Microcystins structure

Microcystis aeruginosa (order Chroococcales) accounts for 92% of the compounds isolated from the genus Microcystis (49 compounds total) with 6% coming from Microcystis viridis and 2% from undetermined species. However, it is likely that some variants of the microcystin structure were not tallied in our analysis. 

In vitro cytotoxicity assays using isolated cells have been applied intermittently to cyanobacterial toxicity testing over several years." Cells investigated for suitability in cyanobacterial toxin assays include primary liver cells (hepatocytes) isolated from rodents and fish, established permanent mammalian cell lines, including hepatocytes, fibroblasts and cancerous cells, and erythrocytes. Earlier work suggested that extracts from toxic cyanobacteria disrupted cells of established lines and erythrocytes," but studies with purified microcystins revealed no alterations in structure or ion transport in fibroblasts or erythrocytes,... 

Fig. 5.1 Common cyanobacterial hepatotoxins. (a) Generalized structure of microcystin, a cyclic heptapeptide. Note that X and Z are L-amino acids. For example, microcystin-LR possesses lysine and arginine residues at X and Z, respectively, (b) Cylindrospermopsin, a hepatotoxic alkaloid from Cylindrospermopsis raceborskii...

Dahlmann, J., Budakowski, W.R. and Luckas, B., Liquid chromatography-electrospray ionisation-mass spectrometry based method for the simultaneous determination of algal and cyanobacterial toxins in phytoplankton from marine waters and lakes followed by tentative structural elucidation of microcystins, /. Chromatogr., 994, 1-2, 45, 2003. 

These natural toxins are heptapeptides produced by cyanobacteria, which are associated with algal blooms. These substances are a hazard to wild and farm animals and sometimes humans who come in contact with contaminated water. There are a number of these toxins, some of which such as microcystin LR are hepatotoxic, causing damage to both hepatocytes and endothelial cells. The toxins have some unusual structural features, incorporating three D-amino acids and two very unusual ones, namely, methyldehydro alanine (Mdha) and amino-methoxy-trimethyl-phenyl-decadi-enoic acid (Adda) (Fig. 7.26). 

Furthermore, the structure of microcystin includes an electrophilic carbon atom, (Fig. 7.26), which is part of the Mdha amino acid. If microcystin is ingested from contaminated water, for example, it is taken up into the liver by an organic anion transporter (OAT) system and therefore is concentrated in the liver. The structure of the microcystins means they are able to associate with the enzymes protein phosphatases, such as PP-1, PP-2A, and PP-2B via hydrophobic and ionic interactions. 

Figure 7.26 The structure of the hepatotoxic cyclic heptapeptide microcystin LR. L-Arginine and L-leucine are variable amino acids. The reactive unsaturated group is indicated by the star. Abbreviations Adda, amino-methoxy-trimethyl-phenyl-decadienoic acid Mdha, methyldehydro-alanine Masp, methyl D-/so-aspartate D-Glu, D-/soglutamate D-Ala, D-alanine.

These natural toxins are heptapeptides produced by cyanobacteria and have unusual structural features, incorporating three D-amino acids. Microcystin LR is hepatotoxic, as a result of inactivation of protein phosphatases, which leads to breakdown of the cytoskeleton and cell death. 

Fig. 16.1. Chemical structure of microcystin-LR. This image is licensed under the http //www.gnu.org/copyleft/fdl.html GNU Free Documentation License. It uses material from the http //en.wikipedia.org/wiki/Cyanotoxin Wikipedia article.

Fig. 7.2 Tlie crystal structure of mammalian Ser/Thr protein phosphatase-1, complexed with the toxin mycrocystin was determined at 2.1 A resolution. PPl has a single domain with a fold, distinct from that of the protein tyrosine phosphatases. The Ser/Thr protein phosphatase-1, is a metalloenzyme with two metal ions positioned at the active site with the help of a p-a-p-o-p scaffold. A dinuclear ion centre consisting of Mn2+ And Fe2+ g situated at the catalytic site that binds the phosphate moiety of the substrate. Ser/Thr phosphatases, PPl and PP2A, are inhibited by the membrane-permeable ocadaic acid and by cyclic hexapeptides, known as microcystins. The toxin molecule is depicted as a ball-and-stick structure. On the left and on the ri t, two different views of the same molecule are shown. Microcystin binds to three distinct regions of the phosphatase to the metaLbinding site, to a hydrophobic groove, and to the edge of a C-terminal groove in the vicinity of the active site. At the surface are binding sites for substrates and inhibitors. These ribbon models are reproduced vnth permission of the authors and Nature from ref. 9.

The microcystins are a group of over 70 structurally related monocyclic heptapeptides containing seven peptide-linked aminoacids with the general structure of Cyclo-(D-alanine -X -D-MeAsp -Y" -Adda -D-glutamate - Mdha ), in which X and Y are variable L aminoacids, D-MeAsp is D-erytro- - methylaspartic acid and Mdha is N-methyldehydroalanine (Fig. 14.1). 

One of the invariant amino acids is a unique P-amino acid called Adda (2S,3S,8S,9S)-3 amino-9 methoxy-2,6,8-trimethyl-10-phei5fldeca-4,6-dienoic acid is the most unusual structure in this group of cyanobacterial cyclic peptide toxins). A two-letter suffix (XY) is ascribed to each individual toxin to denote the two variant amino acids (Carmichael 1988). X is commonly leucine, arginine, or tyrosine. Y is arginine, alanine, or methionine. Variants of all the invarianf amino acids have now been reported, e.g., desmethyl amino acids and/or replacement of the 9-methoxy group of Adda by an acetyl moiety. Currently there are in excess of 60 variants of microcystin that have been characterized (Rinehart 1994 Sivonen and Jones 1999). Of these 60 compounds, microcystin-LR would appear to be the microcystin most commonly found in cyanobacteria. It is also common for more than one microcystin to be found in a particular strain of cyanobacterium (Namikoshi 1992 Lawton 1995). The microcystin variants may also differ in toxicity (Carmichael 1993). The literature indicates that hepatotoxic blooms ofM aeruginosa containing microcystins occur commonly worldwide. 

These two families of toxins are both eyclic peptides (Figure 27.1 A and B), with the same basie eyclic structure involving an amino acid known as ADDA (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid), and four (nodularins) or six (microcystins) other amino acids (Chorus and Bartram, 1999). Only six nodularins have so far been identified. In eontrast, mieroeystins have two variable amino acids (X and Z), two groups (Rl, R2) and two demethylated positions (3 and 7), and as a result there are more than 80 known mieroeystins, ranging in molecular weight from 800 to 1,100 daltons. 

Microcystins are cyclic heptapeptides that share a general structure, as shown in Fig. 1 and Table 1, containing g-hnked D-glutamicacid (d-G1u), D-alanine Co-Ala), p-linked O-erythro-b-methylaspartic acid (o-MeAsp), A-methyldehydroalanine (Mdha), and a unique C20 p-amino acid, (25, 35, 85, 95)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4( ), 6( )-dienoic acid... 

Adda). The other two L-amino acids are variable (denoted X and Z) and are found in positions 2 and 4 of the cyclic structure. The single-letter abbreviation of the variable amino acids is used to distinguish different microcystins for example, the most commonly occurring microcystin... 

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