The rate of macromolecular surface formation in
yeast iso-2 cytochrome c and its site-specific
mutant, N52I iso-2, has been studied using a monoclonal
antibody that recognizes a tertiary epitope including K58
and H39. The results indicate that epitope refolding occurs
after fast folding but prior to slow folding, in contrast
to horse cytochrome c where surface formation
occurs early. The antibody-detected (ad) kinetic phase
accompanying epitope formation has kad
= 0.2 s−1 and is ∼40-fold slower than
the fastest detectable event in the folding of yeast iso-2
cytochrome c (k2f ∼ 8
s−1), but occurs prior to the absorbance-
and fluorescence-detected slow folding steps (k1a
∼ 0.06 s−1; k1b
∼ 0.09 s−1). N52I iso-2 cytochrome
c exhibits similar kinetic behavior with respect
to epitope formation. A detailed dissection of the mechanistic
differences between the folding pathways of horse and yeast
cytochromes c identifies possible reasons for
the slow surface formation in the latter. Our results suggest
that non-native ligation involving H33 or H39 during refolding
may slow down the formation of the tertiary epitope in
iso-2 cytochrome c. This study illustrates that
surface formation can be coupled to early events in protein
folding. Thus, the rate of macromolecular surface formation
is fine tuned by the residues that make up the surface
and the interactions they entertain during refolding.