Refolding of a bifunctional enzyme and its monofunctional fragment.

The renaturation of the bifunctional enzyme aspartokinase II-homoserine dehydrogenase II has been studied by using the reappearance of its two activities. The same kinetics of renaturation are obtained for the dehydrogenase (EC 1.1.1.3) and the kinase activity (EC 2.7.2.4). The mechanism of refolding of the enzyme apparently involves two steps, a folding step occurring within a monomer and a subsequent dimerization step. The reappearance of the two activities depends on this dimerization step, suggesting that monomeric species are inactive. A proteolytic fragment possessing full dehydrogenase activity is shown to be able to renature, as judged by the recovery of its activity. In this case also, the refolding depends on the formation of dimeric species. However, the refolding of this fragment is much faster than that of the dehydrogenase region in the intact enzyme. These results suggest that, although the dehydrogenase region can refold by itself when isolated as a fragment, refolding of this same region in the whole protein involves interactions with the remainder of the protein.