EARLY STEPS IN CYTOCHROME-C FOLDING PROBED BY TIME-RESOLVED CIRCULAR-DICHROISM AND FLUORESCENCE SPECTROSCOPY

The kinetics of protein folding for horse ferricytochrome c was investigated by stopped-flow methods, using far-UV circular dichroism (CD), near-UV CD, and tryptophan fluorescence to probe the formation of secondary structure and tertiary interactions. In the far-UV region of the CD spectrum (222 nm), 44% of the total change associated with refolding occurs within the dead time of the stopped-flow experiment, indicating that a significant amount of helical secondary structure is formed in less than 4 ms. The remaining changes in the ellipticity at 222 nm occur in two kinetic phases with time constants of about 40 ms and 0.7 s, respectively. In contrast, there is no evidence for rapid changes in the ellipticity at 289 nm: an aromatic CD band, which is indicative of the formation of a tightly packed core, only begins to appear in a 400-ms step and is completed in a final 10-s phase. The fluorescence of a single tryptophan at position 59, which becomes quenched upon folding via nonradiative energy transfer to the heme group, provides complementary information on the condensation of the polypeptide chain during refolding. The fluorescence-detected stopped-flow folding kinetics of ferricytochrome c exhibits a 35% decrease in fluorescence during the dead time, suggesting that a substantial decrease in the average tryptophan-heme distance occurs on a submillisecond time scale. The subsequent fluorescence changes exhibit two prominent phases with time constants of about 20 and 300 ms, followed by a minor 5-s phase. Transient peptide CD spectra measured at different folding times (4 ms to 5 s) show no evidence for non-native elements of secondary structure at any stage of folding. Together with previous pulsed amide proton exchange data measured under identical folding conditions [Roder, Elove, & Englander (1988) Nature 335, 700-704], the results suggest that during the early stages of folding (<4 ms), a partially condensed intermediate with a fluctuating core is formed that contains a significant amount of helical secondary structure but no stable hydrogen bonds. A second intermediate, populated on the 20-100-ms time scale, exhibits stable hydrogen-bonded structure in two helical segments near the chain termini and increased compactness, but the aromatic side chains are still in a dynamic or exposed environment. Other helical segments contribute to the helical character of the far-UV CD spectrum but lack sufficiently stable hydrogen bonding to provide protection against NH exchange.