Time-resolved Small-angle X-ray Scattering Investigation of the Folding Dynamics of Heme Oxygenase: Implication of the Scaling Relationship for the Submillisecond Intermediates of Protein Folding

Polypeptide collapse is generally observed as the initial folding dynamics of proteins with more than 100 residues, and is suggested to be caused by the coil–globule transition explained by Floryʹs theory of polymers. To support the suggestion by establishing a scaling behavior between radius of gyration (Rg) and chain length for the initial folding intermediates, the folding dynamics of heme oxygenase (HO) was characterized by time-resolved, small-angle X-ray scattering. HO is a highly helical protein without disulfide bridges, and is the largest protein (263 residues) characterized by the method. The folding process of HO was found to contain a transient oligomerization; however, the conformation within 10 ms was demonstrated to be monomeric and to possess Rg of 26.1(±1.1) Å. Together with the corresponding data for proteins with different chain lengths, the seven Rg values demonstrated the scaling relationship to chain length with a scaling exponent of 0.35±0.11, which is close to the theoretical value of 1/3 predicted for globules in solutions where monomer–monomer interactions are favored over monomer–solvent interactions (poor solvent). The finding indicated that the initial folding dynamics of proteins bears the signature of the coil–globule transition, and offers a clue to explain the folding mechanisms of proteins with different chain lengths.