Bacterial cysteate dissimilatory pathway involves a racemase and D-cysteate sulfo-lyase

The sulfite-reducing bacterium Bilophila wadsworthia , a common human intestinal pathobiont, is unique in its ability to metabolize a wide variety of sulfonates to generate sulfite as a terminal electron acceptor (TEA). The resulting formation of H 2 S is implicated in inflammation and colon cancer. l -cysteate, an oxidation product of l -cysteine, is among the sulfonates metabolized by B. wadsworthia , although the enzymes involved remain unknown. Here we report a pathway for l -cysteate dissimilation in B. wadsworthia RZATAU, involving isomerization of l -cysteate to d -cysteate by a cysteate racemase ( Bw CuyB), followed by cleavage into pyruvate, ammonia and sulfite by a d -cysteate sulfo-lyase ( Bw CuyA). The strong selectivity of Bw CuyA for d -cysteate over l -cysteate was rationalized by protein structural modeling. A homolog of Bw CuyA in the marine bacterium Silicibacter pomeroyi ( Sp CuyA) was previously reported to be a l -cysteate sulfo-lyase, but our experiments confirm that Sp CuyA too displays a strong selectivity for d -cysteate. Growth of B. wadsworthia with cysteate as the electron acceptor is accompanied by production of H 2 S and induction of Bw CuyA. Close homologs of Bw CuyA and Bw CuyB are present in diverse bacteria, including many sulfate- and sulfite-reducing bacteria, suggesting their involvement in cysteate degradation in different biological environments.