Number of ways of joining SH groups to form multi-peptide chain proteins.

The expression for the total number of isomeric structures formed upon oxidation of SH groups has previously been correctly obtained for single chain proteins only. Expressions have now been obtained for molecules consisting of 2 and 4 peptide chains of the types A2 and A2B2. The latter type is particularly important as exemplified by the immunoglobulins and the insulin receptor. For the oxidation of insulin A chain with 4 SH groups, the total number of isomeric A2 structures is 59--this is different to all the values previously reported. Lack of consideration of symmetry problems probably accounts for the erroneous results obtained by earlier workers for the number of ways of randomly joining two identical chains. The total number of isomeric structures formed from the oxidation of two light and two heavy chains with 5 and 11 SH groups respectively of the human immunoglobulin GI has been found to be 4.8 X 10(16).