Hoofdstuk 18 Biochemistry (Oxidative Phosphorylation) PDF

# LLS331VN: Biochemistry - Hoorcollege 1 (E. de Jong) ## Monday, April 20, 2015 ### Chapter 18 Biochemistry (Oxidative Phosphorylation) This lecture is about redox chemistry and electron transport. Proton gradients are the energy supply for a cell. - **Cell respiration (in mitochondria)**:...

# LLS331VN: Biochemistry - Hoorcollege 1 (E. de Jong) ## Monday, April 20, 2015 ### Chapter 18 Biochemistry (Oxidative Phosphorylation) This lecture is about redox chemistry and electron transport. Proton gradients are the energy supply for a cell. - **Cell respiration (in mitochondria)**: - Fuel + O2 -> ATP (energy) - **Photosynthesis (in chloroplasts)**: - Sunlight (energy) -> ATP + glucose Both use electron transport chains. ### Citric Acid Cycle (CAC) The citric acid cycle is the cycle for the breakdown of glucose and fatty acids **Figure 17-15** **Here energy is released in steps. The most important fuel is NADH** ### Mitochondria Mitochondria are organelles that function as energy factories. Fuel and oxygen are supplied via the bloodstream, which are converted to ATP in the mitochondria. **Figure 18-17** - **NADH** (the fuel) is oxidized by oxygen and the released energy is stored in ATP. This process is regulated by enzyme complexes. - The citric acid cycle produces NADH (and FADH2), then these substances take a route through the enzyme complexes I, II, III and IV. The red arrow shows that electrons are transferred from NADH to O2 via a series of redox reactions. - The proton gradient can be compared to charging a battery [proton: H+]. A charge difference and a concentration difference are created by pumping protons through the membrane. ### Redox Reactions Redox reactions consist of two half-reactions that together produce a total reaction: - **Oxidator**: 1/2O2 + 2H+ + 2e- -> H2O ([oxidator takes up electrons]) - **Reductor**: NADH -> NAD+ + 2e- + H+ ([reductor gives away electrons]) - **Total reaction**: 1/2O2 + NADH + H+ -> H2O + NAD+ ([sum of the half reactions]) This reaction releases a lot of energy. The strength of an oxidator can be measured by comparing it to a standard hydrogen electrode. If the electrons move to the left (to the side of the standard hydrogen electrode), there is a stronger oxidator than H+, for example in a Cu2+ solution with a Cu electrode. The value of the standard (H2) electrode is set to 0 V. The strengths of a number of important oxidators and reductors are listed in the table below. **Table 18-1. Standard Reduction Potentials of some reactions** | Oxidant | Reductant | n | E'(V) | |---|---|---|---| | Succinate + CO2 | a-Ketoglutarate | 2 | -0.67 | | Acetate | Acetaldehyde | 2 | -0.60 | | Ferredoxin (oxidized) | Ferredoxin (reduced) | 1 | -0.43 | | 2 H+ | H2 | 2 | -0.42 | | NAD+ | NADH + H+ | 2 | -0.32 | | NADP | NADPH + H+ | 2 | -0.32 | | Lipoate (oxidized) | Lipoate (reduced) | 2 | -0.29 | | Glutathione (oxidized) | Glutathione (reduced) | 2 | -0.23 | | FAD | FADH2 | 2 | -0.22 | | Acetaldehyde | Ethanol | 2 | -0.20 | | Pyruvate | Lactate | 2 | -0.19 | | Fumarate | Succinate | 2 | +0.07 | | Cytochrome b (+3) | Cytochrome b (+2) | 1 | +0.08 | | Dehydroascorbate | Ascorbate | 2 | +0.10 | | Ubiquinone (oxidized) | Ubiquinone (reduced) | 2 | +0.22 | | Cytochrome c (+3) | Cytochrome c (+2) | 1 | +0.77 | | 1/2 O2 + 2H+ | H2O | 2 | +0.82 | *Note: E' is the standard oxidation-reduction potential (pH 7, 25°C) and n is the number of electrons transferred.* *E' refers to the partial reaction written as Oxidant + e- -> Reductant.* ### Total Reaction Electron Transport Chain (Cellular Respiration) 1/2O2 + NADH + H+ -> H2O + NAD+ In the electron transport chain, this occurs in steps. Here, oxidators that are weaker than O2 are used and reformed. In this reaction, Fe3+ can also act as an oxidator instead of O2 (cytochrome B protein, see the table). [Iron ion: Fe2+ (reductor), Fe3+ (oxidator)] **Theoretical example of one step** - **1st reaction:** - **Reductor**: NADH -> NAD+ + 2 e- + H+ - **Oxidator**: 2 Fe3+ + 2 e- -> 2 Fe2+ - **Total reaction**: NADH + 2 Fe3+ -> NAD+ + H+ + 2 Fe2+ - **2nd reaction:** - **Reductor**: 2 Fe2+ -> 2 Fe3+ + 2 e- - **Oxidator**: 1/2O2 + 2 H+ + 2 e- -> H2O - **Total reaction**: 1/2O2 + 2 H+ + 2 Fe2+ -> H2O + 2 Fe3+ 2 Fe3+ reacts away in the first reaction, but is regenerated in the second, and can then take an electron from the next NADH molecule, etc. All intermediate electron carriers in the electron transport chain are regenerated and are therefore not consumed. ### Proton Gradient A proton gradient can therefore be compared to a battery. When the two poles are close to each other, a short circuit occurs. Therefore, the poles are separated by the membrane. The proton gradient provides energy for ATP synthesis, through the ATP synthase complex. **Figure 18-1** A part of the energy from sunlight is converted into ATP. First, a proton gradient is formed, and ATP is generated in an ATP synthase complex. ### Photosynthesis VS Oxidative Phosphorylation **Photosynthesis** Sunlight (energy) -> ATP + glucose **Oxidative Phosphorylation** Fuel + O2 -> ATP (energy) **Figure 19-25** - **Human/Animal:** glucose -> CO2 - **Plant:** CO2 -> glucose *In the CAC, NADH is released* *The Calvin cycle requires NADPH* **Sunlight is the energy source for the production of NADPH (NADH in plants). This reductant is needed to produce glucose from CO2 and H2O.** **In conclusion, the document is about the redox reactions, electron transport chain, proton gradient, ATP production and the differences between photosynthesis and oxidative phosphorylation in the context of cellular respiration.**