How can the cell make such a series of conformational changes unidirectional? To force the entire cycle to proceed in one direction, it is enough to make any one of the changes in shape irreversible. Most proteins that are able to walk in one direction for long distances achieve this motion by coupling one of the conformational changes to the hydrolysis of an ATP molecule that is tightly bound to the protein. The mechanism is similar to the one just discussed that drives allosteric protein shape changes by GTP hydrolysis. Because ATP (or GTP) hydrolysis releases a great deal of free energy, it is very unlikely that the nucleotide-binding protein will undergo the reverse shape change needed for moving backward— since this would require that it also reverse the ATP hydrolysis by adding a phosphate molecule to ADP to form ATP.
In the model shown in Figure 3–75A, ATP binding shifts a motor protein from conformation 1 to conformation 2. The bound ATP is then hydrolyzed to produce ADP and inorganic phosphate (Pi), causing a change from conformation 2 to conformation 3. Finally, the release of the bound ADP and Pi drives the protein back to conformation 1. Because the energy provided by ATP hydrolysis drives the transition 2 → 3, this series of conformational changes is effectively irreversible. Thus, the entire cycle goes in only one direction, causing the protein molecule to walk continuously to the right in this example.
Many motor proteins generate directional movement through the use of a similar unidirectional ratchet, including the muscle motor protein myosin, direction of
Figure 3–74 an allosteric “walking” protein. Although its three different conformations allow it to wander randomly back and forth while bound to a thread or a filament, the protein cannot move uniformly in a single direction.
Figure 3–75 How a protein can walk in one direction. (A) An allosteric motor protein driven by ATP hydrolysis. The transition between three different conformations includes a step driven by the hydrolysis of a bound ATP molecule, creating a “unidirectional ratchet” that makes the entire cycle essentially irreversible. By repeated cycles, the protein therefore moves continuously to the right along the thread. (B) Direct visualization of a walking myosin motor protein by high-speed atomic force microscopy; the elapsed time between steps was less than 0.5 sec (see Movie 16.3). (B, modified from
N. Kodera et al., Nature 468:72–76, 2010. With permission from Macmillan  Publishers Ltd.) which walks along actin filaments (Figure 3–75B), and the kinesin proteins that walk along microtubules (both discussed in Chapter 16). These movements can be rapid: some of the motor proteins involved in DNA replication (the DNA helicases) propel themselves along a DNA strand at rates as high as 1000 nucleotides per second.
Membrane-Bound Transporters Harness Energy to Pump Molecules Through Membranes
We have thus far seen how proteins that undergo allosteric shape changes can act as microprocessors (Src family kinases), as assembly factors (EF-Tu), and as generators of mechanical force and motion (motor proteins). Allosteric proteins can also harness energy derived from ATP hydrolysis, ion gradients, or electron-transport processes to pump specific ions or small molecules across a membrane. We consider one example here that will be discussed in more detail in Chapter 11.
The ABC transporters (ATP-binding cassette transporters) constitute an important class of membrane-bound pump proteins. In humans, at least 48 different genes encode them. These transporters mostly function to export hydrophobic molecules from the cytoplasm, serving to remove toxic molecules at the mucosal surface of the intestinal tract, for example, or at the blood–brain barrier. The study of ABC transporters is of intense interest in clinical medicine, because the overproduction of proteins in this class contributes to the resistance of tumor cells to chemotherapeutic drugs. In bacteria, the same types of proteins primarily function to import essential nutrients into the cell.