Polyanion binding to cytochrome c probed by resonance Raman spectroscopy

Area of Science:

• Biochemistry
• Biophysical Chemistry
• Spectroscopy

Background:

• Ferricytochrome c is a key electron transfer protein.
• Interactions with charged interfaces can alter protein structure and function.
• Heteropolytungstates are polyanionic inorganic clusters.

Purpose of the Study:

• To investigate the interaction between ferricytochrome c and negatively charged heteropolytungstates.
• To characterize the conformational states and coordination equilibria of bound ferricytochrome c.
• To elucidate the structural basis of these interactions using spectroscopy.

Main Methods:

• Resonance Raman spectroscopy was employed to study the heme protein.
• Analysis of low-frequency spectra provided insights into heme pocket flexibility.
• Thermodynamic properties of coordination equilibria were determined.

Main Results:

• Two main conformational states (I and II) were stabilized upon binding.
• State II exhibited three distinct coordination configurations with thermal equilibria.
• The heme pocket in state II was more flexible, indicating an "open" structure.
• Electrostatic interactions with heteropolytungstates destabilized the heme crevice.
• Changes in ionic strength, pH, or polytungstate type modulated these equilibria.
• Conformational changes in state II differed from alkaline transitions but showed similarities to the acid form.

Conclusions:

• Coulombic attractions between heteropolytungstates and lysine residues destabilize the heme crevice.
• These interactions perturb ionic equilibria, affecting the protein's acid-induced transition.
• The study reveals significant structural and functional modulation of ferricytochrome c by charged polyanions.