Uncovering structural features that control substrate specificity in a Lactobacillus chlorogenic acid esterase

The structural determinants of chlorogenic acid esterase (CE) substrate specificity are poorly understood. Here, we establish how a Lactobacillus helveticus CE selects for its substrate, chlorogenic acid (CGA). We determine that a Lys residue in an extended loop over the active site imparts substrate specificity by hydrogen bonding to CGA. Mutation of the Lys residue abolishes CGA specificity. Comparison with other bacterial CEs reveals that the extended loop is not conserved. However, the hydrogen bonding functionality to CGA is preserved thanks to other residues. Structural comparison with ferulic acid esterases (FAEs), a related enzyme class, shows that CEs feature a more restricted active site, reflecting the fact that they hydrolyze smaller substrates compared to FAEs.