Mechanism

So far we have talked about the 2 intermediate conformations that LeuT is capable of adopting so you are well equipped to see how this all ties together in: THE MECHANISM

What conformational changes occur that allow the substrate to move through the transporter?

There is partial symmetry between TMs 1-5 and TM 6-10.  TMs 1, 2, 6 and 7 form a bundle.

These 2 facts inspired the rocking bundle mechanism; essentially the whole core moving as one to retain the symmetry eventually leading to the release of substrate on the intracellular side.

BUT this is only partially true:

 It is true that there is a general rotation of 14° of a fraction of the core domain in relation to the membrane HOWEVER, some TM domains are actually altered to different extents.

Similarly, the intracellular loop 1 and EL4 , which are also supposedly counterparts in the two part symmetry, act independently.

To get from the outward open state to the inward open state the intracellular gate must be disrupted and the extracellular gate must be formed...

How is the displacement of TM helices linked to the sodium/substrate site transformations in order to do this?

Na+ binding keeps TM1a in position for the outward-open state, when there is no Na+ this is not  energetically favourable so the transporter is forced to switch to inward-open.

The absence of sodium therefore leads to the following sequence of events:

1.      When TM1a moves it requires rearrangement of TM5 which does not have a hinge region

2.      TM5 movement therefore causes TM7 and TM1b to move

3.      The same occurs with TM2 and TM6a which in conjunction with TM7 and TM1b lead to the closure of the extracellular gate.

THIS IS IMPERITIVE FOR SUBSTRATE TRANSPORT: Opening of the intracellular gate cannot be facilitated without closure of the extracellular gate.

However it is important you are aware that there may be other possibilies, as there so often is in science, because this is a proposed mechanism.

Just in case you can't get your head around it, we have provided some summary diagrams:

Figure 7: A comparison of the 2 conformational states (top is outward open, bottom is inward open) showing the entry and exit points of the substrate