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Home >> Dictionary of Bioinformatics, Biochemistry, Biotechnology >> Alpha Foetoprotein AFP Alternative RNA Splicing
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Allozyme See allosteric enzyme.
Alpha1 (α1)-acid glycoprotein (AAG, orosomucoid) A major constituent of the seromucoid fraction of plasma. It is a glycoprotein with a high content of carbohydrate and a large number of Sialic acid residues. It is predominantly synthesised by the liver and to a lesser extent by granulocytes and monocytes. AAG is a member of a group of proteins known as lipocalins, which bind lipophilic compounds. AAG is known to bind progesterone and drugs such a chlorpromazine, propranolol and benzodiazepines. Plasma AAG is significantly increased in inflation and is one of the most reliable indicators of ulcerative colitis. Plasma AAG concentrations are low in nephritic syndrome and protein-losing enteropathy.
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Alpha (α)-amanitin Bicyclic octapeptide isolated from the poisonous mushroom Amanita phalloides which inhibits transcription catalysed by eukaryotic RNA polymerase II.
Alpha2 (α2)-antiplasmin The most important inhibitor of plasminogen activation.
Alpha beta (αβ)-barrel A complex supersecondary structure in proteins formed by sequential β-α-β motifs, which roll up into a cylinder in which b-strands are parallel and enclosed by a-helices. This motif is common in enzymes such as aldolase, pyruvate kinase and Triose phosphate IsoMerase (therefore termed a TIM barrel).
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Alpha (α)-foetoprotein (AFP) A glycoprotein of molecular weight 70,000 synthesised initially by the yolk sac and later by the foetal liver. High concentration of AFP is present in foetal serum with peak values at 16 weeks of gestation (3000000 ng/mL), which steadily declines at term (5000-12000 ng/mL). there is an exponential decrease in serum AFP values in the infant and adult values (<2 ng/mL) are reached by the 10th month of life. AFP is capable of diffusing through the capillary membranes and is found in foetal urine and therefore in the amniotic fluid and maternal circulation. AFP becomes detectable in maternal plasma at about 12 weeks of gestation with peak values at 25 weeks followed by a steady decline and subsequent return to normal values after childbirth. Abnormally high concentrations of AFP in amniotic fluid and maternal circulation. AFP becomes detectable in maternal plasma are seen in foetal neural tube defects such as spina bifida. AFP is useful as a tumour marker in hepatocellular carcinoma and testicular teratomas.
Alpha (α)-gliadin A wheat protein, sensitivity to which causes malabsorption. See gluten-sensitive enteropathy.
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Alpha (α)-helix A right-handed helical conformation of a polypeptide chain deduced by Pauling and Corey. It is one of the most common secondary structures found in proteins (eg. hair, wool). The structure is stabilised by intrachain hydrogen bonding. Each peptide bond participates in hydrogen bonding which occurs between the carbonyl of one peptide bond and the –NH of another peptide bond four amino acid residues further down the chain. There are 3.6 amino acid residues/turn of the helix with their R groups protruding outward. Amino acids with bulky R groups as well as proline disrupt helix formation.
Structure of α-helix 
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Intrachain Hydrogenbond |
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Amino Acid Side ChainsOriented Outward |
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Alpha (α)-lactalbumin A protein component of milk.
Alpha2 (α2)-macroglobulin see a2-macroglubulin.
Alpha (α)-peptide Part of the b-galactosidase protein encoded by the lacZ gene fragment.
Alpha1 (α1)-antitrypsin (AAT) An α1 – globulin and a serine proteinase inhibitor synthesised by the liver. It inhibits elastase released from polymorphonuclear leucocytes. Oestrogens increase the synthesis of AAT and high plasma levels are seen during late pregnancy and oestrogen therapy. Plasma AAT an acute phase protein, is increased during inflammation and necrosis.
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Deficiency of AAT causes neonatal hepatitis progressing to cirrhosis. AAT shows genetic polymorphism and over 30 alleles have been described. Deficiency of AAT is commonly due to the PiZZ variant in which there is a single amino acid substitution. Deficiency of AAT can also lead to emphysema and chronic obstructive pulmonary disease.
Alpha (α)-actinin An intracellular anchor protein that links the intracellular domains of integrins to the action filaments.
Alphabet The total number of symbols in a sequence. It is 4 for DNA and 20 for protein sequences.
Alphoid DNA (α-satellite DNA) Cryptic satellite DNA found in centromeric DNA of human chromosomes.
ALT See alanine transaminase.
Alternate (alternative) pathway Complement activation that does not involve activation of the C1, C4, C2 pathway by antigen-antibody complexes, and begins with the activation of C3.
Alternating axis of symmetry A structure possessing this axis, when rotated around the axis by an angle of 2p/n followed by reflection across a plane at right angles to the axis, given rise to another identical structure.
Alternative RNA splicing The generation of different proteins from the same RNA transcript by joining of different exons.
Alternative splice-form One of the possible alternate combinations of exons that gives rise to a folded protein by recombining multiple gene segments during mRNA splicing in higher organisms.
Altman, Sidney A scientist who was awarded the Nobel Prize in Chemistry in 1689 together with Thomas Cech for his work on ribozymes. He demonstrated that RNase P is a catalytic RNA (ribozyme) and the RNA component of RNase P alone is sufficient to function as an endonuclease and catalyse pretRNA cleavage in vitro.
Alu elements Moderately repetitive DNA sequences randomly dispersed throughout the mammalian genome. There are about 500 000 copies of these elements in the human genome. Each Alu element is about 300 bases long. Alu sequences are so named because they contain cleavage sites for the restriction enzyme Alul. These elements are believed to have originated from viral RNA sequences that integrated into human DNA thousands of years ago.
Alzheimer’s disease A neurodegenerative disorder affecting the elderly. It is characterised by progressive and severe memory loss with loss of autonomic functions and eventually death. Characterstic plaques consisting of amyloid protein and nerve tangles (neurofibrillary tangles, NFTs) have been identified in the brains of Alzheimer’s patients. The development of Alzheimer’s disease is thought to be associated, in part, with possessing certain alleles of the gene that encodes Apolipoprotein E.
Amber codon The triplet nucleotide UAG, one of three codons that signal termination of polypeptide chain synthesis.
Amber mutation Generation of an amber codon (one of the termination codons) as a result of mutation in the DNA. This causes premature termination of translation.
Amber suppressors Mutant genes encoding tRNAs whose anticodons have been altered so that they respond to UAG codons. As a result, these tRNAs insert an amino acid in response to a termination codon, allowing translation to continue.
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