Dimeric enzyme IImtl of the E. coli phosphoenolpyruvate‐dependent phosphotransferase system

Abstract

The occurrence of intermolecular dithiols on EII^mtl has been studied with a number of thiol-specific crosslinking reagents. The reaction of EII^mtl with bifunctional maleimide derivatives inactivates the enzyme. At the same time the enzyme is irreversibly cross-linked to a dimeric species. Under optimal conditions 50% of the protein is cross-linked upon reaction with the dimaleimides. The enzyme is also cross-linked under oxidizing conditions in the presence of CuCl₂, presumably by oxidizing an intermolecular dithiol to a disulfide. This oxidation can be reversed by the addition of the reducing agent dithiothreitol. The reaction of phosphorylated EII^mtl with the same sulfhydryl-specific bifunctional reagents does not lead to any crosslinked product. The results are discussed in terms of the association state of the purified protein and the distribution of its thiol groups.