title: Mechanism of substrate specificity in 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidases.
abstract: 5'-Methylthioadenosine/S-adenosylhomocysteine (MTA/SAH) nucleosidase (MTAN) plays a key role in the methionine-recycling pathway of bacteria and plants. Despite extensive structural and biochemical studies, the molecular mechanism of substrate specificity for MTAN remains an outstanding question. Bacterial MTANs show comparable efficiency in hydrolyzing MTA and SAH, while the plant enzymes select preferentially for MTA, with either no or significantly reduced activity towards SAH. Bacterial and plant MTANs show significant conservation in the overall structure, and the adenine- and ribose-binding sites. The observation of a more constricted 5'-alkylthio binding site in Arabidopsis thalianaAtMTAN1 and AtMTAN2, two plant MTAN homologues, led to the hypothesis that steric hindrance may play a role in substrate selection in plant MTANs. We show using isothermal titration calorimetry that SAH binds to both Escherichia coli MTAN (EcMTAN) and AtMTAN1 with comparable micromolar affinity. To understand why AtMTAN1 can bind but not hydrolyze SAH, we determined the structure of the protein-SAH complex at 2.2Å resolution. The lack of catalytic activity appears to be related to the enzyme's inability to bind the substrate in a catalytically competent manner. The role of dynamics in substrate selection was also examined by probing the amide proton exchange rates of EcMTAN and AtMTAN1 via deuterium-hydrogen exchange coupled mass spectrometry. These results correlate with the B factors of available structures and the thermodynamic parameters associated with substrate binding, and suggest a higher level of conformational flexibility in the active site of EcMTAN. Our results implicate dynamics as an important factor in substrate selection in MTAN.
authors.affiliation: Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada.
authors.forename: Karen K W
authors.initials: KK
authors.lastname: Siu
authors.forename: Kyle
authors.initials: K
authors.lastname: Asmus
authors.forename: Allison N
authors.initials: AN
authors.lastname: Zhang
authors.forename: Cathy
authors.initials: C
authors.lastname: Horvatin
authors.forename: Sheng
authors.initials: S
authors.lastname: Li
authors.forename: Tong
authors.initials: T
authors.lastname: Liu
authors.forename: Barbara
authors.initials: B
authors.lastname: Moffatt
authors.forename: Virgil L
authors.initials: VL
authors.lastname: Woods
authors.forename: P Lynne
authors.initials: PL
authors.lastname: Howell
mesh_terms: D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites / Q000235:genetics; D002151:Calorimetry; D002384:Catalysis; D003460:Crystallization; D003839:Deoxyadenosines / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D006868:Hydrolysis; D008958:Models, Molecular; D015394:Molecular Structure; D009699:N-Glycosyl Hydrolases / Q000235:genetics / Q000378:metabolism*; D011487:Protein Conformation; D011683:Purine-Nucleoside Phosphorylase / Q000235:genetics / Q000378:metabolism*; D012435:S-Adenosylhomocysteine / Q000378:metabolism*; D013379:Substrate Specificity; D013816:Thermodynamics; D013872:Thionucleosides / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D003839:Deoxyadenosines; D029968:Escherichia coli Proteins; D013872:Thionucleosides; C008500:5'-methylthioadenosine; D012435:S-Adenosylhomocysteine; D011683:Purine-Nucleoside Phosphorylase; C540168:MTAN1 protein, Arabidopsis; D009699:N-Glycosyl Hydrolases; C505576:5'-methylthioadenosine nucleosidase, E coli