What NAD-Dependent Dehydrogenases Teach Us About the Folding and Association of Oligomeric Proteins

Abstract

The term “protein folding” is commonly used to refer both to the description of the spatial arrangement of the amino acid residues in a functional protein and to the (kinetic) mechanism by which the covaient polypeptide chain achieves its three-dimensional structure (Jaenicke 1980). In this article, we refer to the second alternative which may be considered the postlude of translation. The experimental approach makes use of the reversibility of protein denaturation, focusing on refolding in vitro rather than in vivo folding. The reason is that no direct approach is available to study the acquisition of the native structure of the nascent molecule either in vivo or in the cell-free system (Wetlaufer and Ristow 1973). Evidently the in vitro process must be different from the vectorial in vivo process because refolding starts from the denatured complete chain, while folding of the nascent chain may occur as a co-translational event (Hamlin and Zabin 1972; Bergman and Kuehl 1979). The question of whether refolding indeed reflects the in vivo process cannot be answered unambiguously. However, two criteria suggest that the two reactions follow similar pathways:(1) under certain experimental conditions the kinetics of reconstitution are in the same time range as the folding of nascent polypeptide chains in vivo; (2) the final product of reconstitution after denaturation-renaturation is found to be indistinguishable from the initial native state.