Location of normal and abnormal tyrosyl residues in ribonuclease

Iodination at pH 9.5 has converted three of the six tyrosyl residues of ribonuclease to 3,5-diiodotyrosyl residues. The iodinated ribonuclease is still enzymatically active toward ribonucleic acid. The diiodotyrosyl residues have approximately normal pK's, while the tyrosyl residues remaining have abnormally high pK's. Hence, iodination has taken place on the normal tyrosyl residues.

Two of the iodinated tyrosyl residues have been shown to be No. 115 and No.92; hence, these appear to be normal tyrosyl residues. One of the non-iodinated tyrosyl residues is No. 97, and hence is one of the abnormal tyrosyl residues. The location of the other normal tyrosyl residue (or the other two abnormal ones) remains uncertain.