The effect of methylation on cytochrome c fragment complementation.

The heme propionates of heme peptide 1-25 from horse heart cytochrome c were methylated in acidified methanol. The methylated heme peptide combines with apocytochrome c in a single second order kinetic phase whose rate constant is within a factor of four of that measured for the unmethylated heme peptide. The dissociation constants for the two complexes are also within a factor of four. The methylated and unmethylated complexes exhibit similar fluorescence quenching, far ultraviolet dichroic spectra, and catalytic activities. The methylated complex has a Tm about 10 degrees C lower than that of the unmethylated complex. The charge transfer band of the methylated complex occurs at 720 nm instead of 695 nm as observed for the unmethylated complex. It is proposed that methylation of the outer heme propionate causes a small steric effect which alters the geometry of the methionine 80 ligation with the heme iron.