Differences between the sequential collapse folding pathways of apoleghemoglobin and apomyoglobin

The sequential collapse model (SCM) is applied to reveal the folding pathway of apoleghemoglobin, and the results are compared to the folding pathway of apomyoglobin, a structurally similar protein. The folding pathway of apoleghemoglobin is found to differ significantly from that calculated for apomyoglobin by the SCM in previous work. There are two energetically equivalent initial contacts leading to two distinct folding pathways. One of the possible initial contacts is predicted to form between segments in helices E and H. The other is predicted to form between a segment in the region connecting helices C and E and helix G. In apomyoglobin the dominant initial contact was predicted to form between segments in helices B and G. The predicted differences are shown to be in general agreement with experimental results. The observed differences between the folding pathways of both proteins confirm experimental and theoretical observations that folding pathways are not evolutionarily conserved and that a significant degree of structural homology does not necessarily imply similar folding pathways for different proteins. This result suggests that several folding pathways are available for a given fold and that the dominant one for each protein is a function of the sequence and the experimental conditions.