6.021/Notes/2006-09-29

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4 state model

  • Simplify the model with assumptions
    • α1 = α31 = β3 (binding same on inside and outside)
    • α2 = α42 = β4 (ability for protein to translocate/flip is independent of solute)
    • Binding fast relative to translocation
      • Only care about the dissociation constant as it will always be in steady state
  • Instead of concentrations (which is per volume), it is easier to think about \mathfrak{N}_E (per surface area) \mathfrak{N}_E=c_E\cdot d where d is the membrane thickness
  • This leads to the simple symmetric four state carrier model
  • The solution can be interpreted intuitively
    • The enzyme is first partitioned into facing in or out depending on α,β
    • Then it is partitioned into whether has substrate bound by K and cs
    • The concentration difference between inside and outside is not important. All that matters is the concentration relative to K.


Solution to simple symmetric 4-state carrier model:

\mathfrak{N}^i_{ES}=\frac{\beta}{\alpha+\beta}\frac{c^i_s}{c^i_s+K}\mathfrak{N}_{ET}

\mathfrak{N}^i_{E}=\frac{\beta}{\alpha+\beta}\frac{K}{c^i_s+K}\mathfrak{N}_{ET}

\mathfrak{N}^o_{ES}=\frac{\alpha}{\alpha+\beta}\frac{c^o_s}{c^o_s+K}\mathfrak{N}_{ET}

\mathfrak{N}^o_{E}=\frac{\alpha}{\alpha+\beta}\frac{K}{c^o_s+K}\mathfrak{N}_{ET}

\phi_s=(\phi_s)_{max}(\frac{c^i_s}{c^i_s+K}-\frac{c^o_s}{c^o_s+K}); (\phi_s)_{max}=\frac{\alpha\beta}{\alpha+\beta}\mathfrak{N}_{ET}

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