The Apomyoglobin Folding Pathway Revisited: Structural Heterogeneity in the Kinetic Burst Phase Intermediate

Extensive analysis of accurate quench-flow hydrogen exchange results indicates that the burst phase kinetic intermediate in the folding of apomyoglobin (apoMb) from urea is structurally heterogeneous. The structural variability is associated with the partial folding of the E helix during the burst phase (<6.4 ms) of the folding process. Analysis of the effects of exchange-out of amide proton labels during the labeling pulse (∼pH 10) of the quench-flow process indicates that three of the amide protons in the E helix are in fact largely protected in the burst phase of folding, while the remainder of the E helix has a substantial complement of amide protons that show biphasic kinetics, i.e. are protected partly during the burst phase and partly during the slow phase of folding. The locations of these amide protons can be used to map the sites of structural heterogeneity in the kinetic molten globule. These sites include, besides the E helix, the ends of the A and B helices and part of the C helix. Our results give significant support to the hypothesis that the kinetic molten globule intermediate of apoMb is native-like.