Cytochrome b(562) folding triggered by electron transfer: Approaching the speed limit for formation of a four-helix-bundle protein

Ferrocytochrome b(562) [Fe(II)cyt b(562)] folding can be triggered by photoinduced electron transfer to unfolded Fe(III)cyt b(562) in 2–3 M guanidine hydrochloride solutions. The folding rates increase with decreasing guanidine hydrochloride; the extrapolated time constant for this folding process in the absence of denaturant (5 μs) is near the predicted value for intrachain diffusion. The relatively smooth energy landscape indicated for Fe(II)cyt b(562) folding accords with the helical, highly symmetrical structure of the protein.