DESTABILIZATION OF THE HEME REGION IN MUTANT CYTOCHROME-C BY REPLACEMENT OF PHE-82 WITH 3-(PYREN-1-YL)-L-ALANINE

Using a semisynthetic method, mutant cytochromes c, in which 3-(pyren-1-yl)-L-alanine (PyrAla(82)) or L-phenylglycine (PGL(82)) replaced a conserved phenylalanine residue (Phe-82) located near the heme, were produced. The PyrAla residue in the reduced state of the mutant cytochrome c was shielded from the access of acrylamide in the aqueous phase, while it is exposed to acrylamide in the oxidized state. The denaturation profile of the mutant protein induced by the addition of GdnHCI was the same as that of the native cytochrome c. Therefore, the hydrophobic heme pocket should retain the native structure in terms of accessibility of acrylamide and GdnHCI. However, thermal denaturation of [PyrAla(82)] cytochrome c showed that the deformation of the heme region occurred at 54 degrees C, which was lower than the value of 64 degrees C for the native one. Thermodynamic analysis revealed that the heme pocket was stabilized by the hydrophobic effect of PyrAla. However, the hydrophobic stabilization is overwhelmed by a large entropy change upon denaturation, resulting in destabilization of the heme region. [PGL(82)]cyctochrome c, in which the phenyl group is dislocated by one methylene unit from [Phe-82]cytochrome c, was also destabilized in the heme region compared with the native cytochrome c, primarily duet decrease of the hydrophobic effect.