# 6.021/Notes/2006-09-29

### From OpenWetWare

## 4 state model

- Simplify the model with assumptions
- α
_{1}= α_{3},β_{1}= β_{3}(binding same on inside and outside) - α
_{2}= α_{4},β_{2}= β_{4}(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

- α
- Instead of concentrations (which is per volume), it is easier to think about (per surface area) 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*c*_{s} - 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:

;