Role of ligand substitution in ferrocytochrome c Folding

The ligand substitutions that occur during the folding of ferrocytochrome c [Fe(II)cyt c] have been monitored by transient absorption spectroscopy. Th e folding reaction was triggered by photoinduced electron transfer to unfol ded Fe(III)cyt c in guanidine hydrochloride (GuHCl) solutions. Assignments of ligation states were made by reference to the; spectra of the imidazole and methionine adducts of N-acetylated microperoxidase 8. At pH 7, the heme in unfolded Fe(II)cyt c is ligated by native His18 and HisX (X = 26, 33) r esidues. The native Met80 ligand displaces HisX only in the last stages of folding. The ferroheme is predominantly five-coordinate in acidic solution; it remains five-coordinate until the native methionine binds the heme to g ive the folded protein (the rate of the methionine binding step is 16 +/- 5 s(-1) at pH 5, 3.2 M GuHCl). The evidence suggests that the substitution o f histidine by methionine is strongly coupled to backbone folding.