Structure and Function of S-Adenosylmethionine Synthetase: Crystal Structures of S-Adenosylmethionine Synthetase with ADP, BrADP, and PPi at 2.8 Å Resolution

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• 作者：Fusao Takusagawa ; Shigehiro Kamitori ; and George D. Markham
• 刊名：Biochemistry
• 出版年：1996
• 出版时间：February 27, 1996
• 年：1996
• 卷：35
• 期：8
• 页码：2586 - 2596
• 全文大小：655K
• 年卷期：v.35,no.8(February 27, 1996)
• ISSN：1520-4995

文摘

S-Adenosylmethionine synthetase (MAT,ATP:L-methionine S-adenosyltransferase, EC2.5.1.6)plays a central metabolic role in all organisms. MAT catalyzes thetwo-step reaction which synthesizesS-adenosylmethionine (AdoMet), pyrophosphate(PP_i), and orthophosphate (P_i) from ATP andL-methionine.AdoMet is the primary methyl group donor in biological systems.The first crystal structure of MATfrom Escherichia coli has recently been determined[Takusagawa et al. (1995) J. Biol.Chem. 271, 136-147]. In order to elucidate the active site and possible catalyticreaction mechanism, the MAT structuresin the crystals grown with the substrate ATP (and BrATP) and theproduct PP_i have been determined(space group P6₂22; unit cell a= b = 128.9 Å, c = 139.8 Å, resolutionlimit 2.8 Å; R 0.19; R_free0.26).The enzyme consists of four identical subunits; two subunits forma spherical dimer, and pairs of thesetightly bound dimers form a tetrameric enzyme. Each dimer has twoactive sites which are located betweenthe subunits. Each subunit consists of three domains related toeach other by a pseudo 3-fold symmetry.The crystal structures showed that the ATP molecules werehydrolyzed to ADP and P_i by the enzyme.Those products were found at the active site along with theessential metal ions (K⁺ and Mg²⁺).Thisrather unexpected finding was first confirmed by the structure of thecomplex with PP_i and later by anHPLC analysis. The enzyme hydrolyzed ATP to ADP and P_iin 72 h under the same conditions as thecrystallization of the enzyme. In the active site, the diphosphatemoiety of ADP and P_i interacts extensivelywith the amino acid residues from the two subunits of the enzyme,whereas the adenine and ribose moietieshave little interaction with the enzyme. The enzyme structure islittle changed upon binding ADP. Allamino acid residues involved in the active site are found to beconserved in the 14 reported sequences ofMAT from a wide range of organisms. Thus the structure determinedin this study can be utilized as amodel for other members of the MAT family. On the basis of thecrystal structures, the catalytic reactionmechanisms of AdoMet formation and hydrolysis of tripolyphosphate areproposed.