Abstract
Quinolone alkaloids, found abundantly in the roots of bael (Aegle marmelos), possess various biological activities and have recently gained attention as potential lead molecules for novel drug designing. Here, we report the characterization of a novel Type III polyketide synthase, quinolone synthase (QNS), from A. marmelos that is involved in the biosynthesis of quinolone alkaloid. Using homology-based structural modeling, we identify two crucial amino acid residues (Ser-132 and Ala-133) at the putative QNS active site. Substitution of Ser-132 to Thr and Ala-133 to Ser apparently constricted the active site cavity resulting in production of naringenin chalcone from p-coumaroyl-CoA. Measurement of steady-state kinetic parameters demonstrates that the catalytic efficiency of QNS was severalfold higher for larger acyl-coenzymeA substrates as compared with smaller precursors. Our mutagenic studies suggest that this protein might have evolved from an evolutionarily related member of chalcone synthase superfamily by mere substitution of two active site residues. The identification and characterization of QNS offers a promising target for gene manipulation studies toward the production of novel alkaloid scaffolds.
Highlights
Type III polyketide synthase is hypothesized to produce quinolones, but no such enzyme has been identified so far
We report the characterization of a novel Type III polyketide synthase, quinolone synthase (QNS), from A. marmelos that is involved in the biosynthesis of quinolone alkaloid
A phylogenetic tree was constructed with others type III polyketide synthases (PKS) in the database, including the highly homologous ones to study the evolutionary relationship of QNS (Fig. 2, A and B)
Summary
Type III polyketide synthase is hypothesized to produce quinolones, but no such enzyme has been identified so far. A report by Abe et al [10] demonstrated the ability of benzalacetone synthase (BAS), a type III PKS to accept non-physiological substrate N-methylanthraniloyl CoA and catalyze formation of 4-hydroxy-2(1H)-quinolones. Type III polyketide synthases (PKS) are homodimeric enzymes where the single active site in each monomer iteratively catalyzes the priming, extension, and cyclization reactions to generate the polyketide product. Despite their structural simplicity, type III PKS produces a variety of compounds, including pyrones, resorcinols, acridones, chalcones, stilbenes, and phloroglucinols [13, 14]. The modeling and mutagenesis studies provide an insight into the structural mechanism for the enzyme that could be used to generate pharmaceutically important products
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