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Spectral and electrochemical studies of cytochrome c peptide complexes






Journal of Biological Chemistry 253(9): 3265-3272

Spectral and electrochemical studies of cytochrome c peptide complexes

Hemepeptides 14-21, 11-21, 11-26, 1-38, 1-65, and 1-80 and non-hemepeptides 66-104, 67-104 (guanidinated) and 1-104 (apoprotein) were purified from horse heart cytochrome c. Addition of non-hemepeptide 1-104 to any of the ferrohemepeptides or addition of non-hemepeptides 66-104 or 67-104 to ferrohemepeptides 1-65 or 1-80 produces changes in the visible absorption spectrum of the ferrohemepeptides, indicating ligation of the S of methionine 80 in the non-hemepeptide with the heme Fe of the hemepeptide. The absorption spectra, circular dichroic spectra, formal potential and electron transfer capability of these peptide complexes were measured. Noncovalent peptide complexes 1-38:1-104, 1-65:1-104, and 1-80:1-104 and the covalent complex 1-65-66-104 exhibit the structural parameters and biological function characteristic of the native protein. These results suggest that the native tertiary structure can be generated from overlapping peptides. By contrast, noncovalent complexes 14-21:1-104, 11-21:1-104, 11-26:1-104, 1-65:67-104, and 1-80:66-104, while containing substantial secondary structure as judged by optical measurements, appear to have an exposed heme moiety in both oxidation states, resulting in a low formal potential and no biological function. Combination of ferrohemepeptide 1-65, having a homoserine lactone at position 65, with peptide 66-104, but not with peptide 67-104, produces a peptide linkage between the COOH terminus of the hemepeptide and the NH2 terminus of the non-hemepeptide. This comparison suggests that the peptides are precisely oriented in a noncovalent complex such that a peptide bond cannot be formed across the gap resulting from the absence of residue 66. Comparison of the properties of the noncovalent complex 1-65:66-104 with the covalent complex 1-65-66-104 indicates that, while both complexes in the ferro form have a native tertiary structure, the noncovalent complex in the ferri form is unable to stabilize a buried heme moiety or a methionine ligand. The noncovalent complex formed by hemepeptide 1-38 with non-hemepeptide 1-104 was studied in detail. The formation constants, KF,O and KF,R, for ligation of methionine 80 in the non-hemepeptide with the hemepeptide in both oxidation states are about 4 orders of magnitude greater than for ligation of methionine itself, with KF,R about 1 order of magnitude greater than KF,O. Analysis of the dependence of the formal potential of the hemepeptide on the concentration of non-hemepeptide present indicates that the major increase in potential results from insertion of the heme into its apolar crevice and not from ligation of the methionine.


Accession: 006459491

PMID: 205545



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