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Read the disclaimer. Electron transport chain and oxidative phosphorylation. Summary Oxidative phosphorylation is a metabolic pathway through which cells release the energy stored in carbohydrates , fats, and proteins to produce adenosine triphosphate ATP , the main source of energy for intracellular reactions.
Oxidative phosphorylation and the electron transport chain Electron transport chain Oxidative phosphorylation Definition An electron transport chain composed of a series of four membrane-bound protein complexes complexes I—IV that catalyze redox reactions to power ATP synthesis Creation of an electrochemical proton gradient over the inner mitochondrial membrane , which powers oxidative phosphorylation Function The process of coupling the electron transport chain with ATP synthesis , catalyzed by ATP - synthase complex V Production of ATP , which provides energy for intracellular reactions ATP produced One glucose molecule yields 32 net ATP via aerobic metabolism through the malate - aspartate shuttle, which is mainly found in heart and liver tissue.
One glucose molecule yields 30 net ATP via aerobic metabolism through the glycerolphosphate shuttle, which is mainly found in muscle tissue. References Feher JJ. Quantitative Human Physiology. Ketone-body production and oxidation in fasting obese humans.. J Clin Invest. J Nutr Metab. Kaplan ; Chatterjea M, Shinde R. FMN, which is derived from vitamin B 2 also called riboflavin , is one of several prosthetic groups or co-factors in the electron transport chain.
A prosthetic group is a non-protein molecule required for the activity of a protein. Prosthetic groups can be organic or inorganic and are non-peptide molecules bound to a protein that facilitate its function.
Prosthetic groups include co-enzymes, which are the prosthetic groups of enzymes. Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space; it is in this way that the hydrogen ion gradient is established and maintained between the two compartments separated by the inner mitochondrial membrane.
The compound connecting the first and second complexes to the third is ubiquinone Q. The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced to QH 2 , ubiquinone delivers its electrons to the next complex in the electron transport chain. This enzyme and FADH 2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex.
Since these electrons bypass, and thus do not energize, the proton pump in the first complex, fewer ATP molecules are made from the FADH 2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane. The third complex is composed of cytochrome b, another Fe-S protein, Rieske center 2Fe-2S center , and cytochrome c proteins; this complex is also called cytochrome oxidoreductase.
Cytochrome proteins have a prosthetic heme group. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen.
The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, which makes each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes. Cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time.
The fourth complex is composed of cytochrome proteins c, a, and a 3. This complex contains two heme groups one in each of the cytochromes a and a 3 and three copper ions a pair of Cu A and one Cu B in cytochrome a 3. The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced.
The reduced oxygen then picks up two hydrogen ions from the surrounding medium to produce water H 2 O. The removal of the hydrogen ions from the system also contributes to the ion gradient used in the process of chemiosmosis. Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. Describe how the energy obtained from the electron transport chain powers chemiosmosis and discuss the role of hydrogen ions in the synthesis of ATP.
Chemiosmosis : In oxidative phosphorylation, the hydrogen ion gradient formed by the electron transport chain is used by ATP synthase to form ATP. If the membrane were open to diffusion by the hydrogen ions, the ions would tend to spontaneously diffuse back across into the matrix, driven by their electrochemical gradient. However, many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels.
Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through a membrane protein called ATP synthase. This protein acts as a tiny generator turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient.
The turning of this molecular machine harnesses the potential energy stored in the hydrogen ion gradient to add a phosphate to ADP, forming ATP. 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.
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