Modulation of the hydrophobicity of glutamine synthetase by mixed-function oxidation

Oxidative modification of Escherichia coli glutamine synthetase renders the enzyme susceptible to proteolytic degradation by a specific protease purified from the bacterium; native enzyme is not a substrate for the protease. A model oxidizing system consisting of ascorbate, iron, and oxygen was used to generate a series of glutamine synthetases of increasing oxidative modification. We assessed the effect of oxidative modification on the surface hydrophobicity of the glutamine synthetases, utilizing hydrophobic chromatography on a phenyl matrix. Initial exposure to the oxidizing system caused inactivation of the enzyme and generated a protein that was more hydrophilic than the native form; it was not a substrate for the protease. Continued exposure to the oxidizing system yielded a protein with additional oxidative modification. This form was distinctly more hydrophobic than the native form and it was very susceptible to proteolytic attack by the purified protease. Thus, oxidative modification modulates the surface hydrophobicity of glutamine synthetase, and this modulation can control susceptibility to proteolysis.