It took us a lot of time to understand how chemical energy can be converted in different types to facilitate our lives. Today we use cars to travel or nuclear reactions to get energy. But most of us probably have no idea that there exists much smaller organisms that designed chemistry-driven motion much much earlier.
This is typical bacteria. It consists of one cell that is surrounded by two membranes (called inner and outer). Between those two membranes there is a fluid; we’re gonna call it intramembrane fluid. There is also a long tail protruding from cell. It’s called flagellum. The general idea is that bacteria rotates it at 200-300 rpm rate (best achieves 1700 rpm) to move. Of course, since energy conversion problem is quite young to humankind, scientists were interested how bacteria’s architecture attains such rotation.
Thanks to crystallography and other biophysical methodology we are able to sketch molecular model of flagellum and propose step-by-step mechanism of rotation driven by chemicals.
Well, there is a molecular model of flagellum below; notice MotA and MotB proteins which form statory part of complex (i.e. they don’t rotate). There is also FliG part that is called “rotor“, the main part that rotates flagellum.
All right, so the main issue this text deals with is how bacterias attain this rotation? And in fact we’re still not sure but group of scientists from Berkeley and Oxford suggested convincing theoretical mechanism based on lots of experimental results (arXiv:1501.02883 [physics.bio-ph]). They proposed that rotation is driven by electrostatic and steric forces.
The first idea comes from result that some mutations in MotA charged aminoacids weakens rotation but don’t disable this function (Zhou, 1998, Takewana, 2014). What’s more interesting, some additional mutations in FliG (“rotor”, see model above) were able to counter the effect of mutations in MotA. This suggested that electrical attraction between MotA and FliG is responsible for appropriate positioning and steering of MotA and MotB (statory part). It may look like here:
Now there comes interaction between MotA and MotB. Well, it was shown that proton cations (H+) involve in many interactions with aminoacids in MotA and MotB. Especially, Asp32 on MotB which is highly conserved among different bacterias interacts with cation. This way, through hydrogen bonds MotA interacts closely with MotB and change its conformation. This gives birth to steric push of rotator.
But why hydrogen protons occur in intramembrane fluid? In inner membrane of bacterias scientists found proteins called ion pumps. The model is shown below and mechanism of their action is fully understood and presented for instance here.
The general idea is as follows: bacteria maintain lower concentration of proton ion (H) inside the cell than in intramembrane fluid. Hence, transporting ion from inside the cell to intramembrane fluid requires energy. Now, look at the picture above. Protons are wandering randomly around whole cell. If one occasionally enter the pump, ATP molecule will bind to the bottom of the pump which react with ATP in two steps. The first is called hydrolysis and it gives 30.5 kJ/mol of energy. The second is change of pump conformation (i.e. how it looks) and this requires energy but less than 30.5 kJ/mol so the total reaction will occur. And this second step is important – the change of conformation blocks proton possibility to come back.
Reed more about mechanism (this includes some mathematical models): arXiv:1501.02883 [physics.bio-ph]