Energy Transduction and Transformations in Biology

Gram-negative bacteria are surrounded by two membranes: an inner membrane and an outer membrane that are separated by about 20 nanometers of the periplasmic space. Only the inner membrane generates energy (in the form of ATP or electrochemical gradients). However, the outer membrane also requires energy for several work-demanding processes.
Enzymes residing in the outer membrane have no energy source: there’s no ATP in the periplasm and there are on electrochemical gradients across the outer membrane. So somehow, energy needs to be transmitted from its source (the inner membrane), across 200 nanometers, to where it is needed (the outer membrane). We know this does not happen via shuttling of ATP.

So how is energy transmitted?

The energy transduction complex of ExbB-ExbD-TonB:
ExbB and ExbD reside in the inner membrane where they presumably harvest the electrochemical energy of the proton gradient and somehow transfer this energy to TonB. TonB spans the periplasmic space and presumably delivers this energy to transporters that reside in the outer membrane, like BtuB shown here.
Such outer membrane transporters (like BtuB) need energy for their function but have no accessible energy source of their own.

We do not know, but we have a prime suspect: the TonB/ExbB/ExbD complex.
Very little is known about this protein complex, but what we do know is that it resides in the inner membrane, it has the ability to cross the periplasm and reach the outer membrane, and many energy-requiring processes of the outer membrane completely depend on the TonB/ExbB/ExbD complex.
So there’s good circumstantial evidence that this complex is indeed the energy transducer.
But how?
Due to sequence homology to the rotary motor MotA/B of the flagella, we hypothesize that ExbB/ExbD may also form a rotary motor, turned around and round by the proton electrochemical gradient, like the motor of the ATP synthase.
But without TonB nothing will happen.
So somehow, the speculative rotary motor of ExbB/ExbD needs to turn TonB, or cause some conformational change in it, and TonB needs to deliver it to specific energy-requiring enzymes of the outer membrane.
A lot of speculations, a lot of unknowns.
What we try to do in our lab is reconstruct the whole energy transduction process “bottom up”. We purify the individual components ExbB and ExbD and build the rotary motor. We then add the shaft, or energy transducer, TonB., to form the energy transduction complex. We then add a target enzyme, that needs energy to do its work but cannot access it, and finally we test for energy delivery from the TonB/ExbB/ExbD complex to this enzyme.

Along the way we address such questions as

• Do ExbB/ExbD really form a rotary motor?
• Are they able to harvest the energy of the proton electrochemical gradient?
• Do they transfer this energy to TonB, and how?
• How does TonB transfer this energy to the target?

This is an incredibly challenging project that has never been attempted before, and we are always looking for new people to join in.