Cytochrome c(553), a small heme protein that lacks misligation in its unfolded state, folds with rapid two-state kinetics

Cytochrome c(553) (cyt c(553)) from Desulfovibrio vulgaris is a small helic al heme protein that displays apparent two-state equilibrium-unfolding beha vior. The covalently attached heme is low-spin, ligated by Met and His resi dues, in the native state but becomes high-spin upon unfolding at pH 7. Her e, we show that in contrast to other c-type heme proteins, where misligatio ns in the unfolded states are prominent, cyt c(553) refolding kinetics at p H 7 proceeds rapidly without detectable intermediates. The extrapolated fol ding rate constant in water for oxidized cyt c(553) matches exactly that pr edicted from the cyt c(553) native-state topology: 5300 s(-1) (experimental ) versus 5020 s(-1) (predicted). We therefore conclude that the presence of the oxidized cofactor does not affect the intrinsic formation speed of the cyt C-553 structural motif. (C) 2000 Academic Press.