The protons in water are highly mobile: by rapidly dissociating from one water molecule and associating with its neighbor, they can rapidly flit through a hydrogen-bonded network of water molecules (see Figure 2–5). But how can a pro ton move through the hydrophobic interior of a protein embedded in the lipid bilayer? Proton-translocating proteins contain so-called proton wires, which are rows of polar or ionic side chains, or water molecules spaced at short distances, so that the protons can jump from one to the next (Figure 14–27). Along such predefined pathways, protons move up to 40 times faster than through bulk water. The three-dimensional structure of cytochrome c oxidase indicates two different proton-uptake pathways. This confirmed earlier mutagenesis studies, which had shown that replacing the side chains of particular aspartate or arginine residues, whose side chains can bind and release protons, made the cytochrome c oxidase less efficient as a proton pump.
But how can electron transport cause allosteric changes in protein conformations that pump protons? From the most basic point of view, if electron transport drives sequential allosteric changes in protein conformation that alter the redox state of the components, these conformational changes can be connected to protein wires that allow the protein to pump H+ across the crista membrane. This type of H+ pumping requires at least three distinct conformations for the pump protein, as schematically illustrated in Figure 14–28.
Figure 14–27 Proton movement through water and proteins. (a) protons move rapidly through water, hopping from one h2O molecule to the next by the continuous formation and dissociation of hydronium ions, h3O+ (see Chapter 2). In this diagram, proton jumps are indicated by red arrows.
wires.” these are predefined proton paths consisting of suitably spaced amino acid side chains that accept and release protons easily (asp, Glu) or carry a waterlike hydroxyl group (Ser, thr), along with water molecules trapped in the protein interior.
The respiratory chain embedded in the inner mitochondrial membrane contains three respiratory enzyme complexes, through which electrons pass on their way from NADH to O2. In these complexes, electrons are transferred along a series of protein-bound electron carriers, including hemes and iron–sulfur clusters. The energy released as the electrons move to lower and lower energy levels is used to pump protons by different mechanisms in the three respiratory enzyme complexes, each coupling lateral electron transport to vectorial proton transport across the membrane. Electrons are shuttled between enzyme complexes by the mobile electron carriers ubiquinone and cytochrome c to complete the electron-transport chain. The path of electron flow is NADH → NADH dehydrogenase complex → ubiquinone → cytochrome c reductase → cytochrome c → cytochrome c oxidase complex → molecular oxygen (O2).
As we have just discussed, the three proton pumps of the respiratory chain each contribute to the formation of an electrochemical proton gradient across the inner mitochondrial membrane. This gradient drives ATP synthesis by ATP synthase, a large membrane-bound protein complex that performs the extraordinary feat of converting the energy contained in this electrochemical gradient into biologically useful, chemical-bond energy in the form of ATP (see Figure 14–10). Protons flow down their electrochemical gradient through the membrane part of this proton turbine, thereby driving the synthesis of ATP from ADP and Pi in the extramembranous part of the complex. As discussed in Chapter 2, the formation of ATP from ADP and inorganic phosphate is highly unfavorable energetically. As we shall see, ATP synthase can produce ATP only because of allosteric shape changes in this protein complex that directly couple ATP synthesis to the energetically favorable flow of protons across its membrane.
the Large Negative Value of ∆G for atp hydrolysis Makes atp Useful to the Cell