A Pair of Atypical KAS III Homologues with Initiation and Elongation Functions Program the Polyketide Biosynthesis in Asukamycin

Abstract

β-Ketoacyl-ACP synthase III (KAS III) is a class of important CÀ C bond-forming enzymes that mostly catalyze the initiation of polyketide and fatty acid biosynthesis. In this study, we elucidated an unusual polyketide synthase (PKS) system that involves two unique KAS IIIs (AsuC3 and C4) in the biosynthesis of the upper triene chain of asukamycin. Significantly, AsuC3 and C4 have both initiation and iterative elongation activity, while being functionally biased toward the elongation and initiation steps, respectively. Mutational analysis revealed that their catalytic activities rely on the catalytic triad Cys-His-Asn. Unlike other KAS IIIs, AsuC3 and C4 are very promiscuous and can accept various lengths of acyl-CoAs with either cyclic, branched or linear acyl moieties. By cooperation with the permissive ketoreductase (AsuC7) and dehydratase (AsuC8/C9), a large variety of polyenes can be efficiently synthesized. This study significantly broadens the understanding of KAS IIIs and polyketide biosynthesis.

The shikimate pathway: gateway to metabolic diversity

“…Qu and coworkers recently investigated the unusual PKS machinery that elongates the CHC-CoA starter unit into the polyene "upper chain" portion of asukamycin. 98 b-ketoacyl-ACP synthase III (KAS III) enzymes are canonically associated with the initiation of fatty acid and polyketide biosynthesis, wherein they catalyze the condensation of a malonyl-ACP and an acyl-CoA to generate a b-ketoacyl-ACP product. 99 Notably, the asukamycin BGC contains genes encoding two KAS III enzymes, AsuC3 and AsuC4 (29% AA sequence ID), adjacent to an acyl carrier protein (AsuC5), ketoreductase (AsuC7), and two dehydratases (AsuC8 and AsuC9).…”

“…These small molecules are isolated along with the biotin binding protein streptavidin, resulting in a potent synergistic cocktail that causes biotin deciency in microbes. 181,182 Stravidins, such as stravidin S2 (98), contain the nonproteinogenic amino acid amiclenomycin (Acm) that selectively binds and inhibits mycobacterial biotin biosynthesis protein, BioA, by forming an irreversible, covalent adduct with BioA's PLP cofactor. The characteristic amino-substituted 1,4-cyclohexadiene moiety of Acm is critical for this activity, however the molecular origin of this amino acid, as well as the biosynthesis of stravidins, remained unknown for decades.…”

“…Fig.35The conversion of chorismate to ADC is the branch point for PABA (96) and 4-amino-4-deoxyprephenate (97) biosynthesis. 97 further diverges through either aromatic or non-aromatic decarboxylation reactions to chloramphenicol (99) and stravidin S2, respectively(98).…”