Fast folding kinetics and stabilization of apo‐cytochrome c

It is generally accepted that in the c ‐type cytochromes the covalently bound heme plays a primary role in the acquisition of the folded state. Here, we show that a stabilized site‐directed variant of apo‐cyt c 551 from Pseudomonas aeruginosa ( Pa ‐apocyt F7A/W77F) retains native‐like features in the presence of sodium sulfate even in the absence of heme. By time‐resolved intrinsic fluorescence, we have evidence that Pa ‐apocyt F7A/W77F may acquire a compact, native‐like conformation within microseconds. These results challenge current thinking about the role of the heme group in the folding of c ‐type cytochromes.