S-Adenosylhomocysteine Hydrolase, NAD-binding and catalytic domains
S-adenosyl-L-homocysteine hydrolase (SAHH, AdoHycase) catalyzes the hydrolysis of S-adenosyl-L-homocysteine (AdoHyc) to form adenosine (Ado) and homocysteine (Hcy). The equilibrium lies far on the side of AdoHyc synthesis, but in nature the removal of Ado and Hyc is sufficiently fast, so that the net reaction is in the direction of hydrolysis. Since AdoHyc is a potent inhibitor of S-adenosyl-L-methionine dependent methyltransferases, AdoHycase plays a critical role in the modulation of the activity of various methyltransferases. The enzyme forms homotetramers, with each monomer binding one molecule of NAD+.