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Home >> Dictionary of Bioinformatics, Biochemistry, Biotechnology >>Lac Operon Lactate Dehydrogenase LD
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L phase variants Bacterial variants without a cell wall produced under conditions in which cell wall synthesis is inhibited such as the presence of penicillin. The L phase variants pass through filters that normally retain bacteria.
L1 element The most abundant long interspersed nuclear elements (LINES), of 6kb in length with a copy number of 105. Together with Alu, L1 elements make up 19 per cent of the human genome. They are found in introns and flanking in introns and flanking regions of genes. These sequences constitute parasitic or selfish DNA without any known function. However, they are also postulated to have a role in replication and gene regulation.
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Label An atom or a compound that is linked to or incorporated into a macromolecule and is used to detect the presence of a compound, substance, or macromolecule in a sample. Also called a tag.
Labeling The process of replacing a stable atom in a compound with a radioactive isotope of the same element enabling it to be detected by autoradiography or other techniques. This is used to trace the path of any labelled compound through a biological or chemical system. Radioactive labeling is however being replaced by fluorescent labeling.
Labile enzyme Enzymes involved in metabolic pathways that have short half-lives.
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Lab on a chip A term used to describe microdevices that allow rapid, microanalytical analysis of DNA or protein in a single, fully integrated system. These devices are miniature surfaces, made of silicon, glass or plastic that carry the necessary microdevices such as pumps, valves, microfluidic controllers, and detectors enabling sample separation and analysis. These devices are used in drug discovery and genetic testing.
Lac operon (lactose operon) A paradigm to understand the regulation of gene expression in prokaryotes. The lac operon contains structural genes and regulatory genes. The three structural genes in the lac operon-lacZ, lacy and lacA encode β-galactosidase galactoside Permease and thiogalactoside transacetylase respectively are involved in the metabolism of lactose. The regulatory genes, I gene, operator and promoter influence expression of the structural genes. The I gene codes for lac repressor which binds to the operator inhibiting transcription. Glucose represses lac operon via cAMP and CAP.
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The Lac Operon
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Lac repressor-lac promoter system See IPTG.
Lac repressor protein The protein encoded by the I gene in the lac operon that block lac gene expression by binding to the operator region.
Lactam antibiotics Antibiotics such as penicillin that are derived from the lactam ring.
Lactam ring A molecule with the general structure (CH2)n-CH=O.
Lactamase An enzyme produced by penicillin resistant bacteria that cleaves the lactam ring between the NH and C=O bonds.
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Lactate The end product of anaerobic metabolism of glucose formed from pyruvate by the action of lactated dehydrogenase (LD) with the equilibrium in favour of lactate Lactate is produced by erythrocytes, skin, intestine, brain, renal medulla, retina and muscle.
Lactate dehydrogenase (LD) An enzyme that catalyses the oxidation of L-lactate to pyruvate with NAD+ as the hydrogen acceptor. The reaction is reversible and the equilibrium strongly favours the reverse reaction. LD is a widely distributed cytosolic enzyme with high concentrations in the heart, skeletal muscle, liver, kidney, brain and erythrocytes. Plasma LD is predominantly derived from erythrocytes and platelets. LD is a tetramer composed of two types of monomer units-the H type and the M type which give rise to 5 isoenzymes (LD-1 to LD-5) all of which are detectable in plasma. Two more isoenzymes of LD have been identified-LD-X (or LDc) containing four X (or C) subunits present in prepubertal human tests and LD-6 detected in the sera of severely ill patients. Plasma LD is markedly increased in circulatory failure with shock and hypoxia, myocardial infarction (HBD fraction) and malignancies.
Lactic acid The product formed by the conversion of pyruvate by the enzyme lactate dehydrogenase under anaerobic conditions such as after strenuous muscular exercise.
Lactic acid bacteria Anaerobic bacteria that generate lactic acid during oxidation of sugar. Lactic acid bacteria are responsible for the souring of milk and the sour taste of sauerkraut.
Lactic acidosis One of the common causes of metabolic acidosis. It occurs due to an imbalance between the rate of production and the rate of utilization of lactate. The formation of lactate is increased under anaerobic conditions. Impairment of TCA cycle and gluconegenesis can decrease the utilization of lactate.
Lactoperoxidase labeling A technique for labeling proteins on the outside of cell membranes using radioisotopes of iodine (125I). Lactoperoxidase catalyses the transfer of iodine from iodoacetamide to the tyrosine residues of proteins to be labelled.
Lactose Milk sugar. A disaccharide containing one unit each of glucose and galactose.
Lactose Galactose Glucose
Lactosuria Excretion of lactose in urine. Lactose is found in the urine of women during pregnancy, lactation and the weaning period.
Lag phase 1. A state of inactivity that precedes a response. Also called a latent phase. 2. The initial growth phase during which the cell number remains relatively constant before rapid growth begins. 3. The first of five growth phases of most batch-propagated cell suspension cultures. This is the phase in which the inoculated cells in fresh medium adapt to the new environment and prepare to divide. See growth phases.
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