Where is fadh2 oxidized

One of the principal energy-yielding nutrients in our diet is glucose see structure in Table 1 in the blue box below , a simple six-carbon sugar that can be broken down by the body. When the chemical bonds in glucose are broken, free energy is released.

The complete breakdown of glucose into CO 2 occurs in two processes: glycolysis and the citric-acid cycle. The reactions for these two processes are shown in the blue box below.

The first process in the breakdown of glucose is glycolysis Equation 5 , in which glucose is broken down into two three-carbon molecules known as pyruvate.

The pyruvate is then converted to acetyl CoA acetyl coenzyme A and carbon dioxide in an intermediate step Equation 6. In the second process, known as the citric-acid cycle Equation 7 , the three-carbon molecules are further broken down into carbon dioxide. The energy released by the breakdown of glucose red can be used to phosphorylate add a phosphate group to ADP, forming ATP green.

The net reactions for glycolysis Equation 5 and the citric-acid cycle Equation 7 are shown below. Note: In the equations below, glucose and the carbon compounds into which glucose is broken are shown in red; energy-currency molecules are shown in green, and reducing agents used in the synthesis of ATP are shown in blue. Note: Carbon atoms from glucose are shown in red. Coenzyme A is shown in purple. Note: The part of the molecule that participates in oxidation-reduction reactions is shown in blue.

This table shows the two-dimensional representations of several important molecules in Equations This yield is far below the amount needed by the body for normal functioning, and in fact is far below the actual ATP yield for glucose in aerobic organisms organisms that use molecular oxygen.

For each glucose molecule the body processes, the body actually gains approximately 30 ATP molecules! See Figure 4, below. So, how does the body generate ATP? The process that accounts for the high ATP yield is known as oxidative phosphorylation. These products are molecules that are oxidized i.

As you will see later in this tutorial, it is the free energy from these redox reactions that is used to drive the production of ATP. This flowchart shows the major steps involved in breaking down glucose from the diet and converting its chemical energy to the chemical energy in the phosphate bonds of ATP, in aerobic oxygen-using organisms. Note: In this flowchart, red denotes a source of carbon atoms originally from glucose , green denotes energy-currency molecules, and blue denotes the reducing agents that can be oxidized spontaneously.

In the discussion above, we see that glucose by itself generates only a tiny amount of ATP. How does this work? As discussed in an earlier section about coupling reactions, ATP is used as free-energy currency by coupling its spontaneous dephosphorylation Equation 3 with a nonspontaneous biochemical reaction to give a net release of free energy i. This set of coupled reactions is so important that it has been given a special name: oxidative phosphorylation.

In addition, we must consider the reduction reaction gaining of electrons that accompanies the oxidation of NADH. Oxidation reactions are always accompanied by reduction reactions, because an electron given up by one group must be accepted by another group. In this case, molecular oxygen O 2 is the electron acceptor, and the oxygen is reduced to water Equation 10, below.

The molecular changes that occur upon oxidation are shown in red. In this tutorial, we have seen that nonspontaneous reactions in the body occur by coupling them with a very spontaneous reaction usually the ATP reaction shown in Equation 3. But we have not yet answered the question: by what mechanism are these reactions coupled? Every day your body carries out many nonspontaneous reactions.

As discussed earlier, if a nonspontaneous reaction is coupled to a spontaneous reaction, as long as the sum of the free energies for the two reactions is negative, the coupled reactions will occur spontaneously. How is this coupling achieved in the body? Living systems couple reactions in several ways, but the most common method of coupling reactions is to carry out both reactions on the same enzyme.

Consider again the phosphorylation of glycerol Equations Glycerol is phosphorylated by the enzyme glycerol kinase, which is found in your liver.

The product of glycerol phosporylation, glycerolphosphate Equation 2 , is used in the synthesis of phospholipids. Glycerol kinase is a large protein comprised of about amino acids. X-ray crystallography of the protein shows us that there is a deep groove or cleft in the protein where glycerol and ATP attach see Figure 6, below. Because the enzyme holds the ATP and the glycerol in place, the phosphate can be transferred directly from the ATP to glycerol.

Instead of two separate reactions where ATP loses a phosphate Equation 3 and glycerol picks up a phosphate Equation 2 , the enzyme allows the phosphate to move directly from ATP to glycerol Equation 4. The coupling in oxidative phosphorylation uses a more complicated and amazing!

This is a schematic representation of ATP and glycerol bound attached to glycerol kinase. The enzyme glycerol kinase is a dimer consists of two identical subuits. There is a deep cleft between the subunits where ATP and glycerol bind. Since the ATP and phosphate are physically so close together when they are bound to the enzyme, the phosphate can be transferred directly from ATP to glycerol.

Hence, the processes of ATP losing a phosphate spontaneous and glycerol gaining a phosphate nonspontaneous are linked together as one spontaneous process.

Neglecting any differences in difficulty synthesizing or accessing these molecules by biological systems, rank the molecules in order of their efficiency as a free-energy currency i. In order to couple the redox and phosphorylation reactions needed for ATP synthesis in the body, there must be some mechanism linking the reactions together.

In cells, this is accomplished through an elegant proton-pumping system that occurs inside special double-membrane-bound organelles specialized cellular components known as mitochondria.

A number of proteins are required to maintain this proton-pumping system and catalyze the oxidative and phosphorylation reactions. There are three key steps in this process:. Note: Steps a and b show cytochrome oxidase, the final electron-carrier protein in the electron-transport chain described above. When this protein accepts an electron green from another protein in the electron-transport chain, an Fe III ion in the center of a heme group purple embedded in the protein is reduced to Fe II.

Cells use a proton-pumping system made up of proteins inside the mitochondria to generate ATP. Before we examine the details of ATP synthesis, we shall step back and look at the big picture by exploring the structure and function of the mitochondria, where oxidative phosphorylation occurs. The mitochondria Figure 8 are where the oxidative-phosphorylation reactions occur.

Mitochondria are present in virtually every cell of the body. They contain the enzymes required for the citric-acid cycle the last steps in the breakdown of glucose , oxidative phosphorylation, and the oxidation of fatty acids. This is a schematic diagram showing the membranes of the mitochondrion. The purple shapes on the inner membrane represent proteins, which are described in the section below.

An enlargement of the boxed portion of the inner membrane in this figure is shown in Figure 8, below. The mitochondrial membranes are crucial for this organelle's role in oxidative phosphorylation. As shown in Figure 8, mitochondria have two membranes, an inner and an outer membrane. The outer membrane is permeable to most small molecules and ions, because it contains large protein channels called porins.

The inner membrane is impermeable to most ions and polar molecules. The inner membrane is the site of oxidative phosphorylation. Recall the discussion of protein channels in the " Maintaining the Body's Chemistry: Dialysis in the Kidneys " Tutorial. As shown in Figure 8, inside the inner membrane is a space known as the matrix ; the space between the two membranes is known as the intermembrane space.

This charge difference is used to provide free energy G for the phosphorylation reaction Equation 8. Electrons are not transferred directly from NADH to O 2 , but rather pass through a series of intermediate electron carriers in the inner membrane of the mitochondrion. This allows something very important to occur: the pumping of protons across the inner membrane of the mitochondrion.

As we shall see, it is this proton pumping that is ultimately responsible for coupling the oxidation-reduction reaction to ATP synthesis. Two major types of mitochondrial proteins see Figure 9, below are required for oxidative phosphorylation to occur.

Both classes of proteins are located in the inner mitochondrial membrane. The electron carriers can be divided into three protein complexes NADH-Q reductase 1 , cytochrome reductase 3 , and cytochrome oxidase 5 that pump protons from the matrix to the intermembrane space, and two mobile carriers ubiquinone 2 and cytochrome c 4 that transfer electrons between the three proton-pumping complexes. Gold numbers refer to the labels on each protein in Figure 9, below. Because electrons move from one carrier to another until they are finally transferred to O 2 , the electron carriers shown in Figure 9,below are said to form an electron-transport chain.

Figure 9, below, is a schematic representation of the proteins involved in oxidative phosphorylation. To see an animation of oxidative phosphorylation, click on "View the Movie. This is a schematic diagram illustrating the transfer of electrons from NADH, through the electron carriers in the electron transport chain, to molecular oxygen.

Please click on the pink button below to view a QuickTime animation of the functions of the proteins embedded in the inner mitochondrial membrane that are necessary for oxidative phosphorylation.

Click the blue button below to download QuickTime 4. Ubiquinone Q 2 and cytochrome c Cyt C 4 are mobile electron carriers. Ubiquinone is not actually a protein. All of the electron carriers are shown in purple, with lighter shades representing increasingly higher reduction potentials. The path of the electrons is shown with the green dotted line. ATP synthetase red has two components: a proton channel allowing diffusion of protons down a concentration gradient, from the intermembrane space to the matrix , and a catalytic component to catalyze the formation of ATP.

For a more complete description of each step in oxidative phosphorylation indicated by the gold numbers , click here. Click here for a brief description of each of the electron carriers in the electron-transport chain. It is important to note that, although NADH donates two electrons and O 2 ultimately accepts four electrons, each of the carriers can only transfer one electron at a time.

Hence, there are several points along the chain where electrons can be collected and dispersed. For the sake of simplicity, these points are not described in this tutorial. In the section above, we see that the oxidation-reduction process is a series of electron transfers that occurs spontaneously and produces a proton gradient.

Why are the electron tranfers from one electron carrier to the next spontaneous? What causes electrons to be transferred down the electron-transport chain? As seen in Table 2, below, and Figure 7a, in these carriers, the species being oxidized or reduced is Fe, which is found either in a iron-sulfur Fe-S group or in a heme group. Table 2 shows that the electrons are transferred through the electron-transport chain because of the difference in the reduction potential of the electron carriers.

As explained in the green box below, the higher the electrical potential e of a reduction half reaction is, the greater the tendency is for the species to accept an electron. Hence, in the electron-transport chain, electrons are transferred spontaneously from carriers whose reduction results in a small electrical potential change to carriers whose reduction results in an increasingly larger electrical potential change.

An oxidation-reduction reaction consists of an oxidation half reaction and a reduction half reaction. Every half reaction has an electrical potential e. By convention, all half reactions are written as reductions, and the electrical potential for an oxidation half-reaction is equal in magnitude, but opposite in sign, to the electrical potential for the corresponding reduction i.

The electrical potential for an oxidation-reduction reaction is calculated by. The two FADH2 originate in the citric acid cycle. In complex I, electrons are passed from NADH to the electron transport chain, where they flow through the remaining complexes. When electrons arrive at complex IV, they are transferred to a molecule of oxygen. Since the oxygen gains electrons, it is reduced to water. While these oxidation and reduction reactions take place, another, connected event occurs in the electron transport chain.

The movement of electrons through complexes I-IV causes protons hydrogen atoms to be pumped out of the intermembrane space into the cell cytosol.

As a result, a net negative charge from the electrons builds up in the matrix space while a net positive charge from the proton pumping builds up in the intermembrane space.