Chiral recognition of 2,3-dihydroxypropanesulfonate by bacterial transport proteins adapted to distinct ecological niches

Abstract

2,3-Dihydroxypropanesulfonate (DHPS) is an abundant organosulfonate that links sulfosugar catabolism to sulfur mineralization in marine and gut environments. In surface seawater, DHPS occurs as a dilute, mixed R/S pool, whereas in the anaerobic gut it is produced predominantly as S-DHPS through bacterial sulfoglycolysis pathways. Uptake is proposed to occur via tripartite ATP-independent periplasmic (TRAP) transporters that employ periplasmic substrate-binding proteins (HpsK), but the molecular basis of enantiomer recognition has not been defined. Here, we compare HpsK proteins from the marine bacterium Ruegeria pomeroyi and the gut anaerobe Bilophila wadsworthia using proteomics, biophysical analysis, X-ray crystallography, and bioinformatics. RpHpsK binds both R- and S-DHPS with low-nanomolar affinity (K_D 5-9 nM), whereas BwHpsK binds selectively to S-DHPS (K_D 530 nM), representing an ~100-fold difference in affinity and strict stereoselectivity. Crystal structures reveal two contrasting strategies for chiral recognition: RpHpsK accommodates both enantiomers through subtle side-chain "toggling" within an otherwise conserved binding pocket, whereas BwHpsK achieves stereoselectivity through a distinct hydrogen-bonding network and a binding site that sterically excludes R-DHPS. Sequence similarity and genome neighbourhood analyses place these proteins in separate clusters associated with oxidative (HpsNOP) or glycyl radical enzyme-linked (HpsGH/HpfGH) pathways. These findings show how changes in binding-site architecture tune ligand stereoselectivity and illustrate the adaptation of TRAP-associated substrate binding proteins to distinct ecological and metabolic niches.