Structural features for substrate recognition by bacterial 5′‐methylthioadenosine nucleosidase

The bacterial enzyme 5′‐methylthioadenosine/ S‐ adenosylhomocysteine nucleosidase (MTAN) is a multifunctional enzyme that catalyzes the hydrolysis of the N‐ribosidic bond of at least 3 different adenosine‐based metabolites: S‐ adenosylhomocysteine (SAH), 5′‐deoxyadenosine and 5′‐methylthioadenosine (MTA). These activities place the enzyme at the hub of 5 fundamental bacterial metabolic pathways: S‐ adenosylmethionine (SAM) utilization, the purine salvage pathway, the methionine salvage pathway, the SAM radical pathways and autoinducer‐2 biosynthesis. The last pathway is important in synthesizing diffusible compounds used in bacterial quorum sensing, which affects biofilm formation in many pathogenic bacteria. Although structures of various bacterial and plant MTANs have been described, the interactions between the homocysteine moiety of SAH and the 5′‐alkylthiol binding site of MTAN have never been resolved. To gain information regarding these interactions, we have solved crystal structures of an inactive mutant form of Helicobacter pylori MTAN bound to MTA and SAH to 1.65 and 1.2 Å, respectively. These structures identify specific interactions between the homocysteine moiety and the 5′‐alkylthiol binding site of the enzyme that appear to be conserved in campylobacter but not across all bacterial MTANs. Comparison of the MTA and SAH structures also shows that the localized structure of 5′‐alkylthiol binding site is identical without regard for which substrate is bound. This suggests that Hp MTAN binding of the homocysteine moiety contributes little or no entropic penalty to this binding interaction and that these new data may promote development of species‐specific inhibitors for MTAN that affect biofilm formation.