Functional and Structural Analysis of Programmed C-Methylation in the Biosynthesis of the Fungal Polyketide Citrinin

Abstract

Summary Fungal polyketide synthases (PKSs) are large, multi-domain enzymes that biosynthesize a wide range of natural products. A hallmark of these megasynthases is the iterative use of catalytic domains to extend and modify a series of enzyme-bound intermediates. A subset of these iterative PKSs (iPKSs) contains a C-methyltransferase (CMeT) domain that adds one or more S-adenosylmethionine (SAM)-derived methyl groups to the carbon framework. Neither the basis by which only specific positions on the growing intermediate are methylated (“programming”) nor the mechanism of methylation are well understood. Domain dissection and reconstitution of PksCT, the fungal non-reducing PKS (NR-PKS) responsible for the first isolable intermediate in citrinin biosynthesis, demonstrates the role of CMeT-catalyzed methylation in precursor elongation and pentaketide formation. The crystal structure of the S-adenosyl-homocysteine (SAH) coproduct-bound PksCT CMeT domain reveals a two-subdomain organization with a novel N-terminal subdomain characteristic of PKS C-methyltransferase domains and provides insights into cofactor and ligand recognition.

Genome mining and biosynthesis of a polyketide from a biofertilizer fungus that can facilitate reductive iron assimilation in plant

“…Therefore, the TlnCcatalyzed α-methylation step serves as a checkpoint in TlnB programmed steps to ensure 3, not 8, is produced. α-Methylation as a checkpoint in HRPKS programming has been noted previously (35,36), and is observed here for an NRPKS via interacting with an in trans MT.…”

A Mononuclear Iron-Dependent Methyltransferase Catalyzes Initial Steps in Assembly of the Apratoxin A Polyketide Starter Unit

“…Of the several hundred MT structures in the structure database, AprA MT1 most closely resembles the MTs from PKS extension steps 28, 29 . All class I SAM-dependent MTs are thought to have a common ancestor, but the superfamily has several highly diverged branches.…”

Diels–Alder Reactions During the Biosynthesis of Sorbicillinoids

“…Pyrones 13 and 14 are likely premature offloading shunt products of SorB,s ince similar products are known to be formed when methylation steps are not properly completed by nr-PKS. [28] Next, sorC was added to sorAB in the expectation of forming 2a.H owever, A. oryzae + sorABC produced 15 a as the major product ( Figure 2E). TheU Vabsorption of 15 a is distinct from that of 2a, [9] and the m/z [MÀH] À of 249 indicated areduction.…”