The Nature of the Thermal Equilibrium Affecting the Iron Coordination of Ferric Cytochrome c

In cytochrome c, ligation of the heme iron by the methionine-80 sulfur plays a major role in determining the structure and the thermodynamic stability of the protein. In the ferric state, this bond is reversibly broken by moderately acid or alkaline pH's (pK's 2.5 and 9.4, respectively) and by exogenous ligands. NMR studies of horse ferricytochrome c in which the Met-65 and Met-80 methyl groups were chemically enriched with ¹³C demonstrate that, at 59 °C, a temperature at which the protein is still folded, the sulfur-iron bond is already partially broken. This structural change corresponds to the reversible disappearance upon moderate heating of the 695 nm band, characteristic of the sulfur-iron coordination of this protein. The thermal effect results from a shift in the alkaline pK from 9.4 at 25 °C to 8.2 at 59 °C. The exchange rate from iron-bound to free methionine-80 at 59 °C is 1.8 s^-1, as measured by saturation transfer experiments. The free and bound methionine-80 ε-methyl groups in the spectrum are assigned as (1.87, 2.25) and -21.43, respectively; in the ¹³C spectrum they are assigned as 15.6 and 12.8, respectively (all these values are in ppm from 3-(trimethylsilyl)propionic-2,2,3,3-d₄ acid, sodium salt).