Crystal structure of Fatty Acid Thioesterase A bound by 129 fragments provides diverse development opportunities

To alleviate the growing issue of herbicide resistance, diversification of the herbicide portfolio is necessary. A promising yet underutilised mode of action (MoA) are Fatty Acid Thioesterases (Fat), which terminate de novo fatty acid (FA) biosynthesis by releasing fatty acids from acyl carrier protein (ACP) cofactors. These enzymes impact plant growth and sterility by determining the amount and length of FAs present. To find novel chemical matter targeting this MoA we have solved the crystal structure of Arabidopsis thaliana FatA to 1.5Å and conducted a crystallographic fragment screen which identified 141 hits. Our fragments recapitulated interactions made by current herbicides, but also targeted novel regions, namely the active site and the dimer interface. Ten fragments demonstrated on-scale potency, two of those exploiting different interactions to known herbicides. The screen also revealed large conformational changes exploited by some fragments, including stabilisation of a loop and shifts in the ACP-binding lid domain. These changes alter the accessibility of the substrate binding site by reducing the size of the apparent substrate access channel. Finally, the fragments readily lend themselves to designing compounds that both merge motifs and allow catalogue-based SAR exploration. Elaboration of one of the fragments resulted in an improvement of affinity from ∼20 μM to ∼90nM KD. Thus our data may facilitate accelerated early discovery of alternative inhibitors that are novel in both chemical scaffolds and modes of interaction.