Microcystin-LR
 The chemical structure of microcystin-LR
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| Names | |
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IUPAC name
(5R,8S,11R,12S,15S,18S,19S,22R)-15-[3-(diaminomethylideneamino)propyl]-18-[(1E,3E,5S,6S)-6-Methoxy-3,5-dimethyl-7-phenylhepta-1,3-dienyl]-1,5,12,19-tetramethyl-2-methylidene-8-(2-methylpropyl)-3,6,9,13,16,20,25-heptaoxo-1,4,7,10,14,17,21-heptazacyclopentacosane-11,22-dicarboxylic acid
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| Other names
5-L-Arginine-microcystin LA
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| Identifiers | |
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3D model (Jmol)
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| Abbreviations | MC-LR, MCYST-LR |
| ChEBI | |
| ChemSpider | |
| ECHA InfoCard | 100.150.186 |
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PubChem CID
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| UNII | |
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| Properties | |
| C49H74N10O12 | |
| Molar mass | 995.19 g·mol−1 |
| Appearance | White solid |
| Density | 1.299 g/cm3 |
| Solubility in ethanol | 1 mg/mL |
| log P | -1.44 |
| Pharmacology | |
| Ingestion | |
| Hazards | |
| R-phrases | R26/27/28, R36/37/38, R43 |
| S-phrases | S26, S36/37/39, S45 |
| Lethal dose or concentration (LD, LC): | |
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LD50 (median dose)
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5 mg/kg |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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 (what is   ?) |
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| Infobox references | |
Microcystin-LR (MC-LR) is a toxin produced by cyanobacteria. It is the most toxic of the microcystins.
The Chinese general Zhu-Ge Liang was the first to observe cyanobacteria poisoning about 1000 years ago. He reported the death of troops who drank green coloured water from a river in southern China. The first published report of an incidence of cyanobacteria poisoning dates from the poisoning of an Australian lake in 1878. Also, in China and Brasil, people died after drinking water from a lake. All these incidences have been attributed to cyanobacteria and the toxic compound microcystin-LR. That is the reason why the WHO, World Health Organisation, issued a guideline for microcystins in drinking water. The WHO guideline for microcystins in drinking water, based on microcystin-LR, is 1 μg/L. With the high levels of Eutrophication in South Africa, typical exposures can be as high as 10 μg/L.
Microcystins are cyclic heptapeptides. The seven amino acids that are involved in the structure of a microcystin include a unique β-amino acid (ADDA). It contains alanine (D-ala), D-β-methyl-isoaspartate (D-β-Me-isoAsp), and glutamic acid (D-glu). Furthermore, microcystins contain two variable residues, which make the differentiation between variants of microcystins. These two variable elements are always standard L-amino acids. In microcystin-LR these are leucine and arginine.
Eighty microcystins have been identified to date, representing differences in the two variable residues and some modifications in the other amino acids. These modifications include demethylation of Masp and Mdha and methylesterification of D-Glu. Different microcystins have different toxicity profiles, with microcystin-LR found to be the most toxic.
Microcystins are small nonribosomal peptides. In Microcystis aeruginosa microcystin-LR is synthesized by proteins that encoded by a 55 kb microcystin-gene cluster (mcy) that contains 6 large (over 3 kb) genes that encode proteins with polyketide synthase activity, nonribosomal peptide synthase activity (mcyA-E and G) and 4 smaller genes (mcyF and H-J). These large proteins are made up of different protein domains, coined 'modules', that each have their own specific enzymatic function. Although the enzyme systems involved in the biosynthesis of microcystins is not identical among all cyanobacteria, there are large similarities and most of the essential enzymes are conserved.
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