Biology Reference
In-Depth Information
Figure 6.3
Theproposedbiosynthesisformicrocystin-LR.TheactualstarterofthePKS
McyG(Rontheigure)isnotknown.Thedomainabbreviationsaredeinedintheabbre-
viationslist.
in vitro experiments showed that this racemase uses aspartate rather than
glutamate (
Sielaff et al., 2003
). Thus, the glutamate racemase gene is not part
of the
mcy
cluster. The following steps are catalysed by three NRPSs, McyA,
B, and C. They sequentially add five amino acids,
N
-methyldehydroala-
nine, D-alanine, L-leucine, D-
erythro
-3-methylaspartate with a β-peptide
bond, and L-arginine. The origin of dehydroalanine is not known, but the
N
-methylation probably occurs on McyA as well as the epimerization of
the next alanyl residue. The McyI and McyF proteins are thought to be
involved in the biosynthesis of D-
erythro
-3-methylaspartate. The function of
McyI was studied in vitro and it was shown that it is an NADPH-dependent
dehydrogenase likely involved in the formation of L-
threo
-3-methylaspartate
(
Pearson, Barrow, & Neilan, 2007
). It has been shown, in vitro, that McyF
catalyses the racemization of aspartate, but it is not known if this enzyme
epimerizes L-
threo
-3-methylaspartate to give D-
erythro
-3-methylaspartate.
