Fast folding of cytochromec

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Abstract
Native iso-2 cytochrome c contains two residues (His 18, Met 80) coordinated to the covalently attached heme. On unfolding of iso-2, the His 18 ligand remains coordinated to the heme iron, whereas Met 80 is displaced by a non-native heme ligand, His 33 or His 39. To test whether non-native His-heme ligation slows folding, we have constructed a double mutant protein in which the non-native ligands are replaced by asparagine and lysine, respectively (H33N,H39K iso-2). The double mutant protein, which cannot form non-native histidine-heme coordinate bonds, folds significantly faster than normal iso-2 cytochrome c: t = 14-26 ms for H33N,H39K iso-2 versus r = 200-1,100 ms for iso-2. These results with iso-2 cytochrome c strongly support the hypothesis that non-native His-heme ligation results in a kinetic barrier to fast folding of cytochrome c. Assuming that the maximum rate of a conformational search is about 10″ s1, the results imply that the direct folding pathway of iso-2 involves passage through on the order of 109 or fewer partially folded conformers.
Funding Information
  • National Institute of General Medical Sciences (GM32980)
  • National Center for Research Resources (RR05043)
  • Robert A. Welch Foundation (AQ838)

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