Effect of varying polyglutamate chain length on the structure and stability of ferricytochrome c

The effect of varying polyglutamate chain length on local and global stability of horse heart ferricytochrome c was studied using scanning calorimetry and spectroscopy methods. Spectral data indicate that polyglutamate chain lengths equal or greater than eight monomer units significantly change the apparent pK(a) for the alkaline transition of cytochrome c. The change in pK(a) is comparable to the value when cytochrome c is complexed with cytochrome bc(1). Glutamate and diglutamate do not significantly alter the temperature transition for cleavage of the Met(80)-heme iron bond of cytochrome c. At low ionic strength, polyglutamates consisting of eight or more glutamate monomers increase midpoint of the temperature transition from 57.3 +/- 0.2 to 66.9 +/- 0.2 degreesC. On the other hand, the denaturation temperature of cytochrome c decreases from 85.2 +/- 0.2 to 68.8 +/- 0.2 degreesC in the presence of polyglutamates with number of glutamate monomers n greater than or similar to 8. The rate constant for cyanide binding to the heme iron of cytochrome c of cytochrome c-polyglutamate complex also decreases by similar to 42.5% with n greater than or similar to 8. The binding constant for the binding of octaglutamate (m.w. similar to 1000) to cyt c was found to be 1.15 X 10(5) M-1 at pH 8.0 and low ionic strength. The results indicate that the polyglutamate (n greater than or similar to 8) is able to increase the stability of the methionine sulfur-heme iron bond of cytochrome c in spite of structural differences that weaken the overall stability of the cyt c at neutral and slightly alkaline pH. (C) 2002 Elsevier Science B.V All rights reserved.