Structural insights into the reaction mechanism of S-adenosyl-L-homocysteine hydrolase

S-adenosyl-L-homocysteine hydrolase (SAH hydrolase or SAHH) is a highly conserved enzyme that catalyses the reversible hydrolysis of SAH to L-homocysteine (HCY) and adenosine (ADO). High-resolution crystal structures have been reported for bacterial and plant SAHHs, but not mammalian SAHHs. Here, we report the first high-resolution crystal structure of mammalian SAHH (mouse SAHH) in complex with a reaction product (ADO) and with two reaction intermediate analogues-3'-keto-aristeromycin (3KA) and noraristeromycin (NRN)-at resolutions of 1.55, 1.55, and 1.65 Å. Each of the three structures constitutes a structural snapshot of one of the last three steps of the five-step process of SAH hydrolysis by SAHH. In the NRN complex, a water molecule, which is an essential substrate for ADO formation, is structurally identified for the first time as the candidate donor in a Michael addition by SAHH to the 3'-keto-4',5'-didehydroadenosine reaction intermediate. The presence of the water molecule is consistent with the reaction mechanism proposed by Palmer &Abeles in 1979. These results provide insights into the reaction mechanism of the SAHH enzyme.