Rotary Motors in Biology - the ATP Synthase F1-ATPase

 


 

The amazing, rotating powerhouse - the proton-driven ATP-synthase

ATP (adenosine triphosphate) is the “energy currency” of the cell. It is the chemical equivalent of a loaded spring and is used to power very many cellular functions.

The ATP synthase is a complex enzyme that spans a membrane. Oxidation of foods (burning of fuels) by the cell creates excess protons on one side of a "coupling" membrane. This energizes the membrane.

As excess protons move through the ATP synthase membrane sector (Fo), they cause a rotation of the c-subunits which in turn moves the gamma subunit in the F1-ATPase catalytic core. This spinning of gamma in between the alpha and beta hexameric subunits drives conformational changes that allow ATP to form and be released from the enzyme.

A normal human makes his or her body weight in ATP each day by this process!

The ATP synthase is one of Nature's most powerful and most efficient molecular motors.

See below for an introduction to our current studies on this fascinating enzyme.


Important remaining questions about how this enzyme functions:

What is the structure of the b-dimer stator subunits?

Rotary mechanisms require a stator to resist the rotation of the catalytic subunits.

Without a stator, rotation would not be able to perform the work necessary for ATP synthesis.

How does the b-dimer interact with the a and c subunits in the membrane and with the F1 catalytic subunits to help drive ATP synthesis?


Background to the problem:

The rotating catalytic subunits of the ATP synthase

 

 

Above: Looking from below on the F1 catalytic subunits: Overview showing rotation - note blue gamma movement and the ATP being released.

Alpha subunits are shown in red, beta subunits in yellow and the gamma subunit is in blue. ATP and ADP molecules are gray. Note the asymmetric gamma structure. This subunit rotates within the alpha-beta-hexamer! The rotation is powered by a gradient of protons across a membrane.


 

 

 

Above: The F1 catalytic subunits - side view. Coordinates are from Abrahams, J.P., Leslie, A.G.W., Lutter, R. and Walker, J.E. (1994) Nature 370 621-628.


 

Above: Cartoon of the overall structure showing membrane and proton gradient. Note the unknown structure of the stator b-dimer subunits.


 

Structures for the subunit b dimer:

Theoretical studies on the b-dimer show that it can adopt a classical left-handed coiled coil.

Above:

A model of the b-dimer stator calculated from ideal left-handed coiled coil helix packing. This structure shows the stator in a classic left-handed coiled coil structure.

Left panel: The backbone helices of the dimerization domain of the E. coli b2 are shown in silver and residues identified as a and d heptads are shown in spacefilling representations in blue or red, respectively.

Right panel. A superposition of the monomeric X-ray crystal structure (yellow backbone) determined by Del Rizzo, Paul A;Bi, Yumin;Dunn, Stanley D;Shilton, Brian, Biochemistry 41, 6875-84, 2002 with the “A” chain of the left handed coiled coil presented in the left panel (silver backbone). The heptad a and d residues are shown in blue and red spacefilling representations, respectively. The arrows show the position of Arg83 in the proteins.


 

Experimental Validation of the left-handed coiled coil b-dimer stator model

We used a technique called site specific spin labeling and electron spin resonance spectroscopy validate our model.

By measuring distances between residues in the actual b-dimer protein and comparing these measured distances to distances predicted by our model, we showed that the model predicts the measurements with a high degree of accuracy.

The figure below shows a comparison of the measured and modeled data.

See Wise, J.G. and Vogel, P.D. “Subunit b dimer of the Escherichia coli ATP synthase can form left-handed coiled coils” (2008) Biophysical J. 94, 5040-5052 for further details.


Conclusions:

The subunit b-dimer stator can exist as a left-handed coiled coil structure.


Current research directions:

We are currently studying how the dimer of b subunits interacts with the F1 catalytic subunits and how it interacts with the subunits a and c of the Fo membrane proton pore.


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