Alpha-1 antitrypsin

SERPINA1
Available structures PDB Ortholog search: PDBe RCSB List of PDB id codes 1ATU, 1D5S, 1EZX, 1HP7, 1IZ2, 1KCT, 1OO8, 1OPH, 1PSI, 1QLP, 1QMB, 2D26, 2QUG, 3CWL, 3CWM, 3DRM, 3DRU, 3NDD, 3NDF, 3NE4, 3T1P, 7API, 8API, 9API, 4PYW, 5IO1
Identifiers
Aliases	SERPINA1, A1A, A1AT, AAT, PI, PI1, PRO2275, alpha1AT, serpin family A member 1, nNIF
External IDs	OMIM: 107400 MGI: 891968 HomoloGene: 20103 GeneCards: SERPINA1
Gene location (Human) Chr. Chromosome 14 (human)[1] Band 14q32.13 Start 94,376,747 bp[1] End 94,390,693 bp[1]
Gene location (Mouse) Chr. Chromosome 12 (mouse)[2] Band 12 E|12 52.98 cM Start 103,729,853 bp[2] End 103,739,851 bp[2]
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in right lobe of liver blood islet of Langerhans monocyte gallbladder duodenum kidney upper lobe of left lung appendix right lung Top expressed in proximal tubule hepatobiliary system liver kidney Hindgut lobe of liver yolk sac midgut islet of Langerhans white adipose tissue More reference expression data BioGPS More reference expression data
Gene ontology Molecular function peptidase inhibitor activity protease binding protein binding identical protein binding serine-type endopeptidase inhibitor activity Cellular component Golgi apparatus endoplasmic reticulum lumen COPII-coated ER to Golgi transport vesicle extracellular exosome endoplasmic reticulum-Golgi intermediate compartment membrane platelet alpha granule lumen extracellular region endoplasmic reticulum Golgi membrane extracellular space intracellular membrane-bounded organelle ficolin-1-rich granule lumen collagen-containing extracellular matrix Biological process hemostasis negative regulation of peptidase activity platelet degranulation endoplasmic reticulum to Golgi vesicle-mediated transport COPII vesicle coating acute-phase response negative regulation of endopeptidase activity blood coagulation neutrophil degranulation post-translational protein modification Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 5265 20703 Ensembl ENSG00000197249 ENSG00000277377 ENSMUSG00000071177 UniProt P01009 Q00897 P07758 RefSeq (mRNA) NM_001127707 NM_000295 NM_001002235 NM_001002236 NM_001127700 NM_001127701 NM_001127702 NM_001127703 NM_001127704 NM_001127705 NM_001127706 NM_009246 RefSeq (protein) NP_000286 NP_001002235 NP_001002236 NP_001121172 NP_001121173 NP_001121174 NP_001121175 NP_001121176 NP_001121177 NP_001121178 NP_001121179 NP_033272 NP_001239498 NP_033269 Location (UCSC) Chr 14: 94.38 – 94.39 Mb Chr 12: 103.73 – 103.74 Mb PubMed search [3] [4]
Wikidata
View/Edit Human View/Edit Mouse

Alpha-1 antitrypsin or α₁-antitrypsin (A1AT, α₁AT, A1A, or AAT) is a protein belonging to the serpin superfamily. It is encoded in humans by the SERPINA1 gene. A protease inhibitor, it is also known as alpha₁–proteinase inhibitor (A1PI) or alpha₁-antiproteinase (A1AP) because it inhibits various proteases (not just trypsin).^[5] In older biomedical literature it was sometimes called serum trypsin inhibitor (STI, dated terminology), because its capability as a trypsin inhibitor was a salient feature of its early study. As a type of enzyme inhibitor, it protects tissues from enzymes of inflammatory cells, especially neutrophil elastase, and has a reference range in blood of 0.9–2.3 g/L (in the US the reference range is expressed as mg/dL or micromoles), but the concentration can rise manyfold upon acute inflammation.^[6]

When the blood contains inadequate amounts of A1AT or functionally defective A1AT (such as in alpha-1 antitrypsin deficiency), neutrophil elastase is excessively free to break down elastin, degrading the elasticity of the lungs, which results in respiratory complications, such as chronic obstructive pulmonary disease, in adults. Normally, A1AT leaves its site of origin, the liver, and joins the systemic circulation; defective A1AT can fail to do so, building up in the liver, which results in cirrhosis in either adults or children.

In addition to binding to neutrophil elastase released by inflammatory cells, A1AT also binds to elastase localized on the cell surface in which case elastase does not act as an enzyme, but instead acts to signal cells to undergo locomotion.^[7] Besides liver cells, A1PI is produced in bone marrow, by lymphocytic and monocytic cells in lymphoid tissue, and by the Paneth cells of the gut.^[8]

Inactivation of A1AT by enzymes other than elastase due to inflammation/infection causes the migration of T cells to halt precisely at the site where the pathologic insult exists. This suggests a role for α1PI not only in locomotion of lymphocytes through tissue, but as a consequence of infection, a primary role as a sentinel in immune vigilance.^[9]

A1AT is both an endogenous protease inhibitor and an exogenous one used as medication. The pharmaceutical form is purified from human donor blood and is sold under the nonproprietary name alpha₁–proteinase inhibitor (human) and under various trade names (including Aralast NP, Glassia, Prolastin, Prolastin-C, and Zemaira). Recombinant versions are also available but are currently used in medical research more than as medication.

Function

A1AT is a 52-kDa serpin and, in medicine, it is considered the most prominent serpin; the terms α1-antitrypsin and protease inhibitor (P_i) are often used interchangeably.

Most serpins inactivate enzymes by binding to them covalently. These enzymes are released locally in relatively low concentrations where they are immediately cleared by proteins such as A1AT. In the acute phase reaction, a further elevation is required to "limit" the damage caused by activated neutrophil granulocytes and their enzyme elastase, which breaks down the connective tissue fiber elastin.

Besides limiting elastase activity to limit tissue degradation, A1PI also acts to induce locomotion of lymphocytes through tissue including immature T cells through the thymus where immature T cells mature to become immunocompetent T cells that are released into tissue to elevate immune responsiveness.^[10]

Like all serine protease inhibitors, A1AT has a characteristic secondary structure of beta sheets and alpha helices. Mutations in these areas can lead to non-functional proteins that can polymerise and accumulate in the liver (infantile hepatic cirrhosis).

Role in disease

Disorders of this protein include alpha-1 antitrypsin deficiency, an autosomal co-dominant hereditary disorder in which a deficiency of alpha-1 antitrypsin leads to a chronic uninhibited tissue breakdown. This causes the degradation especially of lung tissue and eventually leads to characteristic manifestations of pulmonary emphysema.^[11] Evidence has shown^[12] that cigarette smoke can result in oxidation of methionine 358 of α₁-antitrypsin (382 in the pre-processed form containing the 24 amino acid signal peptide), a residue essential for binding elastase; this is thought to be one of the primary mechanisms by which cigarette smoking (or second-hand smoke) can lead to emphysema. Because A1AT is expressed in the liver, certain mutations in the gene encoding the protein can cause misfolding and impaired secretion, which can lead to liver cirrhosis.

An extremely rare form of P_i, termed P_iPittsburgh, functions as an antithrombin (a related serpin), due to a mutation (Met358Arg). One person with this mutation has been reported to have died of a bleeding diathesis.^[13]

A liver biopsy will show abundant PAS-positive globules within periportal hepatocytes.

Patients with rheumatoid arthritis (RA) have been found to make autoantibodies toward the carbamylated form of A1AT in the synovial fluid. This suggests that A1AT may play an anti-inflammatory or tissue-protecting role outside the lungs. These antibodies are associated with a more severe disease course, can be observed years before disease onset, and may predict the development of RA in arthralgia patients. Consequently, carbamylated A1AT is currently being developed as an antigenic biomarker for RA.^[14]

Nomenclature

The protein was initially named "antitrypsin" because of its ability to bind and irreversibly inactivate the enzyme trypsin in vitro covalently. Trypsin, a type of peptidase, is a digestive enzyme active in the duodenum and elsewhere.

The term alpha-1 refers to the protein's behavior on protein electrophoresis. On electrophoresis, the protein component of the blood is separated by electric current. There are several clusters, the first being albumin, the second being the alpha, the third beta and the fourth gamma (immunoglobulins). The non-albumin proteins are referred to as globulins.

The alpha region can be further divided into two sub-regions, termed "1" and "2". Alpha-1 antitrypsin is the main protein of the alpha-globulin 1 region.

Another name used is alpha-1 proteinase inhibitor (α₁-PI).

Genetics

The gene is located on the long arm of chromosome 14 (14q32.1).

Over 100 different variants of α₁-antitrypsin have been described in various populations. North-Western Europeans are most at risk for carrying one of the most common mutant forms of A1AT, the Z mutation (Glu342Lys on M1A, rs28929474).^[15]

Biochemical properties

A1AT is a single-chain glycoprotein consisting of 394 amino acids in the mature form and exhibits many glycoforms. The three N-linked glycosylations sites are mainly equipped with so-called diantennary N-glycans. However, one particular site shows a considerable amount of heterogeneity since tri- and even tetraantennary N-glycans can be attached to the Asparagine 107 (UniProtKB amino acid nomenclature). These glycans carry different amounts of negatively charged sialic acids; this causes the heterogeneity observed on normal A1AT when analysed by isoelectric focusing. Also, the fucosylated triantennary N-glycans were shown to have the fucose as part of a so-called Sialyl Lewis x epitope,^[16] which could confer this protein particular protein-cell recognition properties. The single cysteine residue of A1AT in position 256 (UniProtKB nomenclature) is found to be covalently linked to a free single cysteine by a disulfide bridge.^[16]

Analysis

The level of A1AT in serum is most often determined by adding an antibody that binds to A1AT, then using turbidimetry to measure how much A1AT is present. Other detection methods include the use of enzyme-linked-immuno-sorbent-assays and radial immunodiffusion.

Different analytical methods are used to determine A1AT phenotype. As protein electrophoresis is imprecise, the A1AT phenotype is analysed by isoelectric focusing (IEF) in the pH range 4.5-5.5, where the protein migrates in a gel according to its isoelectric point or charge in a pH gradient.

Normal A1AT is termed M, as it migrates toward the center of such an IEF gel. Other variants are less functional and are termed A-L and N-Z, dependent on whether they run proximal or distal to the M band. The presence of deviant bands on IEF can signify the presence of alpha-1 antitrypsin deficiency. Since the number of identified mutations has exceeded the number of letters in the alphabet, subscripts have been added to most recent discoveries in this area, as in the Pittsburgh mutation described above.

As every person has two copies of the A1AT gene, a heterozygote with two different copies of the gene may have two different bands showing on electrofocusing, although heterozygote with one null mutant that abolishes expression of the gene will only show one band.

In blood test results, the IEF results are notated as in P_iMM, where P_i stands for protease inhibitor and "MM" is the banding pattern of that patient.

Alpha-1 antitrypsin levels in the blood depend on the genotype. Some mutant forms fail to fold properly and are, thus, targeted for destruction in the proteasome, whereas others have a tendency to polymerise, being retained in the endoplasmic reticulum. The serum levels of some of the common genotypes are:

• PiMM: 100% (normal)
• PiMS: 80% of normal serum level of A1AT
• PiSS: 60% of normal serum level of A1AT
• PiMZ: 60% of normal serum level of A1AT
• PiSZ: 40% of normal serum level of A1AT
• PiZZ: 10-15% (severe alpha-1 antitrypsin deficiency)
• PiZ is caused by a glutamate to lysine mutation at position 342 (366 in pre-processed form)
• PiS is caused by a glutamate to valine mutation at position 264 (288 in pre-processed form)

Other rarer forms have been described; in all, there are over 80 variants.

Medical uses

Routes of
administrationIntravenousATC codeLegal statusLegal status IdentifiersCAS NumberDrugBankChemSpiderUNIIChEMBLPDB ligandCompTox Dashboard (EPA)ECHA InfoCard100.029.919 Chemical and physical dataFormulaC₂₀₀₁H₃₁₃₀N₅₁₄O₆₀₁S₁₀Molar mass44324.65 g·mol⁻¹  (verify)

Alpha-1 antitrypsin concentrates are prepared from the blood plasma of blood donors. The US Food and Drug Administration (FDA) has approved the use of four alpha-1 antitrypsin products derived from a human plasma: Prolastin, Zemaira, Glassia, and Aralast.^{[24][25][26][27][28][29]} These products for intravenous augmentation A1AT therapy can cost up to $100,000 per year per patient.^[30] They are administered intravenously at a dose of 60 mg/kg once a week; higher doses do not provide additional benefit although they can be used in anticipation of an interruption of weekly administration, such as for a vacation.^[31]

Alpha1-proteinase inhibitor (Respreeza) was approved for medical use in the European Union in August 2015.^[32] It is indicated for maintenance treatment, to slow the progression of emphysema in adults with documented severe alpha1-proteinase inhibitor deficiency (e.g., genotypes PiZZ, PiZ (null), Pi (null, null), PiSZ).^[32] People are to be under optimal pharmacologic and non-pharmacologic treatment and show evidence of progressive lung disease (e.g. lower forced expiratory volume per second (FEV1) predicted, impaired walking capacity or increased number of exacerbations) as evaluated by a healthcare professional experienced in the treatment of alpha1-proteinase inhibitor deficiency.^[32]

The most common side effects include dizziness, headache, dyspnoea (shortness of breath) and nausea.^[32] Allergic reactions have been observed during treatment, some of which were severe.^[32]

Aerosolized-augmented A1AT therapy is under study.^[when?] This involves inhaling purified human A1AT into the lungs and trapping the A1AT into the lower respiratory tract. However, inhaled A1AT may not reach the elastin fibers in the lung where elastase injury occurs. Further study is currently underway.^[33] Recombinant alpha-1 antitrypsin is not yet available for use as a medication but is under development.^[34]

History

Axelsson and Laurell first investigated the possibility of allelic variants of A1AT leading to disease in 1965.^[35]

See also

• Alpha 1-antichymotrypsin, another serpin that is analogous for protecting the body from excessive effects of its own inflammatory proteases
• Orosomucoid is a related alpha 1 protein

References

Further reading

External links

• The MEROPS online database for peptidases and their inhibitors: I04.001
• Proteopedia: Alpha-1-antitrypsin
• Human SERPINA1 genome location and SERPINA1 gene details page in the UCSC Genome Browser.