S ‐Adenosylmethionine ( SAM ) hydrolases counter increased SAM 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 a metabolic salvage pathway. To further assess the biological relevance of this pathway, 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.