|
Home >> Dictionary of Bioinformatics, Biochemistry, Biotechnology >> Helical Wheel - Heparin
|
|
Heavy isotopes
Forms of atoms that contain greater number of neutrons (eg. 15N, 13C).
Hedgehog (Hh)
A family of secreted signaling proteins that play a role in animal tissue development. The Hh proteins are synthesized as long precursors. The C-terminal region of the precursor, which has autoproteolytic activity, cleaves the molecule to release the N-terminal signaling moiety.
|
During this process, a cholesterol molecule is covalently joined to the carboxy terminus of the signalling peptide. The patched transmembrane protein functions as a receptor. Mutations in Hh signal transduction have been documented in human birth defects and cancer.
HeLa cell
A cell line derived from a human cervical carcinoma that grows indefinitely in culture. It is named after the patient Henrietta Lacks, from whom the tumour cell were isolated.
|
α – helix
See alpha helix.
Helix (helical) parameters
A universal set of parameters that describe details of the positional relationship between stacked and paired bases. These include translational (displacement) and rotational parameters that describe the relative positions of bases in base pairs, the relative position of successive base pairs in base stacks and the absolute position of base pairs relative to the helical axis.
Helix
A spiral structure with a repeating pattern.
Helix pi
Similar to α – helix but more loosely wound (5-residuces per turn) and rarely observed in natural proteins.
|
|
Helix morphology
The classification of DNA and RNA helices by gross morphological criteria such as helical diameter, helical sense (direction of rotation), pitch (number of base pairs per helical turn) and the width and depth of the major and minor grooves.
Helix-loop-helix (HLH)
A nucleic acid-binding protein motif that consists of two monomers, each of which contains two α-helices. The tow monomers are held together in a four-helix bundle. Each monomer contains a short α-helix linked by a loop to a second, longer α-helix. The connecting loop is flexible and permits one helix to fold back and pack against the other. The two longer α-helices of the two monomers bind to each other and to specific DNA sequences. The two monomers may be similar (homodimers) or distinct (heterodimers).
|
|
|
Helix-turn-helix (HTH)
This is the first nucleic acid-binding protein motif to be studied in detail. The motif contains 2 α – helices separated by a short β-turn. The second helix, which interacts with the major groove of DNA is known as the recognition helix. It makes sequence-specific contacts with the bases in the major groove. The first helix anchors the recognition helix in position and is required for stability. The HTH motif is found in E.coli lac and trp repressors, cAMP receptor protein (CRP), bacteriophage cro protein and ANtennapedia transcription factor of Drosophila. All proteins bind to DNA as dimers. The two monomers associate by means of antiparallel β-sheets to form a dimer.
|
Helical wheel
A circular graph depicting five turns of a helix around which the residues of a protein sequence are plotted.
Helicase
An enzyme that catalyses DNA unwinding. See DNA helicase.
Helminthes
Parasitic worms. These include internal parasites of humans and animals.
Helper cells
T cells that help other cell types (B and T lymphocytes) to elicit an immune response in the presence of an antigen. Also termed helper T lymphocytes.
Helper plasmid
A plasmid that provides function(s) to another plasmid in the same cell.
Helper T cells
See helper cells.
Helper T lymphocytes
See helper cells.
Helper virus
A virus that provides function(s) to another (defective) virus in the same cell, enabling it to complete the infection cycle.
Heme
See haem.
Hemiacetal
The product of the condensation of an aldehyde with an alcohol. This contains two oxygen atoms linked to the same central carbon atom; while one oxygen is linked in a hydroxyl fashion, the other is attached by an ether linkage.
Hemicellulase
A commercially available enzyme that is capable of degrading hemicellulose to galactose.
Hemicellulose
A cellulose-like carbohydrate that forms the cell wall matrix together with pectin and lignin. It is a polymer of D-xylose, D-mannose or D-galactose which provide a matrix in which cellulose fibres are embedded.
Hemidemosome
An anchoring cell junction between an epithelial cell and the underlying basal lamina.
Hemizygote
An individual who is a diploid but has lost one copy of a particular gene due perhaps to chromosome loss or aneuploidy.
Hemizygous
Possessing only one copy of a particular gene. For example, human males are hemizygous for genes on the Y chromosome.
Hemoglobin
See haemoglobin.
Hemophilia
See haemophilia.
Henderson-Hasselbalch equation
An equation relating the pKa to the pH and the ratio of the proton acceptor (A-) and the proton donor (HA) species of a conjugate acid-base pair.
pH = pK’ + log 
Henderson – Hasselbalch equation
HEPA filter
(high efficiency particulate air filter) A filter that can screen out particles larger than 0.3μm. These filters are used in laminar air flow cabinets (hoods) for sterile transfer work.
Heparin
A polyanionic glycoprotein rich in sulphated acidic polysaccharides. On hydrolysis, heparin gives rise to glucuronic acid and glucosamine as well as acetic and sulphuric acid. Although found in several tissues, it is particularly abundant in the mast cells lining the endothelium of blood vessels. Heparin is used as an anticoagulant. It binds to and enhances the inhibitory activity of antithrombin III. The antithrombin III-heparin complexes inhibit many clotting factors such as IIa, IXa, Xa, XIa and XIIa. Heparin is used in vivo during blood transfusions, in surgical procedures where blood is passed through a heart lung machine or artificial kidney, as a prophylactic anticoagulant to prevent venous thrombosis and in patients with prosthetic implants. Heparin is also used as an anticoagulant in vitro. The major complication of heparin administration is haemorrhage, which can be managed by administration of protamine that inactivates heparin by virtue of its positive charge.
Heptic lipase
An enzyme that hydrolyses triacylglycerols and phospholipids n HDL2 releasing cholesterol for uptake by the liver.
Hepatitis
Inflammation of the liver.
Hepatitis A (HAV)
Also called infectious hepatitis. It is transmitted by the faecal-oral route and is a food-borne and water-borne infection. Presence of lgM anti-HAV in serum is diagnostic.
|
|
