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 methyltransf erase M.TaqI with the inhibitor Sinefungin and the reaction product S- adenosyl-L-homocysteine were determined, both at 2.6 Angstrom, resolut ion. Structural comparison of these binary complexes with the complex formed by M.TaqI and the cofactor S-adenosyl-L-methionine suggests tha t the key element for molecular recognition of these ligands is the bi nding of their adenosine part in a pocket, and discrimination between cofactor, reaction product and inhibitor is mediated by different conf ormations of these molecules; the methionine part of S-adenosyl-L-meth ionine is located in the binding cleft, whereas the amino acid moietie s of Sinefungin and S-adenosyl-L-homocysteine are in a different orien tation and interact with the active site amino acid residues (NPPY.108 )-N-105 Dissociation constants for the complexes of M.TaqI with the th ree ligands were determined spectrofluorometrically . Sinefungin binds more strongly than S-adenosyl-L-homocysteine or S-adenosyl-L-methioni ne, with K-D=0.34 mu M, 2.4 mu M and 2.0 mu M, respectively. (C) 1997 Academic Press Limited