Cooh

COOH CH2

COOH COOH

oxaloacetate citric acid

A series of eight reactions complete the cycle, in which the two carbons added by the first step are effectively removed, along with the four pairs of hydrogen. (Some of the hydrogen originates in H2O, which is incorporated at several steps.) Some of the intermediates may be used to synthesize other compounds needed by the cell. The reducing power of the NADH2 may be used in synthesis reactions of other pathways. Both of these reduce the energy yield of the Krebs cycle; however, this is not necessarily inefficient since useful products are made. However, the cell must maintain a supply of oxaloacetate so that the cycle can continue. If intermediates are drawn off, oxaloacetate can be formed from pyruvate and CO2.

The Krebs cycle is normally controlled by feedback inhibition by the products of the cycle. High levels of the ratios NADH2/NAD, ATP/ADP, or acetyl-CoA/CoA indicate that the cell has ample energy, and slow the cycle.

The Cytochrome System The six carbons from the glucose have been disposed of, but the substantial reducing power in the form of NADH2 and FADH2 has yet to be converted to energy in a useful form: namely, as ATP. In eukaryotes this conversion occurs with a series of enzymes and other compounds bound to the inner mitochondrial membrane, which comprise the cytochrome system (Figure 5.7). NADH2 reduces the first of these compounds, flavin mononucleotide (FMN), which spans the membrane from one side to the other. FMN takes the two hydrogens, passes their electrons to the next membrane compound (at a lower energy), and releases the protons to the outside of the membrane. It may be useful to think of the electrons as dropping to a lower voltage, to use an electronic analogy, each time they pass from one electron carrier to the next. Thus, the net effect is to use some of the energy from the electrons to move a pair of protons to the outer compartment of the mitochondrion.

The compound that receives the electrons does a similar thing; it passes them on at a yet lower energy, which may or may not result in moving protons to the outer compartment. In all, six protons are transported. Consider the result: Energy is used to transport protons, creating a pH gradient. This pumping of protons across a membrane into an area of increasing proton concentration stores energy, just as to pumping air into a tank produces a pressure difference that also stores energy. But there isn't only a pH gradient. Since the protons are transported without their electrons or complementary anions, an electric charge gradient, an actual voltage, is also accumulated. The combined chemical and electrical potential across the inner membrane of the mitochondrion is called the chemiosmotic potential.

Figure 5.7 Cytochrome electron transport system, with a relationship to glycolysis and the Krebs cycle.

The energy of the chemiosmotic potential is used when the protons flow back across the membrane to the inner compartment through a complex of transmembrane proteins called ATP synthase, producing ATP from ADP. Think of air that has been pumped into a pressure vessel then flowing back out through a turbine to produce electricity. Altogether, 3 mol of ATP is produced for each mole of NADH2. The FADH2 acts similarly, except that its electrons have a lower energy to start with, so it enters the chain farther along. It provides only enough energy to produce 2 mol of ATP per mole of FADH2.

Finally, what becomes of the energy-depleted electrons in the transport chain? The final components of the transport chain are a series of membrane-bound compounds called cytochromes. The final cytochrome performs one last reduction, that of oxygen. Each 1 mol of O2 receives 2 mol of electrons and 2 mol of H+, forming H2O and completing the process of respiration. Because of the use of an electron acceptor, ATP production in this system is called oxidative phosphorylation.

Assuming that none of the intermediate compounds or NADH2 have been shunted off for other cellular purposes, the final tally for ATP is a maximum of 36 mol per mole of glucose oxidized. Now the efficiency based on standard Gibbs free energy is 38%, a big improvement over glycolysis or the Krebs cycle. Since organisms will use every opportunity for growth, a more typical number of ATPs actually produced by glycolysis and respiration is 20 to 25.

In prokaryotes, the electron transport chain is located in the cell membrane. Bacteria perform the function of the electron transport system without mitochondria by pumping protons outside the cell, depleting it within to create the chemiosmotic potential.

Alternative Electron Acceptors Some bacteria can switch to other electron acceptors, such as nitrate, when oxygen is absent. The nitrate becomes reduced ultimately to nitrogen by the process of denitrification. It is thought that this occurs in a series of steps as follows:

However, the nitrate requires a higher-energy electron for its reduction. It gets it at an earlier point in the electron transport system, so that only 2 mol of ATP is formed per mole of NADH2. This is why organisms that can will always use oxygen when it is present and switch to nitrate only in the absence of oxygen.

Sulfate and carbon dioxide can also function as electron acceptors for certain microorganisms. However, the organisms that do so cannot also use oxygen. When sulfate is reduced, the product is hydrogen sulfide, a poisonous gas. Carbon dioxide is reduced to methane by molecular hydrogen by a special group of organisms within the archaea. Oxidized metal ions such as ferric (III) iron can also be an electron acceptor. This occurs, for instance, in the sediments of wetlands, where oxygen is limiting. The iron becomes reduced to ferrous (II) iron. If an environment contains oxygen, nitrate, sulfate, and carbon dioxide, the oxygen will tend to be used up first, producing H2O. Then the nitrate, then sulfate, and then CO2 will be used. In homogeneous environments, one electron acceptor will not be utilized until the energetically more favorable one is depleted. However, in some situations microenvironments may form, allowing several of these reactions to proceed simultaneously. For example, in biological slime layers in aquatic systems, microorganisms near the slime layer surface may have access to oxygen while organisms below the surface may be depleted of oxygen and will use nitrate.

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