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Home >> Dictionary of Bioinformatics, Biochemistry, Biotechnology >>Lipid Storage Diseases Lipidoses Lipofection
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Linking number The number of times the two strands of a closed DNA duplex cross over each other. This describes DNA underwinding. The linking number of relaxed DNA termed Lk0 serves as reference. The change in linking number is expressed in terms of a specific linking difference also termed superhelical density. This is a measure of the turns removed relative to those present in relaxed DNA and is a length-independent quantity. The discrepancy between the existence of -2 supercoils in the path of DNA on the Nucleosome compared to the -1 supercoil released when histones are removed.
Linking number paradox The discrepancy between the values obtained by different methods of determination of the number of turns of DNA around each Nucleosome. This is of theoretical significance. For example, digestion of nucleosomes by endonucleases gives a value of 1.8 DNA coils per nucleosome whereas measurements based on DNA supercoiling give a value of about 1 coil per nucleosome.
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LINUS (Local Independency Nucleated Units of Structure) A programme for predicting protein structure from the amino acid sequence without making an explicit reference to any known structure or sequence-structure relationships.
LINUX A UNIX-like operating system named after Linus Thorvald, the inventor.
Lipaemia Excess lipid in the blood.
Lipases Enzymes that degrade lipids into their component fatty acid and head group moieties. The lipases used in biotechnology are usually digestive lipases that break down dietary fats.
Lipid Fat. A water-insoluble biomolecule containing fatty acids, sterols or isoprenoids.
Lipid-anchored membrane protein A protein that is bound to a cellular membrane by covalent attachment of one or more lipid groups embedded in the phospholipid bilayer.
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Lipid bilayer See Bilayer.
Lipid peroxidation The process of reactive oxygen species (ROS)-induced destruction of membrance lipids especially polyunsaturated fatty acids (PUFA). Lipid peroxidation is a complex free radical chain reaction involving three distinct stages-initiation, propagation and termination. Initiation involves attack of fatty acid chains of membrance lipids by free radicals to leave an unpaired electron. Addition of molecular oxygen to form lipid hydroperoxides, alkoxy and peroxy radicals constitutes the propagation phase. The peroxyl radical can form cyclic peroxides and cytotoxic aldehydes such as malondialdehyde (MDA), 4-hydroxy-2-nonenal (4-HNE) acrolein and crotonaldehyde. The chain reaction continues until the PUFA substrate is completely consumed or until the radical self-annihilates (termination). A primary effect of lipid peroxidation is loss of PUFA, decreased membrane fluidity and disrupted functions of membrane-bound proteins.
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Lipid raft A small region of the plasma membrance that is rich is sphingolipids and cholesterol.
Lipid storage diseases (lipidosis) A group of inherited disorders that are part of a larger group of lysosomal diseases. These are characterised by accumulation of complex lipids, which have ceramide as a portion of their structure due to a deficiency of the lysosomal enzyme responsible for degrading the lipid. The clinical symptoms that occur due to damage caused by the stored material include neurological dysfunction, skeletal abnormalities and visceromegaly. The disorders are progressive and many are fatal. Infantile, juvenile and adult forms have been recognised for many of these disorders. Diagnosis can be made by enzyme assay on fibroblasts, serum and leukocytes.
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Lipidomics A rapidly expanding research field in which multiple techniques are utilised to quantitate the chemically distinct lipids in cells and determine the molecular mechanisms through which they facilitate cellular function. Recent developments in electrospray ionisation mass spectrometry (ESI/MS) have made possible, for the first time, the precise identification and quantification of alterations in a cells lipidome after cellular perturbations.
Lipidosis See lipid storage diseases.
Lipoate (lipoic acid) A vitamin of the B complex that functions as an intermediate carrier of hydrogen atoms and acyl groups.
Lipocalins A group of proteins which bind lipophilic compounds.
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Lipofection Liposome mediated gene transfer. Transfer of DNA, RNA or other compounds encapsulated in an artifical phospholipid vesicle (liposome) into eukaryotic cells.
Lipoic acid A cofactor, essential for the oxidation to keto acids, such as pyruvate during metabolism.
Lipolysis The breakdown of triacylglycerols and fatty acids.
Lipophilic See hydrophilic.
Lipopolysaccharide (LPS) A lipid bound to a polysaccharide. It functions as a structural component of microbial cell walls.
Lipoprotein(a) or LP(a) Lipoprotein synthesised in the liver and normally present in low concentration in plasma. It has a composition similar to that of LDL but with higher protein content. LP(a) is also known as sinking pre-β, because it has pre-β mobility. However, unlike VLDL, the density at which it sediments on ultracentrifugation is 1.008. LP(a) levels are elevated in patients with coronary heart disease.
Lipoprotein lipase An enzyme bound to endothelial surfaces of the capillary walls that is released into circulation following injection of heparin. It catalyses hydrolysis of triacylglycerols and requires phospholipids and apoC-ll as cofactors.
Lipoprotein-X A lipoprotein not detectable in normal plasma but seen in the plasma of patients with obstructive jaundice and lecithin cholesterol acyltransferase (LCAT) deficiency. The lipoprotein is composed of 66% phospholipids, 22% free cholesterol, 3% ester cholesterol, 6% protein, 3% triglycerides and 2-3% bile acids.
Lipoproteins Lipid-protein complexes that transports lipids in blood. These consist of an insoluble nonpolar core of triglycertides and cholesterol esters surrounded by a polar surface coat of free cholesterol, phospholipids and proteins, with the polar groups oriented outwards. The protein part of the lipoprotein is known as Apolipoprotein.
Structure of a Lipoprotein
| 1. Intergral Apoprotein |
2. Free Cholesterol |
| 3. Triacylglycerol |
4. Phospholipid |
| 5. Cholesterol Ester |
6. Peripheral Apoprotein |
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