Structure and Function of BorB, the Type II Thioesterase from the Borrelidin Biosynthetic Gene Cluster

α/β hydrolases make up a large and diverse protein superfamily. In natural product biosynthesis, cis-acting thioesterase α/β hydrolases can terminate biosynthetic assembly lines and release products by hydrolyzing or cyclizing the biosynthetic intermediate. Thioesterases can also act in trans, removing aberrant intermediates and restarting stalled biosynthesis. Knockout of this "editing" function leads to reduced product titers. The borrelidin biosynthetic gene cluster from Streptomyces parvulus Tü4055 contains a hitherto uncharacterized stand-alone thioesterase, borB. In this work, we demonstrate that purified BorB cleaves acyl substrates with a preference for propionate, which supports the hypothesis that it is also an editing thioesterase. The crystal structure of BorB shows a wedgelike hydrophobic substrate binding crevice that limits substrate length. To investigate the structure-function relationship, we made chimeric BorB variants using loop regions from characterized homologues with different specificities. BorB chimeras slightly reduced activity, arguing that the modified region is a not major determinant of substrate preference. The structure-function relationships described here contribute to the process of elimination for understanding thioesterase specificity and, ultimately, engineering and applying trans-acting thioesterases in biosynthetic assembly lines.