The Respiratory Chain: Electrons, Protons, and ATP Production

Pyruvate oxidation and the operation of the citric acid cycle generate large amounts of reduced electron carriers containing trapped energy. To liberate this energy and produce ATP, something must happen to these reduced carriers. Furthermore, without NAD+ and FAD, the oxidative steps of glycolysis, pyruvate oxidation, and the citric acid cycle could not occur. To regenerate NAD+ and FAD, the reduced forms of these carriers must have some way to get rid of their hydrogens (H+ + e–). The fate of these protons and electrons is the rest of the story of cellular respiration. The story hasthree parts:

1. The electrons pass through a series of membrane-associated electron carriers called the respiratory chain or the electron transport chain.

2. The flow of electrons along the chain accomplishes the active transport of protons across the inner mitochondrial membrane, out of the matrix, creating a proton concentration gradient.

3. The protons diffuse back into the mitochondrial matrix through a proton channel, which couples this diffusion to the synthesis of ATP.

The overall process of ATP synthesis resulting from electron transport through the chain is called oxidative phosphorylation.

Before we proceed with the details of oxidative phosphorylation, let’s reflect on an important question: Why should the respiratory chain have so many components and complex processes? Why, for example, don’t cells use the following single step?

NADH + H+ + 1⁄2 O2 ®NAD+ + H2O

Fundamentally, this would be an untamable reaction. It would be very exergonic—rather like setting off a stick of dynamite in the cell. There is no biochemical way to harvest that burst of energy efficiently and put it to physiological use (that is, no metabolic reaction that is so endergonic as to consume a significant fraction of that energy in a single step). To control the release of energy during the oxidation of glucose in a cell, evolution has produced the lengthy electron transport chain we observe today: a series of reactions, each releasing a small, manageable amount of energy.

The respiratory chain transports electrons and releases energy

The respiratory chain contains large integral proteins, smaller mobile proteins, and even a smaller lipid molecule:

Four large protein complexes containing carrier molecules and their associated enzymes are integral proteins of the inner mitochondrial membrane in eukaryotes.

Cytochrome c is a small peripheral protein that lies in the space between the inner and outer mitochondrial membranes. It is loosely attached to the inner membrane.

A nonprotein component called ubiquinone (abbreviated Q) is a small, nonpolar molecule that moves freely within the hydrophobic interior of the phospholipids bilayer of the inner membrane.

NADH + H+ passes electrons to Q by way of the first large protein complex, called NADH-Q reductase, which contains twenty-six polypeptides and attached prosthetic groups. NADH-Q reductase passes the electrons to Q, forming QH₂. The second complex, succinate dehydrogenase, passes electrons to Q from FADH₂ during the formation of fumarate from succinate in reaction 6 of the citric acid cycle. These electrons enter the chain later than those from NADH. The third complex, cytochrome c reductase, with ten subunits, receives electrons from QH₂ and passes them to cytochrome c. The fourth complex, cytochrome c oxidase, with eight subunits, receives electrons from cytochrome c and passes them to oxygen, which with these extra electrons (1⁄2 O₂ –) picks up two hydrogen ions (H+) to form H₂O. The electron carriers of the respiratory chain (including those contained in the three protein complexes) differ as to how they change when they become reduced. NAD+, for example, accepts H– (a hydride ion — one proton and two electrons), leaving the proton from the other hydrogen atom to float free: NADH + H+. Other carriers, including Q, bind both protons and both electrons becoming, for example, QH2. The remainder of the chain, however, is only an electron transport process. Electrons, but not protons, are passed from Q to cytochrome c. An electron from QH₂ reduces a cyto-chrome’s Fe3+ to Fe2+. The fate of the protons will be discussed below. Electron transport within each of the three protein complexes results, as we’ll see, in the pumping of protons across the inner mitochondrial membrane, and the return of the protons across the membrane is coupled to the formation of ATP. Thus the energy originally contained in glucose and other foods is finally captured in the cellular energy currency, ATP. For each pair of electrons passed along the chain from NADH + H+ to oxygen, three molecules of ATP are formed. If only electrons are carried through the final reactions of the respiratory chain, what happens to the protons? How are proton movements coupled to the production of ATP?

Date: 2014-12-22; view: 1090