Pathway of Oxidative Folding of Bovine α-Interferon: Predominance of Native Disulfide-Bonded Folding Intermediates

Diverse pathways of oxidative folding of disulfide proteins: underlying causes and folding models

The pathway of oxidative folding of disulfide proteins exhibits a high degree of diversity, which is manifested mainly by distinct structural heterogeneity and diverse rearrangement pathways of folding intermediates. During the past two decades, the scope of this diversity has widened through studies of more than 30 disulfide-rich proteins by various laboratories. A more comprehensive landscape of the mechanism of protein oxidative folding has emerged. This review will cover three themes. (1) Elaboration of the scope of diversity of disulfide folding pathways, including the two opposite extreme models, represented by bovine pancreatic trypsin inhibitor (BPTI) and hirudin. (2) Demonstration of experimental evidence accounting for the underlying mechanism of the folding diversity. (3) Discussion of the convergence between the extreme models of oxidative folding and models of conventional conformational folding (framework model, hydrophobic collapse model).

Diselenides as universal oxidative folding catalysts of diverse proteins

Small-molecule diselenides show considerable potential as catalysts of oxidative protein folding. To explore their scope, diselenide-containing redox buffers were used to promote the folding of proteins that varied in properties such as size, overall tertiary structure, number of disulfide bonds, pI value, and difficulty of in vitro folding. Diselenides are able to catalyze the oxidative folding of all proteins tested, providing significant increases in both rate and yield relative to analogous disulfides. Compared to the disulfide-linked dimer of glutathione (the most commonly used oxidant for in vitro protein folding), selenoglutathione provided markedly improved efficiencies in the folding of biotechnologically important proteins such as hirudin, lysozyme, human epidermal growth factor and interferon α-2a. Selenoglutathione also enhances the renaturation of more challenging targets such as bovine serum albumin, whose native state contains 17 disulfide bonds, and the Fab fragment of an antibody. In the latter case, micromolar amounts of selenoglutathione are able to match the modest yield provided by a previously optimized redox buffer, which contains millimolar levels of glutathione. Taken together, the folding reactions of these diverse proteins exemplify the advantages and limitations of diselenide catalysts.

How general is the nucleation-condensation mechanism?

We investigate the structures of the major folding transition states of nine proteins by correlation of published Phi-values with inter-residue contact maps. Combined with previous studies on six proteins, the analysis suggests that at least 10 of the 15 small globular proteins fold via a nucleation-condensation mechanism with a concurrent build-up of secondary and tertiary structure contacts, but a structural consolidation that is clearly nonuniformly distributed over the molecule and most intense in a single structural region suggesting the occurrence of a single folding nucleus. However, on average helix- and sheet-forming residues show somewhat larger Phi-values in the major transition state, suggesting that secondary structure formation is one important driving force in the nucleation-condensation in many proteins and that secondary-structure forming residues tend to be more prominent in folding nuclei. We synthesize the combined information on these 10 of 15 proteins into a unified nucleation-condensation mechanism which also accounts for effects described by the framework, hydrophobic collapse, zipper, and funnel models.

Diversity of folding pathways and folding models of disulfide proteins

Comprehensive understanding of the mechanism of protein folding requires the elucidation of both a folding pathway and a folding model. This entails characterization of the properties and structures of folding intermediates populated along the folding pathway, as well as the formation and interplay of secondary structures and tertiary structures along the course of folding. Using the conventional unfolding-refolding technique, there are limitations of acquiring these data in detail because of the inherent difficulty of trapping and analysis of folding intermediates. The technique of oxidative folding, in contrast, permits trapping, isolation, and further structural characterization of folding intermediates at any stage of the folding process. In this brief review, we present the potential of the technique of oxidative folding for concurrent analysis of both folding pathways and folding models.