Shedding light on a family of broad-spectrum α-hydroxy acid and polyhydroxy acid (sugar acid) isomerases

α-Hydroxy acids (AHAs) and polyhydroxy acids (PHAs) are of key importance in organic synthesis and find numerous applications in various industries. However, the stereoselective synthesis of these compounds, whether enzymatic or chemical, remains a major challenge. In this study, we uncover a previously uncharacterized family of lactate racemase homologs (LarAHs) within the LarA superfamily and show their ability to catalyze the interconversion of AHA and PHA stereoisomers. Through systematic biochemical characterization, we investigated the substrate specificity, catalytic properties, and structural modelling of four representative LarAHs from this novel family. Our findings demonstrate that these enzymes act as broad-spectrum hydroxy acid isomerases (BSHIs) and exhibit racemase activity on up to 15 structurally diverse AHAs (including aliphatic, aromatic, and polycarboxylic AHAs), and C2-epimerase activity on up to 24 distinct PHAs (including aldonic acids, uronic acids, aldaric acids, and other sugar-derived acids). Notably, 20 of the sugar acid C2-epimerization reactions identified were previously unreported. Structural analysis revealed that BSHIs possess an original C-terminal fold that forms a substrate-binding site adapted for bulky substrates, with considerable active-site flexibility likely contributing to their broad substrate specificity. Collectively, these findings suggest the biotechnological potential of BSHIs for stereoselective interconversion of valuable AHA and PHA stereoisomers.