Residue-Specific Kinetic Insights into the Transition State in Slow Polypeptide Topological Isomerization by NMR Exchange Spectroscopy

The
characterization of the transition state is a central issue
in biophysical studies of protein folding. NMR is a multiprobe measurement
technique that provides residue-specific information. Here, we used
exchange spectroscopy to characterize the transition state of the
two-state slow topological isomerization of a 27-residue lantibiotic
peptide. The exchange kinetic rates varied on a per-residue basis,
indicating the reduced kinetic cooperativity of the two-state exchange,
as well as the previously observed reduced thermodynamic cooperativity.
Furthermore, temperature-dependent measurements revealed large variations
in the activation enthalpy and entropy terms among residues. Interestingly,
we found a linear relationship between the logarithm of the equilibrium
constants and that of the exchange rates. Because the data points
are derived from amino acid residues in one polypeptide chain, we
refer to the linear relationship as the residue-based linear free
energy relationship (rbLFER). The rbLFER offers information about
the transition state of the two-state exchange.