Oxidative phosphorylation

10 Key excerpts on "Oxidative phosphorylation"

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  eBook - ePub

  Plant Biochemistry

  • Hans-Walter Heldt, Birgit Piechulla(Authors)
  • 2010(Publication Date)
  • Academic Press

    (Publisher)

  5 Mitochondria are the power station of the cell

  In the process of biological oxidation, substrates such as carbohydrates are oxidized to form water and CO2 . Biological oxidation can be seen as a reversal of the photosynthesis process. It evolved only after oxygen accumulated in the atmosphere during photosynthesis. Both biological oxidation and photosynthesis serve the purpose of generating energy in the form of ATP. Biological oxidation involves a transport of electrons through a mitochondrial electron transport chain, which is in part similar to the photosynthetic electron transport discussed in Chapter 3 . The present chapter will show that the machinery of mitochondrial electron transport is also assembled of three modules. The second complex has the same basic structure as the cytochrome-b 6 /f complex of the chloroplasts. As in photosynthesis, the mitochondrial oxidative electron transport and ATP synthesis are coupled to each other via a proton gradient. The synthesis of ATP proceeds by an F-ATP synthase, which was described in Chapter 4 .

  5.1 Biological oxidation is preceded by a degradation of substrates to form bound hydrogen and CO2

  The overall reaction of biological oxidation is equivalent to a combustion of substrates. In contrast to technical combustion, however, biological oxidation proceeds in a sequence of partial reactions, which allows the utilization of the major part of the free energy for ATP synthesis.

  The principle of biological oxidation was formulated in 1932 by the Nobel Prize winner Heinrich Wieland (Germany):

  First, hydrogen is removed from substrate XH2 and afterwards oxidized to water. The oxidation of carbohydrates [CH2 O]

  n

  involves a degradation by reaction with water to form CO2 and bound hydrogen [H], which is oxidized to water:

  In 1934 Otto Warburg (Berlin, winner of the 1931 Nobel Prize in Medicine) showed that the transfer of bound hydrogen from substrates to the site of oxidation occurs in the form of NADH . From studies with homogenates from pigeon muscles, Hans Krebs formulated the citrate cycle (also called the Krebs cycle) in England in 1937, as a mechanism for substrate degradation yielding NADH for biological oxidation. In 1953 he was awarded the Nobel Prize in Medicine for this discovery. The operation of the citrate cycle will be discussed in detail in section 5.3

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  Landmark Experiments in Protein Science

  • Pascal Leclair(Author)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  9 The Energy of Cells Part II Oxidative phosphorylation

  DOI: 10.1201/9781003379058-9

  We saw in Chapter 5 that foodstuff initially gets broken down in the glycolytic pathway, quickly yielding a small burst of energy usable by cells. During anaerobic respiration, which includes the process of fermentation, this pathway results in the accumulation of lactic acid in mammals or ethanol and CO2 in yeast and other fermenting organisms. However, if oxygen is available, the end-product of glycolysis, the three-carbon molecule, pyruvic acid, is converted to a high-energy compound, acetyl-CoA, which then combines with oxaloacetate to form citric acid, the first molecule of the Krebs cycle. The Krebs cycle is a series of redox reactions that create FADH2 and NADH as by-products and its end-product, oxaloacetate, is recycled to form citrate in a new cycle. The end result of glycolysis and the Krebs cycle is the net synthesis of ATP, whose terminal phosphates can be used as a signaling tag (discussed in Chapters 12 and 13 ) or as a high-energy bond to enable cellular reactions. In this chapter, we discuss the experiments which allowed the elucidation of the mechanism of Oxidative phosphorylation, that is, how FADH2 and NADH donate electrons to the electron transfer chain of mitochondria—the powerhouse of the cell and the main site of ATP synthesis—which creates a H+ gradient across the mitochondrial membrane that fuels the synthesis of ATP.

  In 1932, Otto Warburg and W. Christian identified a new class of molecules involved in the transfer of hydrogen ions. Warburg had always been suspicious about the reactions occurring in Thunberg tubes and questioned whether they were physiologically relevant.1 However, a visit to a colleague’s laboratory in the U.S. was to change his mind. A colleague performed some experiments on red blood cells for him which showed that methylene blue was reduced in the metabolism of glucose to CO2 and pyruvate. Warburg was intrigued by this phenomenon and, therefore, began an investigation of the chemistry of methylene blue.1 As such, he found that incubation of ATP, glucose-6-phosphate (the first intermediate of glycolysis), and methylene blue with yeast cell extracts led to an O2 uptake, and dialysis of the extracts revealed that two components were required for this hydrogen-transferring process: a heat-labile enzyme and a heat-stable co-enzyme.1 The enzyme component was further fractionated into two components, one of which had a yellow color when oxidized and became clear when reduced—this molecule was appropriately named “yellow enzyme”.2 This substance, which Warburg and Christian succeeded in isolating a few years later, was later called lumiflavin and was determined to be part of a greater family of molecules called flavoproteins which are used as carriers of hydrogen (Figure 9.1

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  The Chlamydomonas Sourcebook: Organellar and Metabolic Processes

  Volume 2

  • David Stern(Author)
  • 2009(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 13. Oxidative phosphorylation - Building Blocks and Related Components

  I. Introduction

  Oxidative phosphorylation (OXPHOS) is defined as an electron transfer chain driven by substrate oxidation that is coupled to the synthesis of ATP through an electrochemical transmembrane gradient (Figure 13.1 ). Historically, bovine heart mitochondria have been the system of choice for the structural characterization of eukaryotic OXPHOS complexes (Saraste, 1999 ), because they can be purified in relatively large quantities. The yeast Saccharomyces cerevisiae, which is amenable to a large variety of molecular genetic tools (Bonnefoy and Fox, 2002 ), has become the model organism to study the biogenesis of mitochondrial complexes (Barrientos et al., 2002 ; Herrmann and Neupert, 2003 ) and the effect of mutations on OXPHOS components (Fisher et al., 2004 ). By contrast, mitochondria of photosynthetic organisms have been poorly characterized from a biochemical point of view, mainly due to the difficulties in obtaining preparations free of chloroplast contaminants. Nevertheless, the characterization of Arabidopsis mitochondrial components through proteomic approaches has advanced significantly (Millar et al., 2005 ).

  Figure 13.1. The oxidative phosporylation (OXPHOS) system in the inner membrane of plant mitochondria. Electrons are transferred from NADH and NAD(P)H to coenzyme Q (Q) via type-I NADH:ubiquinone oxidoreductase or complex I (I) and type-II alternative NADH dehydrogenases (NDA; located on the internal (int) or external (ext) faces of the inner membrane). Some electrons from organic acid oxidation are also transferred to Q via the succinate dehydrogenase or complex II (II). Electrons are thereafter transferred from reduced coenzyme Q (QH2) to cytochrome c (cyt c) via the ubiquinol:cytochrome c oxidoreductase or complex III (III) and then to oxygen via the cytochrome c oxidase or complex IV (IV). An alternative oxidase (AOX) can bypass complexes III and IV by transferring electrons directly from QH2 to oxygen. The oxidoreduction reactions mediated by complexes I, III, and IV are coupled with protons (H1) transfer from the matrix to the intermembrane space. The energy associated with the proton gradient is used for the production of ATP by complex V (ATP synthase) or dissipated by uncoupling proteins (UCP).

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  Biochemistry

  • Donald Voet, Judith G. Voet(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  20 Electron Transport and Oxidative phosphorylation

  1 The Mitochondrion

  A. Mitochondrial Anatomy

  B. Mitochondrial Transport Systems

  2 Electron Transport

  A. Thermodynamics of Electron Transport

  B. The Sequence of Electron Transport

  C. Components of the Electron-Transport Chain

  3 Oxidative phosphorylation

  A. Energy Coupling Hypotheses

  B. Proton Gradient Generation

  C. Mechanism of ATP Synthesis

  D. Uncoupling of Oxidative phosphorylation

  4 Control of ATP Production

  A. Control of Oxidative phosphorylation

  B. Coordinated Control of ATP Production

  C. Physiological Implications of Aerobic versus Anaerobic Metabolism

  In 1789, Armand Séguin and Antoine Lavoisier (the father of modern chemistry) wrote:

  . . . in general, respiration is nothing but a slow combustion of carbon and hydrogen, which is entirely similar to that which occurs in a lamp or lighted candle, and that, from this point of view, animals that respire are true combustible bodies that burn and consume themselves.

  Lavoisier had by this time demonstrated that living animals consume oxygen and generate carbon dioxide. It was not until the early twentieth century, however, after the rise of enzymology, that it was established, largely through the work of Otto Warburg, that biological oxidations are catalyzed by intracellular enzymes. As we have seen, glucose is completely oxidized to CO2 through the enzymatic reactions of glycolysis and the citric acid cycle. In this chapter we shall examine the fate of the electrons that are removed from glucose by this oxidation process.

  The complete oxidation of glucose by molecular oxygen is described by the following redox equation: To see more clearly the transfer of electrons, let us break this equation down into two half-reactions. In the first half-reaction the glucose carbon atoms are oxidized:

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  Fundamentals of Biochemistry, Integrated E-Text with E-Student Companion

  Life at the Molecular Level

  • Donald Voet, Judith G. Voet, Charlotte W. Pratt(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  0 components of ATP synthase. Which parts move? Which are stationary? Which are mostly stationary but undergo conformational changes?
• Summarize the steps of the binding change mechanism.
• Describe how protons move from the intermembrane space into the matrix. How is proton translocation linked to ATP synthesis?
• Explain why the P/O ratio for a given substrate is not necessarily an integer.
• Explain how Oxidative phosphorylation is linked to electron transport and how the two processes can be uncoupled.
• Summarize the energy transformations that occur in converting the free energy of glucose to the free energy of ATP.

4 Control of Oxidative Metabolism

KEY CONCEPTS

• The rate of Oxidative phosphorylation is coordinated with the cell’s other oxidative pathways.
• Although aerobic metabolism is efficient, it leads to the production of reactive oxygen species.

An adult woman requires some 1500 to 1800 kcal (6300–7500 kJ) of metabolic energy per day. This corresponds to the free energy of hydrolysis of over 200 mol of ATP to ADP and

P i

. Yet the total amount of ATP present in the body at any one time is <0.1 mol; obviously, this sparse supply of ATP must be continually recycled. As we have seen, when carbohydrates serve as the energy supply and aerobic conditions prevail, this recycling involves glycogenolysis, glycolysis, the citric acid cycle, and Oxidative phosphorylation.

Box 18-4 Perspectives in Biochemistry

Uncoupling in Brown Adipose Tissue Generates Heat

Heat generation is the physiological function of brown adipose tissue (brown fat). This tissue is unlike typical (white) adipose tissue in that it contains numerous mitochondria whose cytochromes cause its brown color. Newborn mammals that lack fur, such as humans, as well as hibernating mammals, all contain brown fat in their neck and upper back that generates heat by nonshivering thermogenesis. Other sources of heat are the ATP hydrolysis that occurs during muscle contraction (in shivering or any other movement) and the operation of ATP-hydrolyzing substrate cycles (see Section 15-4B