Crystal structures of S-adenosylhomocysteine hydrolase from the thermophilic bacterium Thermotoga maritima

S-adenosylhomocysteine (SAH) hydrolase catalyzes the reversible hydrolysis of SAH into adenosine and homocysteine by using NAD+ as a cofactor. The enzyme from Thermotoga maritima (tmSAHH) has great potentials in industrial applications because of its hyperthermophilic properties. Here, two crystal structures of tmSAHH in complex with NAD+ show both open and closed conformations despite the absence of bound substrate. Each subunit of the tetrameric enzyme is composed of three domains, namely the catalytic domain, the NAD+-binding domain and the C-terminal domain. The NAD+ binding mode is clearly observed and a substrate analogue can also be modeled into the active site, where two cysteine residues in mesophilic enzymes are replaced by serine and threonine in tmSAHH. Notably, the C-terminal domain of tmSAHH lacks the second loop region of mesophilic SAHH, which is important in NAD+ binding, and thus exposes the bound cofactor to the solvent. The difference explains the higher NAD+ requirement of tmSAHH because of the reduced affinity. Furthermore, the feature of missing loop is consistently observed in thermophilic bacterial and archaeal SAHHs, and may be related to their thermostability.