Differential binding of S-adenosylmethionine S-adenosylhomocysteine and Sinefungin to the adenine-specific DNA methyltransferase M.TaqI

The crystal structures of the binary complexes of the DNA methyltransferase M.TaqI with the inhibitor Sinefungin and the reaction product S-adenosyl-L-homocysteine were determined, both at 2.6 Angstrom, resolution. Structural comparison of these binary complexes with the complex formed by M.TaqI and the cofactor S-adenosyl-L-methionine suggests that the key element for molecular recognition of these ligands is the binding of their adenosine part in a pocket, and discrimination between cofactor, reaction product and inhibitor is mediated by different conformations of these molecules; the methionine part of S-adenosyl-L-methionine is located in the binding cleft, whereas the amino acid moieties of Sinefungin and S-adenosyl-L-homocysteine are in a different orientation and interact with the active site amino acid residues (NPPY.108)-N-105 Dissociation constants for the complexes of M.TaqI with the three ligands were determined spectrofluorometrically . Sinefungin binds more strongly than S-adenosyl-L-homocysteine or S-adenosyl-L-methionine, with K-D=0.34 mu M, 2.4 mu M and 2.0 mu M, respectively. (C) 1997 Academic Press Limited