Structural studies on the cytochrome c oxidase proton pump using a spin-label probe

Area of Science:

• Biochemistry
• Structural Biology
• Biophysics

Background:

• Cytochrome c oxidase is a crucial enzyme in cellular respiration, responsible for pumping protons across the membrane.
• Understanding the structural basis of its proton pump activity is key to elucidating energy transduction mechanisms.
• N,N'-dicyclohexylcarbodiimide (DCCD) is a known inhibitor that binds to cytochrome c oxidase, but its precise binding site and structural implications are not fully understood.

Purpose of the Study:

• To investigate the structural aspects of the cytochrome c oxidase proton pump using a novel spin-label analogue of DCCD.
• To determine the binding site of the spin label and its relationship to DCCD binding.
• To gain insights into the spatial arrangement of enzyme subunits in both reconstituted and free forms.

Main Methods:

• Utilized N-(2,2,6,6-tetramethylpiperidyl-l-oxyl)-N'-cyclohexylcarbodiimide, a spin-label analogue of DCCD.
• Employed Electron Spin Resonance (ESR) spectroscopy to study the bound spin label.
• Investigated enzyme binding in both reconstituted and free forms of cytochrome c oxidase.

Main Results:

• The spin label binds to the same site as DCCD in the reconstituted enzyme, located within subunit III.
• ESR studies indicate the spin label's binding site is in an apolar region near the surface of the reconstituted enzyme.
• Binding of the spin label to the free oxidase differs from the reconstituted enzyme, showing spin-spin exchange.
• DCCD binds to subunits III and IV in the free oxidase.

Conclusions:

• The spin label effectively probes the structural environment within subunit III of cytochrome c oxidase.
• The spatial proximity of subunits III and IV in the free oxidase was determined to be at most 20 Å.
• These findings contribute to a better understanding of the structural dynamics and functional mechanisms of the proton pump in cytochrome c oxidase.