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 labeli ng proteins, Using high-resolution X-ray crystallographic techniques, the c hemical and three-dimensional structures of iodotyrosines formed by non-enz ymatic incorporation of I atoms into tyrosine residues of a crystalline pro tein are described. Acetylxylan esterase (AXE II; 207 amino-acid residues) from Penicillium purpurogenum has substrate specificities towards acetate e sters of D-xylopyranose residues in xylan and belongs to a new class of alp ha/beta hydrolases. The crystals of the enzyme are highly ordered, tightly packed and diffract to better than sub-angstrom resolution at 85 K. The iod ination technique has been utilized to prepare an isomorphous derivative of the AXE II crystal. The structure of the enzyme determined at 1.10 Angstro m resolution exclusively by normal and anomalous scattering from I atoms, a long with the structure of the iodinated complex at 1.80 Angstrom resolutio n, 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.