Cytochalasins are fungal metabolites that have the ability to bind to actin filaments and block polymerization and the elongation of actin. As a result of the inhibition of actin polymerization, cytochalasins can change cellular morphology, inhibit cellular processes such as cell division, and even cause cells to undergo apoptosis. In Magnaporthe grisea the cytochalasan Pyrichalasin H is synthetized by the polyketide synthase non ribosomal peptide synthase (PKS-NRP) PyiS. PyES release an aldehyde intermedite PyE-Substrate which is cycled by a Knoevenagel reaction by PyiE. The product of this reaction would undergo a Diels Alder reaction by PyiF and give the cytochalasan skeleton
Title: Chemical and Genetic Studies on the Formation of Pyrrolones During the Biosynthesis of Cytochalasans Zhang H, Hantke V, Bruhnke P, Skellam E, Cox RJ Ref: Chemistry, 10:3106, 2021 : PubMed
A key step during the biosynthesis of cytochalasans is a proposed Knoevenagel condensation to form the pyrrolone core, enabling the subsequent 4+2 cycloaddition reaction that results in the characteristic octahydroisoindolone motif of all cytochalasans. Here we investigate the role of the highly conserved alpha-beta -hydrolase enzymes PyiE and ORFZ during the biosynthesis of pyrichalasin H and the ACE1 metabolite respectively, using gene knockout and complementation techniques. Using synthetic aldehyde models we demonstrate that the Knoevenagel condensation proceeds spontaneously but results in the 1,3-dihydro-2H-pyrrol-2-one tautomer, rather than the required 1,5-dihydro-2H-pyrrol-2-one tautomer. Taken together our results suggest that the alpha-beta -hydrolase enzymes are essential for first ring cyclisation, but the precise nature of the intermediates remains to be determined .
