The Streptomyces viridochromogenes product template domain represents an evolutionary intermediate between dehydratase and aldol cyclase of type I polyketide synthases

Abstract

The product template (PT) domains act as an aldol cyclase to control the regiospecific aldol cyclization of the extremely reactive poly-β-ketone intermediate assembled by an iterative type I polyketide synthases (PKSs). Up to now, only the structure of fungal PksA PT that mediates the first-ring cyclization via C4–C9 aldol cyclization is available. We describe here the structural and computational characterization of a bacteria PT domain that controls C2–C7 cyclization in orsellinic acid (OSA) synthesis. Mutating the catalytic H949 of the PT abolishes production of OSA and results in a tetraacetic acid lactone (TTL) generated by spontaneous O-C cyclization of the acyl carrier protein (ACP)-bound tetraketide intermediate. Crystal structure of the bacterial PT domain closely resembles dehydrase (DH) domains of modular type I PKSs in the overall fold, dimerization interface and His-Asp catalytic dyad organization, but is significantly different from PTs of fungal iterative type I PKSs. QM/MM calculation suggests that the catalytic H949 abstracts a proton from C2 and transfers it to C7 carbonyl to mediate the cyclization reaction. According to structural similarity to DHs and functional similarity to fungal PTs, we propose that the bacterial PT represents an evolutionary intermediate between the two tailoring domains of type I PKSs.

CryoEM structure of the di-domain core ofMycobacterium tuberculosispolyketide synthase 13, essential for mycobacterial mycolic acid synthesis

“…The structure of Ms-Pks13 (PDB: 8CV1) comprising the N-terminal ACP, KS and AT domains was superimposed, resulting in the positioning of the ACP (cyan) into the density between the KS and AT domains. B Superposition of the AF2-generated model of the DH* dimer (through co-folding) with the low resolution cryoEM map and with of a DALI-derived structural neighbour, the dehydratase-like AviM product template domain from Streptomyces viridochromogenes (PDB: 7VWK) (Feng et al, 2022). See supplementary information for details of the DALI search.…”

Structural enzymology of iterative type I polyketide synthases: various routes to catalytic programming

“…Recently, the crystal structure of a bacterial PT domain of AviM that catalyzes C2–C7 cyclization in orsellinic acid synthesis has been reported 72 (Fig. 15A, bottom).…”