Unfolding and refolding occur much faster for a proline-free proteins than for most proline-containing proteins.

Abstract

The kinetics for unfolding and refolding of a parvalbumin [carp white muscle] (band 5) were examined as a function of pH near the transition region, using stopped-flow techniques. This protein is rather unusual in that it has no proline residues, and therefore serves as a good example to test the hypothesis that the rate-limiting step seen in denaturation reactions is due to the cis-trans isomerization of proline peptide bonds in the denatured state. The kinetics for parvalbumin unfolding and refolding are complex, with the data being resolvable into 2 fast phases at 25.degree. C. The slower of the 2 phases seen for the parvalbumin is about 100-500 times faster than the slow phase seen for proline-containing proteins under the same conditions! These results argue strongly in support of the proline isomerization hypothesis. The slower phase seen for parvalbumin and the 2nd-slowest phase seen for proline-containing proteins might be due to the cis-trans isomerization of peptide bonds of non-proline residues.