A major kinetic trap for the oxidative folding of human epidermal growth factor

The folding pathway of human epidermal growth factor (EGF) has been characterized by structural and kinetic analysis of the acid-trapped folding intermediates. Oxidative folding of the fully reduced EGF proceeds through 1-disulfide intermediates and accumulates rapidly as a single stable 2-disulfide intermediate (designated as EGF-II), which represents up to more than 85% of the total protein along the folding pathway. Among the five 1-disulfide intermediates that have been structurally characterized, only one is native, and nearly all of them are bridges by neighboring cysteines. Extensive accumulation of EGF-II indicates that it accounts for the major kinetic trap of EGF folding. EGF-II contains two of the three native disulfide bonds of EGF, Cys(14)-Cys(31) and Cys(33)-Cys(42). However, formation of the third native disulfide (Cys(6)-Cys(20)) for EGF-II is slow and does not occur directly. Kinetic analysis reveals that an important route for EGF-II to reach the native structure is via rearrangement pathway through 3-disulfide scrambled isomers. The pathway of EGF-II to attain the native structure differs from that of three major 2-disulfide intermediates of bovine pancreatic trypsin inhibitor (BPTI). The dissimilarities of folding mechanism(s) between EGF, BPTI, and hirudin are discussed in this paper.