Key role of phenylalanine 20 in cytochrome c(3): Structure, stability, andfunction studies

Aromatic residues in c-type cytochromes might have an important function in the folding and/or electron transferring properties of the molecule. In th e tetraheme cytochrome c(3) (M-r 13 000) from Desulfovibrio vulgaris Hilden borough, Phe20, is located between heme 1 and heme 3 with its aromatic ring close and almost parallel to the ring plane of heme 1. We replaced this re sidue by a nonaromatic hydrophobe residue, leucine, and analyzed the effect s in terms of functional, structural, and physicochemical properties. While the F20L replacement did not have any strong effects on the heme region st ability, a decrease of the thermostability of the whole molecule was observ ed, In the same way, the four macroscopic redox potentials were affected by the mutation as well as the flexibility of the surface loop around heme 4. The F20L replacement itself and/or this structural modification might be r esponsible for the loss of the intermolecular cooperativity between F20L cy tochrome cs molecules.