Bovine glutamate dehydrogenase immobilization on magnetic nanoparticles: conformational changes and catalysis†
Glutamate dehydrogenase (GDH) is a well known homohexameric enzyme and its performance in immobilized form was systematically investigated in this work, in order to provide a better understanding of the multimeric enzyme immobilization effects in relation to the monomeric ones. For this purpose, GDH was immobilized on four different magnetic supports and the outcome from such immobilization was characterized in terms of their stabilization and activity. Immobilization procedures involving amine coupling via glutaraldehyde cross-linking yielded the least stable ones, even lower than free GDH, showing no recoverability. However, the immobilization procedures using larger aldehyde cross-linkers presented higher thermal stabilities than free GDH and could be recycled at least 10 times, with a nearly constant activity. Such differences in stability and activity were thoroughly evaluated in terms of the enzymatic structure, which has guided the reasoning behind the intriguing allosteric behavior of the immobilized GDH. The atypical allosteric response exhibited by MagNP–APTS/GDH, MagNP@SiO2–APTS/GDH and MagNP–APTS/glyoxyl-agarose/GDH led us to invoke the intrinsic disorder theory to explain the results. This theory has proved to be an excellent tool to guide research on immobilized enzymes and understand its effects.