(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.