrophenol(PNP) and inorganic phosphate. PNP's yellow color, and its λmax at 410 allows spectrophotometry to determine important information about enzymatic activity.[19] Some complexities of bacterial regulation and metabolism suggest that other, more subtle, purposes for the enzyme may also play a role for the cell. In the laboratory, however, mutant Escherichia coli lacking alkaline phosphatase survive quite well, as do mutants unable to shut off alkaline phosphatase production.[20]

The optimal pH for the activity of the E. coli enzyme is 8.0[21] while the bovine enzyme optimum pH is slightly higher at 8.5.[22] Alkaline phosphatase accounts for 6% of all proteins in derepressed cells.[17]

Use in research

By changing the amino acids of the wild-type alkaline phosphatase enzyme produced by Escherichia coli, a mutant alkaline phosphatase is created which not only has a 36-fold increase in enzyme activity, but also retains thermal stability.[23] Typical uses in the lab for alkaline phosphatases include removing phosphate monoesters to prevent self-ligation, which is undesirable during plasmid DNA cloning.[24]

Common alkaline phosphatases used in research include:

  • Shrimp alkaline phosphatase (SAP), from a species of Arctic shrimp (Pandalus borealis). This phosphatase is easily inactivated by heat, a useful feature in some applications.
  • Calf-intestinal alkaline phosphatase (CIP)
  • Placental alkaline phosphatase (PLAP) and its C terminally truncated version that lacks the last 24 amino acids (constituting the domain that targets for GPI membrane anchoring) – the secreted alkaline phosphatase (SEAP). It presents certain characteristics like heat stability, substrate specificity, and resistance to chemical inactivation.[25]
  • Human-intestinal alkaline phosphatase. The human body has multiple types of alkaline phosphatase present, which are determined by a minimum of three gene loci. Each one of these three loci controls a different kind of alkaline phosphatase isozyme. However, the development of this enzyme can be strictly regulated by other factors such as thermostability, electrophoresis, inhibition, or immunology.[26]

Human-intestinal ALPase shows around 80% homology with bovine intestinal ALPase, which holds true their shared evolutionary origins. That same bovine enzyme has more than 70% homology with human placental enzyme. However, the human intestinal enzyme and the placental enzyme only share 20% homology despite their structural similarities.[27]

Alkaline phosphatase has become a useful tool in molecular biology laboratories, since DNA normally possesses phosphate groups on the 5' end. Removing these phosphates prevents the DNA from ligating (the 5' end attaching to the 3' end), thereby keeping DNA molecules linear until the next step of the process for which they are being prepared; also, removal of the phosphate groups allows radiolabeling (replacement by radioactive phosphate groups) in order to measure the presence of the labeled DNA through further steps in the process or experiment. For these purposes, the alkaline phosphatase from shrimp is the most useful, as it is the easiest to inactivate once it has done its job.

Another important use of alkaline phosphatase is as a label for enzyme immunoassays.

Because undifferentiated pluripotent stem cells have elevated levels of alkaline phosphatase on their cell membrane, therefore alkaline phosphatase staining is used to detect these cells and to test pluripotency (i.e., embryonic stem cells or embryonal carcinoma cells).[28]

There is a positive correlation between serum bone alkaline phosphatase (B-ALP) levels and bone formation in humans, although its use as a biomarker in clinical practice is not recommended.[29]

Ongoing research

Current researchers are looking into the increase of tumor necrosis factor-α and its direct effect on the expression of alkaline phosphatase in vascular smooth muscle cells as well as how alkaline phosphatase (AP) affects the inflammatory responses and may play a direct role in preventing organ damage.[30]

  • Alkaline phosphatase (AP) affects the inflammatory responses in patients with Chronic kidney disease and is directly associated with Erythropoiesis stimulating agent resistant anemia.[31]
  • Intestinal alkaline phosphatase (IAP) and the mechanism it uses to regulate pH and ATP hydrolysis in rat duodenum.[32]
  • Testing the effectiveness of the inhibitor and its impact on IAP in acute intestinal inflammation as well as explore the molecular mechanisms of IAP in "ameliorating intestinal permeability."[33]

Dairy industry

Alkaline phosphatase is commonly used in the dairy industry as an indicator of successful pasteurization. This is because the most heat stable bacterium found in milk, Mycobacterium paratuberculosis, is destroyed by temperatures lower than those required to denature ALP. Therefore, ALP presence is ideal for indicating failed pasteurization.[34][35]

Pasteurization verification is typically performed by measuring the fluorescence of a solution which becomes fluorescent when exposed to active ALP. Fluorimetry assays are required by milk producers in the UK to prove alkaline phosphatase has been denatured,[36] as p-Nitrophenylphosphate tests are not considered accurate enough to meet health standards.

Alternatively the colour change of a para-Nitrophenylphosphate substrate in a buffered solution (Aschaffenburg Mullen Test) can be used.[37] Raw milk would typically produce a yellow colouration within a couple of minutes, whereas properly pasteurised milk should show no change. There are exceptions to this, as in the case of heat-stable alkaline phophatases produced by some bacteria, but these bacteria should not be present in milk.


All mammalian alkaline phosphatase isoenzymes except placental (PALP and SEAP) are inhibited by homoarginine, and, in similar manner, all except the intestinal and placental ones are blocked by levamisole.[38] Phosphate is another inhibitor which competitively inhibits alkaline phosphatase.[39]

Another known example of an alkaline phosphatase inhibitor is [(4-Nitrophenyl)methyl]phosphonic acid.[40]

In metal contaminated soil, alkaline phosphatase are inhibited by Cd (Cadmium). In addition, temperature enhances the inhibition of Cd on ALP activity, which is shown in the increasing values of Km.[41]



In humans, alkaline phosphatase is present in all tissues throughout the body, but is particularly concentrated in the liver, bile duct, kidney, bone, intestinal mucosa and placenta. In the serum, two types of alkaline phosphatase isozymes predominate: skeletal and liver. During childhood the majority of alkaline phosphatase are of skeletal origin.[42] Humans and most other mammals contain the following alkaline phosphatase isozymes:

  • ALPI – intestinal (molecular weight of 150 kDa)
  • ALPL – tissue-nonspecific (expressed mainly in liver, bone, and kidney)
  • ALPP – placental (Regan isozyme)
  • GCAP – germ cell

Four genes encode the four isozymes. The gene for tissue-nonspecific alkaline phosphatase is located on chromosome 1, and the genes for the other three isoforms are located on chromosome 2.[4]

Intestinal alkaline phosphatase

Intestinal alkaline phosphatase is secreted by enterocytes, and seems to play a pivotal role in intestinal homeostasis and protection[43][44] as well as in mediation of inflammation[45] via repression of the downstream Toll-like receptor (TLR)-4-dependent and MyD88-dependent inflammatory cascade. It dephosphorylates toxic/inflammatory microbial ligands like lipopolysaccharides, unmethylated cytosine-guanine dinucleotides, flagellin, and extracellular nucleotides such as uridine diphosphate or ATP. Thus, altered IAP expression has been implicated in chronic inflammatory diseases such as inflammatory bowel disease (IBD).[46][47] It also seems to regulate lipid absorption[48] and bicarbonate secretion[49] in the duodenal mucosa, which regulates the surface pH.

In cancer cells

Studies show that the alkaline phosphatase protein found in cancer cells is similar to that found in nonmalignant body tissues and that the protein originates from the same gene in both. One study compared the enzymes of liver metastases of giant-cell lung carcinoma and nonmalignant placental cells. The two were similar in NH2-terminal sequence, peptide map, subunit molecular weight, and isoelectronic point.[50]

In a different study in which scientists examined alkaline phosphatase protein presence in a human colon cancer cell line, also known as HT-29, results showed that the enzyme activity was similar to that of the non-malignant intestinal type. However, this study revealed that without the influence of sodium butyrate, alkaline phosphatase activity is fairly low in cancer cells.[51] A study based on sodium butyrate effects on cancer cells conveys that it has an effect on androgen receptor co-regulator expression, transcription activity, and also on histone acetylation in cancer cells.[52] This explains why the addition of sodium butyrate show increased activity of alkaline phosphatase in the cancer cells of the human colon.[51] In addition, this further supports the theory that alkaline phosphatase enzyme activity is actually present in cancer cells.

In another study, choriocarcinoma cells were grown in the presence of 5-bromo-2’-deoxyuridine (BrdUrd) and results conveyed a 30- to 40- fold increase in alkaline phosphatase activity. This procedure of enhancing the activity of the enzyme is known as enzyme induction. The evidence shows that there is in fact activity of alkaline phosphatase in tumor cells, but it is minimal and needs to be enhanced. Results from this study further indicate that activities of this enzyme vary among the different choriocarcinoma cell lines and that the activity of the alkaline phosphatase protein in these cells is lower than in the non-malignant placenta cells.[53][54] but levels are significantly higher in children and pregnant women. Blood tests should always be interpreted using the reference range from the laboratory that performed the test. High ALP levels can occur if the bile ducts are obstructed.[55]

Also, ALP increases if there is active bone formation occurring, as ALP is a byproduct of osteoblast activity (such as the case in Paget's disease of bone).

Levels are also elevated in people with untreated coeliac disease.[56] Lowered levels of ALP are less common than elevated levels. The source of elevated ALP levels can be deduced by obtaining serum levels of gamma glutamyltransferase (GGT). Concomitant increases of ALP with GGT should raise the suspicion of hepatobiliary disease.[57]

Some diseases do not affect the levels of alkaline phosphatase, for example, hepatitis C. A high level of this enzyme does not reflect any damage in the liver, even though high alkaline phosphatase levels may result from a blockage of flow in the biliary tract or an increase in the pressure of the liver.[58]

Elevated levels

If it is unclear why alkaline phosphatase is elevated, isoenzyme studies using electrophoresis can confirm the source of the ALP. Skelphosphatase (which is localized in osteoblasts and extracellular layers of newly synthesized matrix) is released into circulation by a yet unclear mechanism.[59] Placental alkaline phosphatase is elevated in seminomas[60] and active forms of rickets, as well as in the following diseases and conditions:[61]

Lowered levels

The following conditions or diseases may lead to reduced levels of alkaline phosphatase:

In addition, oral contraceptives have been demonstrated to reduce alkaline phosphatase.[63]

Prognostic uses

Measuring alkaline phosphatase (along with prostate specific antigen) during, and after six months of hormone treated metastatic prostate cancer was shown to predict the survival of patients.[64]

Leukocyte alkaline phosphatase

Leukocyte alkaline phosphatase (LAP) is found within mature white blood cells. White blood cell levels of LAP can help in the diagnosis of certain conditions.

Structure and properties

Alkaline phosphatase is homodimeric enzyme, meaning it is formed with two molecules. Three metal ions, two Zn and one Mg, are contained in the catalytic sites, and both types are crucial for enzymatic activity to occur. The enzymes catalyze the hydrolysis of monoesters in phosphoric acid which can additionally catalyze a transphosphorylation reaction with large concentrations of phosphate acceptors. While the main features of the catalytic mechanism and activity are conserved between mammalian and bacterial alkaline phosphate, mammalian alkaline phosphatase has higher a specific activity and Km values thus a lower affinity, more alkaline pH optimum, lower heat stability, and are typically membrane bound and are inhibited by l-amino acids and peptides via a means of uncompetitive mechanism. These properties noticeably differ between different mammalian alkaline phosphatase isozymes and therefore showcase a difference in in vivo functions.

Alkaline phosphatase has homology in a large number of other enzymes and composes part of a superfamily of enzymes with several overlapping catalytic aspects and substrate traits. This explains why most salient structural features of mammalian alkaline are the way they are and reference their substrate specificity and homology to other members of the nucleoside pyrophosphatase/phosphodiesterase family of isozyme.[4] Research has shown a relationship between members of the alkaline phosphatase family with aryl sulfatases. The similarities in structure indicate that these two enzyme families came from a common ancestor. Further analysis has linked alkaline phosphates and aryl sulfatases to a larger superfamily. Some of the common genes found in this superfamily, are ones that encode phosphodiesterases as well as autotoxin.[66]

See also


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  48. ^ Narisawa S, Huang L, Iwasaki A, Hasegawa H, Alpers DH, Millán JL (November 2003). "Accelerated fat absorption in intestinal alkaline phosphatase knockout mice". Molecular and Cellular Biology. 23 (21): 7525–30. doi:10.1128/mcb.23.21.7525-7530.2003. PMC 207564. PMID 14560000.
  49. ^ Akiba Y, Mizumori M, Guth PH, Engel E, Kaunitz JD (December 2007). "Duodenal brush border intestinal alkaline phosphatase activity affects bicarbonate secretion in rats". American Journal of Physiology. Gastrointestinal and Liver Physiology. 293 (6): G1223–33. doi:10.1152/ajpgi.00313.2007. PMID 17916646.
  50. ^ Greene PJ, Sussman HH (October 1973). "Structural comparison of ectopic and normal placental alkaline phosphatase". Proceedings of the National Academy of Sciences of the United States of America. 70 (10): 2936–40. Bibcode:1973PNAS...70.2936G. doi:10.1073/pnas.70.10.2936. JSTOR 63137. PMC 427142. PMID 4517947.
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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.

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This tab holds annotation information from the InterPro database.

InterPro entry IPR001952

This entry represents alkaline phosphatases ( EC ) (ALP), which act as non-specific phosphomonoesterases to hydrolyse phosphate esters, optimally at high pH. The reaction mechanism involves the attack of a serine alkoxide on a phosphorus of the substrate to form a transient covalent enzyme-phosphate complex, followed by the hydrolysis of the serine phosphate. Alkaline phosphatases are found in all kingdoms of life, with the exception of some plants. Alkaline phosphatases are metalloenzymes that exist as a dimer, each monomer binding metal ions. The metal ions they carry can differ, although zinc and magnesium are the most common. For example, Escherichia coli alkaline phosphatase (encoded by phoA) requires the presence of two zinc ions bound at the M1 and M2 metal sites, and one magnesium ion bound at the M3 site [ PUBMED:15938627 ]. However, alkaline phosphatases from Thermotoga maritima and Bacillus subtilis require cobalt for maximal activity [ PUBMED:11910033 ].

In mammals, there are four alkaline phosphatase isozymes: placental, placental-like (germ cell), intestinal and tissue-nonspecific (liver/bone/kidney). All four isozymes are anchored to the outer surface of the plasma membrane by a covalently attached glycosylphosphatidylinositol (GPI) anchor [ PUBMED:17520090 ]. Human alkaline phosphatases have four metal binding sites: two for zinc, one for magnesium, and one for calcium ion. Placental alkaline phosphatase (ALPP or PLAP) is highly polymorphic, with at least three common alleles [ PUBMED:11124260 ]. Its activity is down-regulated by a number of effectors such as l-phenylalanine, 5'-AMP, and by p-nitrophenyl-phosphonate (PNPPate) [ PUBMED:15946677 ]. The placental-like isozyme (ALPPL or PLAP-like) is elevated in germ cell tumours. The intestinal isozyme (ALPI or IAP) has the ability to detoxify lipopolysaccharide and prevent bacterial invasion across the gut mucosal barrier [ PUBMED:18292227 ]. The tissue-nonspecific isozyme (ALPL) is, and may play a role in skeletal mineralisation. Defects in ALPL are a cause of hypophosphatasia, including infantile-type (OMIM:241500), childhood-type (OMIM:241510) and adult-type (OMIM:146300). Hhypophosphatasia is an inherited metabolic bone disease characterised by defective skeletal mineralisation [ PUBMED:17719863 ].

This entry also contains the related enzyme streptomycin-6-phosphate phosphatase ( EC ) (encoded by strK) from Streptomyces species. This enzyme is involved in the synthesis of the antibiotic streptomycin, specifically cleaving both streptomycin-6-phosphate and, more slowly, streptomycin-3-phosphate [ PUBMED:1654502 ].

Gene Ontology

The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.

Domain organisation

Below is a listing of the unique domain organisations or architectures in which this domain is found. More...

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Pfam Clan

This family is a member of clan Alk_phosphatase (CL0088), which has the following description:

The members of this clan all share a common structure of their catalytic domains, which contain conserved metal binding residues [1].

The clan contains the following 10 members:

Alk_phosphatase DUF1501 DUF229 DUF4976 Metalloenzyme PglZ Phosphodiest Phosphoesterase Sulfatase Sulfatase_C


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 and the UniProtKB sequence database. More...

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You can also download a FASTA format file containing the full-length sequences for all sequences in the full alignment.

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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...


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.

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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.

Curation View help on the curation process

Seed source: Prosite
Previous IDs: alk_phosphatase;
Type: Domain
Sequence Ontology: SO:0000417
Author: Finn RD
Number in seed: 9
Number in full: 7381
Average length of the domain: 358.30 aa
Average identity of full alignment: 27 %
Average coverage of the sequence by the domain: 77.65 %

HMM information View help on HMM parameters

HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 57096847 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 24.4 24.4
Trusted cut-off 24.5 24.4
Noise cut-off 24.2 24.3
Model length: 418
Family (HMM) version: 22
Download: download the raw HMM for this family

Species distribution

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Archea Archea Eukaryota Eukaryota
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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 adjacent tab. More...

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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 Alk_phosphatase domain has been found. There are 253 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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