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Fig. 6.5.1. Top: ATP synthase complex inserted coupled with rotation of the central rod.

into the membrane separating high from low proton concentration. The arrows are a schematic representation of the helical proton channel in the F0 subunit of ATP synthase. Motion of the protons through the channel is

Fig. 6.5.1. Top: ATP synthase complex inserted coupled with rotation of the central rod.

into the membrane separating high from low proton concentration. The arrows are a schematic representation of the helical proton channel in the F0 subunit of ATP synthase. Motion of the protons through the channel is

Bottom: Schematic drawing of the conformational changes induced by the rotating axle in the catalytically active b subunits of F1 ATP synthase.

the center of the cyclic Fj unit that moves in 120° steps and interacts with the three b-subunits in three different ways (Figure 6.5.1). Thus, each complete rotational cycle induces three different stages in the b-subunits producing a total of three ATP molecules.

But how does nature achieve rotation of this axle? What is its driving force? Figure 6.5.1 shows the F0 part of ATP synthase as a multiprotein complex spanning the tylacoid membrane. The axle is again located at the center of this cyclic array. The F0 ATP synthase bears a helical channel through which protons can flow. The protons enter on the side of the membrane distal from the F1 part, then follow the channel for almost a complete circle around the axle, and finally leave the membrane at the other side, where the F1 part is located. The proton flow is coupled with the rotation of the axle through temporary binding of protons to carboxylates appropriately positioned at the axle's lower section.

Several questions arise from these considerations. First, it is by no means clear how and why unidirectional rotation is achieved in the natural system. A hint has been given above. The proton channel is helical and is thus chiral. If one takes into account that a clockwise rotation is nothing other than a mirror image of a counterclockwise rotation, it becomes obvious why chirality is of such great importance. In the presence of an additional element of chirality the two enantiomeric senses of rotation become diastereomeric and, consequently, one might be preferred energetically over the other [2]. The second question is that of the driving force. Protons only flow through the proton channel if there is a concentration gradient between the plasma on one side of the membrane and the volume on the other side. Indeed, in natural ATP synthase this is realized and for most living organisms translocation of 12 protons is necessary to rotate the axle by 360°, producing three ATP molecules. These considerations now enable us to understand the importance of the membrane and the reason why ATP synthase is a trans-membrane protein complex. For correct function, it is necessary to build up the proton gradient by compartmentation and prevent it from leveling out. In this respect, a third aspect is pivotal - all ATP synthase multiprotein complexes must be inserted into the membrane in the same orientation, because otherwise each ATP synthase complex with the wrong orientation would consume ATP and accelerate proton migration through the membrane.

These few paragraphs on the natural ATP synthase rotary motor should suffice to give a rough impression of how it works. We have followed ATP synthesis back step by step from the final product through different energy conversion processes to a proton gradient as the final energy source. In principle ATP synthase can be regarded as a motor [3] (the F0 subunit), converting chemical energy into, molecular motion coupled to a generator (the F1 part), that uses the mechanical energy of the rotating axle to convert it back into chemical energy (ATP synthesis). Several key properties, which will lead to the discussion of artificial approaches to such machines in the following sections should be summarized here.

A functional system for ATP generation, requires a membrane which separates areas of high proton concentration from those of lower acidity. The orientation of the trans-membrane ATP synthase protein complex in the membrane is of major importance.

To provide a mechanical motor, an axle must be able to rotate inside a stator, and unidirectionality requires the presence of an element of chirality. Finally, a machine needs to be coupled to the motor to convert the mechanical energy back into chemical energy.

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