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The bc1 complex uses a phenylalanine residue (Phe90) as an electron tunneling gate. Ligand binding at specific sites on the bc1 complex dynamically controls Phe90 positioning, regulating electron transfer rates.

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Area of Science:

  • Biochemistry and Molecular Biology
  • Bioenergetics and Electron Transport Chain
  • Computational Biophysics

Background:

  • Respiratory complex III (bc1 complex) is crucial for aerobic respiration, mediating electron transport chain (ETC) function.
  • The bc1 complex features complex electron tunneling (ET) processes, including ET-bifurcation and Rieske domain mobility.
  • A phenylalanine residue (Phe90) was previously identified as a key regulator of electron tunneling in the bc1 complex's low potential arm.

Purpose of the Study:

  • To investigate the dynamic role of the Phe90 residue in electron tunneling between heme bL and heme bH.
  • To elucidate how the occupancy of the Qo and Qi binding sites influences Phe90 dynamics and electron transfer rates.

Main Methods:

  • Molecular dynamics (MD) simulations of four distinct respiratory complex III models with varying Qo and Qi site ligand occupancy.
  • Calculation of electron tunneling rate constants between heme bL and heme bH along the MD trajectories.
  • Analysis of conformational changes and distances related to Phe90 positioning and redox center interactions.

Main Results:

  • Binding of aromatic ligands at the Qo site induces a conformational cascade, repositioning Phe90 to reduce the through-space ET distance (approx. 7 Å to 5.5 Å) and enhance electron transfer.
  • Aromatic ligand binding at the Qi site stabilizes Phe90 conformational variations, reducing its dynamic range (approx. 1.5 Å to 0.5 Å).
  • MD simulations reveal a two-step, on-demand conformational coupling between Qo/Qi site occupancy and Phe90 dynamics, confirming its role as an electron tunneling gate.

Conclusions:

  • The Phe90 residue functions as a dynamic switch, controlling electron transfer rates between heme bL and heme bH in the bc1 complex.
  • Ligand binding at the Qo and Qi sites allosterically modulates Phe90 conformation and dynamics, providing a mechanism for regulating electron tunneling.
  • These findings offer critical insights into the intricate regulatory mechanisms governing electron transport in respiratory complexes.