AMIDE PROTON-EXCHANGE RATES OF OXIDIZED AND REDUCED SACCHAROMYCES-CEREVISIAE ISO-1-CYTOCHROME-C

Proton NMR spectroscopy was used to determine the rate constant, k(obs), for exchange of labile protons in both oxidized (Fe(III)) and reduced (Fe(II)) iso-1-cytochrome c. We find that slowly exchanging backbone amide protons tend to lack solvent-accessible surface area, possess backbone hydrogen bonds, and are present in regions of regular secondary structure as well as in OMEGA-loops. Furthermore, there is no correlation between k(obs) and the distance from a backbone amide nitrogen to the nearest solvent-accessible atom. These observations are consistent with the local unfolding model. Comparisons of the free energy change for denaturation, DELTAG(d), at 298 K to the free energy change for local unfolding, DELTAG(op), at 298 K for the oxidized protein suggest that certain conformations possessing higher free energy than the denatured state are detected at equilibrium. Reduction of the protein results in a general increase in DELTAG(op). Comparisons of DELTAG(d) to DELTAG(op) for the reduced protein show that the most open states of the reduced protein possess more structure than its chemically denatured form. This persistent structure in high-energy conformations of the reduced form appears to involve the axially coordinated heme.