THERMODYNAMIC PROPERTIES OF THE UNFOLDED ENSEMBLE OF PROTEINS
Abstract
A random coil, whose size is determined by its excluded volume, and net energetic interactions with its environment, has served as a representation of the unfolded ensemble of proteins. The work in this thesis involves equilibrium, nuclear magnetic resonance and time-resolved kinetics spectroscopic studies on the unfolded ensemble of BBL, a globally downhill folding 40-residue protein involved the Krebs cycle of <italic>E. coli</italic>, in its acid-denatured state, and on a sequence-randomized version of this protein.
The effect of variability in thermodynamic conditions, such as temperature and the presence of added chaotropes or kosmotropes, on the equilibrium properties and reconfiguration dynamics of the unfolded state, have been deduced in the absence of competition with folding events at low pH. The unfolded ensemble experiences expansion and collapse to varying degrees in response to changes in these conditions. Individual interactions of residues of the protein with the solvent and the cosolvent (direct interactions), and the properties of the solution itself (indirect interactions) are together critical to the unfolded chain's properties and have been quantitatively estimated.
Unfolded, protonated BBL can be refolded by tuning the properties of the solvent by addition of kosmotropic salts. Electrostatic interactions turn out to be essential for folding cooperativity, while solvent-mediated changes in the hydrophobic effect can promote structure formation but cannot induce long-range thermodynamic connectivity in the protein. The effect of sequence on the properties of heteropolymers is also tested with a randomized version of BBL's sequence. Chain radii of gyration, and the degree and rate of hydrophobic collapse depend on the composition of the sequence, viz. hydrophilic versus hydrophobic content. However, the ability to maximize stabilizing interactions and adopt compact conformations is more evident in naturally selected protein sequences versus designed heteropolymers.
Chain reconfiguration of unfolded BBL takes place in ∼1/(100 ns), in agreement with theoretical estimates of homopolymer collapse rates. The refolding dynamics of salt-refolded BBL in the range of 1/(6 μs) at 320 K, emerge as being independent of the degree of folding or protonation of the chain, a result in keeping with the description of dynamics in BBL as oscillations in a single, smooth harmonic potential well, which only varies in its position and curvature with varying thermodynamic conditions.