A look at the face of the molten globule: Structural model of the Helicobacter pylori apoflavodoxin ensemble at acidic pH

Molten globule (MG) is the name given to a compact, non-native conformation of proteins that has stimulated the imagination and work in the protein folding field for more than 40 years. The MG has been proposed to play a central role in the folding reaction and in important cell functions, and to be related to the onset of misfolding diseases. Due to its inherent intractability to high-resolution studies, atomistic structural models have not yet been obtained. We present here an integrative atomistic model of the MG formed at acidic pH by the apoflavodoxin from the human pathogen Helicobacter pylori. This MG has been previously shown to exhibit the archetypical expansion, spectroscopic and thermodynamic features of a molten conformation. To obtain the model, we have analyzed the stability of wild-type and 55 apoflavodoxin mutants to derive experimental equilibrium phi values that have been used in biased molecular dynamics simulations to convert the native conformation into an MG ensemble. The ensemble has been refined to reproduce the experimental hydrodynamic radius and circular dichroism (CD) spectrum. The refined ensemble, deposited in PDB-Dev, successfully explains the characteristic H-1-nuclear magnetic resonance (NMR) and near-UV CD spectral features of the MG as well as its solvent-accessible surface area (SASA) change upon unfolding. This integrative model of an MG will help to understand the energetics and roles of these elusive conformations in protein folding and misfolding. Interestingly, the apoflavodoxin MG is structurally unrelated to previously described partly unfolded conformations of this protein, exemplifying that equilibrium MGs need not to reflect the properties of kinetic intermediates.