<i>S</i> ‐Adenosylmethionine ( <scp>SAM</scp> ) hydrolases counter increased <scp>SAM</scp> 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.