Loops linking secondary structure elements affect the stability of the molten globule intermediate state of apomyoglobin.

Apomyoglobin is a widely used model for studying the molecular mechanisms of globular protein folding. This work aimed to analyze the effects of rigidity and length of loops linking protein secondary structure elements on the stability of the molten globule intermediate state. For this purpose, we studied folding/unfolding of mutant apomyoglobin forms with substitutions of loop-located proline residues to glycine and with loop extension by three or six glycine residues. The kinetic and equilibrium experiments performed gave an opportunity to calculate free energies of different apomyoglobin states. Our analysis revealed that the mutations introduced into the apomyoglobin loops have a noticeable effect on the stability of the intermediate state compared to the unfolded state.