CRYSTAL-STRUCTURE OF S-ADENOSYLMETHIONINE SYNTHETASE

The structure of S-adenosylmethionine synthetase (MAT, ATP:L-methionin e S-adenosyltransferase, EC 2.5.1.6.) from Escherichia coli has been d etermined at 3.0 Angstrom resolution by multiple isomorphous replaceme nt using a uranium derivative and the selenomethionine form of the enz yme (SeMAT). The SeMAT data (9 selenomethionine residues out of 383 am ino acid residues) have been found to have a sufficient phasing power to determine the structure of the 42,000 molecular weight protein by c ombining them with the other heavy atom derivative data (multiple isom orphous replacement). The enzyme consists of four identical subunits; two subunits form a spherical tight dimer, and pairs of these dimers f orm a peanut-shaped tetrameric enzyme. Each pair dimer has two active sites which are located between the subunits. Each subunit consists of three domains that are related to each other by pseudo-3-fold symmetr y. The essential divalent (Mg2+/Co2+) and monovalent (K+) metal ions a nd one of the product, P-i ions, were found in the active site from th ree separate structures.