Determination of a protein structure by iodination:: the structure of iodinated acetylxylan esterase

Enzymatic and non-enzymatic iodination of the amino acid tyrosine is a well known phenomenon. The iodination technique has been widely used for labeling proteins, Using high-resolution X-ray crystallographic techniques, the chemical and three-dimensional structures of iodotyrosines formed by non-enzymatic incorporation of I atoms into tyrosine residues of a crystalline protein are described. Acetylxylan esterase (AXE II; 207 amino-acid residues) from Penicillium purpurogenum has substrate specificities towards acetate esters of D-xylopyranose residues in xylan and belongs to a new class of alpha/beta hydrolases. The crystals of the enzyme are highly ordered, tightly packed and diffract to better than sub-angstrom resolution at 85 K. The iodination technique has been utilized to prepare an isomorphous derivative of the AXE II crystal. The structure of the enzyme determined at 1.10 Angstrom resolution exclusively by normal and anomalous scattering from I atoms, along with the structure of the iodinated complex at 1.80 Angstrom resolution, demonstrate the formation of covalent bonds between I atoms and C atoms at ortho positions to the hydroxyl groups of two tyrosyl moieties, yielding iodotyrosines.