Cytochrome b562 folding triggered by electron transfer: approaching the speed limit for formation of a four-helix-bundle protein.

Ferrocytochrome b562 [Fe(II)cyt b562] folding can be triggered by photoinduced electron transfer to unfolded Fe(III)cyt b562 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 micros) is near the predicted value for intrachain diffusion. The relatively smooth energy landscape indicated for Fe(II)cyt b562 folding accords with the helical, highly symmetrical structure of the protein.