一分子生理学で化学-力学エネルギー変換の仕組みを探る
2004 年 39 巻 supplement1 号 p. 1-4
詳細
2004 年 39 巻 supplement1 号 p. 1-4
The protein F1-ATPase is a rotary motor made of a single molecule. Its central γ subunit rotates against a surrounding cylinder made of α3β3 subunits. This rotary motor is powered by sequential ATP hydrolysis in the three β subunits, and reverse rotation of the motor drives ATP synthesis. We have shown, by single-molecule imaging, that (i) the rotary torque is nearly independent of the rotation angle, (ii) 80-90pN nm of mechanical work can be done per ATP hydrolyzed, (iii) binding of ATP causes 80-90° rotation, and (iv) release of the last hydrolysis product causes further 40-30° rotation. Point ii implies that the efficiency of chemo-mechanical conversion may reach100%. Points i-iv allowed us to infer the angle-dependent potential energies for γ rotation for each of chemical intermediates that appear during rotation. We can now explain, on the basis of these potential energies and using a toy model for illustration, how the free energy of ATP hydrolysis may be converted to mechanical torque, and more importantly, how the reverse rotation of the motor by an external force may lead to ATP synthesis. The most important aspect of our experimental finding is that binding (and release) of a nucleotide, rather than hydrolysis per se, is the major source of mechanical output. An equally important corollary is that mechanical motion (rotation) changes the affinity for a nucleotide by orders of magnitude, the essential ingredient of Boyer's binding-change model for ATP synthesis. Using magnetic tweezers, we have shown that reverse rotation of F1 indeed produces ATP. Possibly for the first time, energetically uphill chemical synthesis has been accomplished by the action of mechanical force generated by human artifacts (with the aid of the nature's nano machine). We are also imaging binding and release of individual ATP molecules while F1 rotates or is rotated by magnets. Details of the coupling scheme between chemical reactions on the three catalytic sites and mechanical rotation of the rotor are beginning to unravel.
