STRUCTURE AND FUNCTION OF S-ADENOSYLMETHIONINE SYNTHETASE - CRYSTAL-STRUCTURES OF S-ADENOSYLMETHIONINE SYNTHETASE WITH ADP, BRADP, AND PPI AT 2.8 ANGSTROM RESOLUTION

S-Adenosylmethionine synthetase (MAT, ATP:L-methionine S-adenosyltrans ferase, EC 2.5.1.6) plays a central metabolic role in all organisms. M AT catalyzes the two-step reaction which synthesizes S-adenosylmethion ine (AdoMet), pyrophosphate (PPi), and orthophosphate (P-i) from ATP a nd L-methionine. AdoMet is the primary methyl group donor in biologica l systems. The first crystal structure of MAT from Escherichia coli ha s recently been determined [Takusagawa et al. (1995) J. Biol. Chem. 27 1, 136-147]. In order to elucidate the active site and possible cataly tic reaction mechanism, the M.4T structures in the crystals grown with the substrate ATP (and BrATP) and the product PPi have been determine d (space group P6(2)22; unit cell a = b = 128.9 Angstrom, c = 139.8 An gstrom, resolution limit 2.8 Angstrom; R 0.19; R(free) 0.26). The enzy me consists of four identical subunits; two subunits form a spherical dimer, and pairs of these tightly bound dimers form a tetrameric enzym e. Each dimer has two active sites which are located between the subun its, Each subunit consists of three domains related to each other by a pseudo 3-fold symmetry. The crystal structures showed that the ATP mo lecules were hydrolyzed to ADP and P-i b4 the enzyme, Those products w ere found at the active site along with the essential metal ions (K+ a nd Mg2+). This rather unexpected finding was first confirmed by the st ructure of the complex with PPi and later by an HPLC analysis, The enz yme hydrolyzed ATP to ADP and P-i in 72 h under the same conditions as the crystallization of the enzyme. In the active site, the diphosphat e moiety of ADP and P-i interacts extensively with the amino acid resi dues from the two subunits of the enzyme, whereas the adenine and ribo se moieties have little interaction with the enzyme. The enzyme struct ure is little changed upon binding ADP. All amino acid residues involv ed in the active site are found to be conserved in the 14 reported seq uences of MAT from a wide range of organisms. Thus the structure deter mined in this study can be utilized as a model for other members of th e MAT family. On the basis of the crystal structures, the catalytic re action mechanisms of AdoMet formation and hydrolysis of tripolyphospha te are proposed.