Crystal structure of YecO from Haemophilus influenzae (HI0319) reveals a methyltransferase fold and a bound S-adenosylhomocysteine

The crystal structure of YecO from Haemophilus influenzae (HI0319), a prote in annotated in the sequence databases as hypothetical, and that has not be en assigned a function, has been determined at 2.2-Angstrom resolution. The structure reveals a fold typical of S-adenosyl-L-methionine-dependent (Ado Met) methyltransferase enzymes. Moreover, a processed cofactor, S-adenosyl- L-homo-cysteine (AdoHcy), is bound to the enzyme, further confirming the bi ochemical function of HI0319 and its sequence family members. An active sit e arginine, shielded from bulk solvent, interacts with an anion, possibly a chloride ion, which in turn interacts with the sulfur atom of AdoHcy. The AdoHcy and nearby protein residues delineate a small solvent-excluded subst rate binding cavity of 162 Angstrom (3) in volume. The environment surround ing the cavity indicates that the substrate molecule contains a hydrophobic moiety and an anionic group. Many of the residues that define the cavity a re invariant in the HI0319 sequence family but are not conserved in other m ethyltransferases. Therefore, the substrate specificity of YecO enzymes is unique and differs from the substrate specificity of all other methyltransf erases sequenced to date. Examination of the Enzyme Commission list of meth yltransferases prompted a manual inspection of 10 possible substrates using computer graphics and suggested that the ortho-substituted benzoic acids f it best in the active site. (C) 2001 Wiley-Liss, Inc.