Spectroscopic and Magnetic Properties of the Complexes of the Heme Octapeptide from Cytochrome C

摘要

Detailed ligand binding effects have been examined in a model system of cytochrome c, the N-acetylated ferric heme octapeptide (N-H8PT) directly isolated from horse heart cytochrome c. The room temperature absorption and magnetic circular dichroism (MCD) spectra of the complexes of N-H8PT with various external ligands such as F exp - , H sub 2 O, OH exp - , N sub 3- , imidazole (Im) and CN exp - are found to behave generally as predicted from ligand field considerations. There is a direct correlation between the Soret absorption band position, beta band intensity or the Soret MCD intensity and susceptibility. The N-acetylated methionine complex of the N-H8PT, however, exhibits thermal equilibrium of spins between high and low spin states, while cytochrome c exists in the purely low spin state. Temperatre dependence of paramagnetic susceptibility of the methionine complex yields delta H exp 0 = -7.6 kcal/mole and delta S exp 0 = -25.9 e.u. for a high spin to low spin transition demonstrating a compensation effect between the two thermodynamic parameters. The low temperature ESR spectrum of the methionine complex indicates a low spin ground state with g values at 2.91, 2.31, and 1.51 which are distinct from the g values of cytochrome c. The axial ( delta ) and rhombic (V) distortion parameters in the t/sub 2g/ set of orbitals correspond to 1200 cm exp -1 and 780 cm exp -1 , respectively. From these results, a model is proposed to account for the uniqueness of the methionine complex: a change in Fe-S distance may play a role in regulating the redox properties of cytochrome c. (ERA citation 05:014256)