Microcystins consist of a seven-membered peptide ring which is made up of five
non-protein amino acids and two protein amino acids. It is these two protein
amino acids that distinguish microcystins from one another, while the other
amino acids are more or less constant between variant microcystins. Using
amino acid single letter code nomenclature, each microcystin is designated a
name depending on the variable amino acids which complete their structure. The
most common and potently toxic microcystin-LR contains the amino acids Leucine
(L) and Arginine (R) in these variable positions.
The molecular structure of microcystin-LR can be seen below, with the variable amino acids (leucine and arginine) in red.
The 3D structure of microcystin-LR can be viewed by following this link
The "Adda" amino acid has become a useful tool in microcystin research as it provides the molecule with a characterstic wavelength absorbance at 238nm. This is believed to be attributable to the conjugated diene group in the long carbon chain of this uncommon amino acid. The Adda moiety is also required for toxicity and is important in the binding of the toxin to protein phosphatases. The stereochemistry about the dienes of the Adda group have also been shown to influence toxicity, as too have the levels of methylation of various structures in the cyclic peptide. As such, the relative toxicities of microcystins can differ greatly. The absorbance characteristics of Adda provide a means of analysis of microcystins after separating them by reverse phase HPLC.