Structure, function and substrate preferences of archaeal S-adenosyl-l-homocysteine hydrolases

S -Adenosyl- l -homocysteine hydrolase (SAHH) reversibly cleaves S -adenosyl- l -homocysteine, the product of S -adenosyl- l -methionine-dependent methylation reactions. The conversion of S -adenosyl- l -homocysteine into adenosine and l -homocysteine plays an important role in the regulation of the methyl cycle. An alternative metabolic route for S -adenosyl- l -methionine regeneration in the extremophiles Methanocaldococcus jannaschii and Thermotoga maritima has been identified, featuring the deamination of S -adenosyl- l -homocysteine to S -inosyl- l -homocysteine. Herein, we report the structural characterisation of different archaeal SAHHs together with a biochemical analysis of various SAHHs from all three domains of life. Homologues deriving from the Euryarchaeota phylum show a higher conversion rate with S -inosyl- l -homocysteine compared to S -adenosyl- l -homocysteine. Crystal structures of SAHH originating from Pyrococcus furiosus in complex with S l H 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 of our study support the understanding of an alternative metabolic route for S -adenosyl- l -methionine and offer insights into the evolutionary progression and diversification of SAHHs involved in methyl and purine salvage pathways.