( R )- S -Adenosyl-L-methionine hydrolases counter sulfonium epimerisation in thermophilic archaea

S -Adenosyl-L-methionine (SAM) is the second most used enzyme cofactor and vital for numerous cellular reactions such as methylation or polyamine synthesis. While most stereocentres of the biologically active ( S _S ,S _Cα )-SAM are fixed, epimerisation at the methyl sulfonium centre is driven by heat, yielding biologically inactive ( R _S ,S _Cα )-SAM. This SAM diastereomer disturbs SAM-dependent pathways, posing a metabolic threat especially to thermophilic organisms . In vitro analysis shows that SAM hydrolases cleave the biologically inactive ( R _S ,S _Cα )-SAM, thereby constituting to a metabolic salvage pathway. For further analysis of the biological relevance, we characterised two archaeal SAM hydrolases from the thermophilic Sulfolobus acidocaldarius and the halophilic Haloferax volcanii, confirming their selectivity towards ( R _S ,S _Cα )-SAM in vitro. Genetic manipulation in the native hosts supports a significant role of the SAM-hydrolases in decreasing the share of intracellular ( R _S ,S _Cα )-SAM to sustain cellular functions in thermophilic organisms .