Divergence of Substrate Specificity and Function in the <i>Escherichia coli</i> Hotdog-fold Thioesterase Paralogs YdiI and YbdB

The work described in this paper, and its companion paper (Wu, R., Latham, J. A., Chen, D., Farelli, J., Zhao, H., Matthews, K. Allen, K. N., and Dunaway-Mariano, D. (2014) Structure and Catalysis in the <i>Escherichia coli</i> Hotdog-fold Thioesterase Paralogs YdiI and YbdB. <i>Biochemistry</i>, DOI: 10.1021/bi500334v), focuses on the evolution of a pair of paralogous hotdog-fold superfamily thioesterases of <i>E. coli</i>, YbdB and YdiI, which share a high level of sequence identity but perform different biological functions (viz., proofreader of 2,3-dihydroxybenzoyl-<i>holo</i>EntB in the enterobactin biosynthetic pathway and catalyst of the 1,4-dihydoxynapthoyl-CoA hydrolysis step in the menaquinone biosynthetic pathway, respectively). In vitro substrate activity screening of a library of thioester metabolites showed that YbdB displays high activity with benzoyl-<i>holo</i>EntB and benzoyl-CoA substrates, marginal activity with acyl-CoA thioesters, and no activity with 1,4-dihydoxynapthoyl-CoA. YdiI, on the other hand, showed a high level of activity with its physiological substrate, significant activity toward a wide range of acyl-CoA thioesters, and minimal activity toward benzoyl-<i>holo</i>EntB. These results were interpreted as evidence for substrate promiscuity that facilitates YbdB and YdiI evolvability, and divergence in substrate preference, which correlates with their assumed biological function. YdiI support of the menaquinone biosynthetic pathway was confirmed by demonstrating reduced anaerobic growth of the <i>E. coli</i> <i>ydiI</i>-knockout mutant (vs wild-type <i>E. coli</i>) on glucose in the presence of the electron acceptor fumarate. Bioinformatic analysis revealed that a small biological range exists for YbdB orthologs (i.e., limited to Enterobacteriales) relative to that of YdiI orthologs. The divergence in YbdB and YdiI substrate specificity detailed in this paper set the stage for their structural analyses reported in the companion paper