Archaeal S-adenosyl-l-homocysteine hydrolases: structure, function and substrate preferences

S -Adenosyl-l-homocysteine hydrolase (SAHH) reversibly cleaves S -adenosyl-l-homocysteine (SAH), the product of S -adenosyl-l-methionine (SAM)-dependent methylation reactions. The conversion of SAH into adenosine and l-homocysteine (Hcy) plays an important role in the regulation of the methyl cycle. An alternative metabolic route for SAM regeneration in the extremophiles Methanocaldococcus jannaschii and Thermotoga maritima was identified with the deamination of SAH to S -inosyl-l-homocysteine (SIH). Herein, we report the first structural characterisation of different archaeal SAHHs together with a biochemical analysis of various SAHHs from all three domains of life. We found that homologues deriving from the Euryarchaeota phylum show a higher conversion rate with SIH compared to SAH. Crystal structures of SAHH originating from Pyrococcus furiosus in complex with SIH and inosine as ligands, show architectural flexibility in the active site and offer deeper insights into the binding mode of hypoxanthine-containing substrates. Altogether, the findings presented in this study support the understanding of an alternative metabolic route for SAM and offer insights into the evolutionary progression and diversification of SAHHs involved in methyl and purine salvage pathways. ### Competing Interest Statement The authors have declared no competing interest.