A new assignment of the disulfide linkage in stellacyanin.

Which cysteine of apostellacyanine reacts with 4-vinylpyridine in 6 M guanidine-HC1 depends upon the pH. We infer that a disulfide switch occurs at higher pH and that in the native protein the disulfide bridge occurs between Cys-59 and Cys-93. Thus Cys-87, which is homologous to the cysteine ligands of azurin and plastocyanin, is available to bind copper.