📒 Bazil 2013

Catalytic coupling of oxidative phosphorylation, ATP demand, and reactive oxygen species generation1



  • The biochemical equation of Complex III for the net reaction is shown as

  • Previous models are either too simple or too complex

  • The model developed here maintains the features presented above in a tenable, well-constrained representation of the bc1 complex


Model structure

  • This model includes the redox biochemistry that occurs at the Qo-site and Qi-site of the complex and couples cyt c reduction with the first electron transfer from ubiquinol
  • This first electron transfer at the Qo-site is the one of the primary, rate-limiting steps in the catalytic cycle
  • we assume that up to two mobile electrons can exist at both the Qo-site and Qi-site
  • 6 states
  • The fractional substate occupancies and the state transitions are governed by the thermodynamic driving force of the redox biochemistry defined by the midpoint potentials some of them being pH-dependent
  • Binding polynomials for Qo, Qi site, and ISP FeS
  • Under the right circumstances when cyt bL is reduced, there is a small but significant level of SQ at the Qo-site
  • State transitions are governed by two primary Gibb’s free energies of reaction.
  • The net turnover flux through the enzyme:
  • Superoxide production rate:
  • analytic expressions for the states at steady state
  • The analytical solution for each state contains ∼1000 terms, so they are not explicitly presented here (using KA method ?)

Results and Discussion

  • Table1: fixed parameter values obtained from the literature and used to simulate the model
    • midpoint potentials, stability constants, pKa values, and other parameters
  • Table2: fitted parameter values and sesitivity

  • The cyt c binding constants were assumed to be similar to the fitted constants for horse heart cyt c, based on its apparent universal nature as a substrate for bc1 complexes from other species


  1. Bazil JN, Vinnakota KC, Wu F, Beard DA. Analysis of the kinetics and bistability of ubiquinol:cytochrome c oxidoreductase. Biophys J. 2013;105(2):343-55. PMC3714890 ↩︎