Plant Mitochondrial Electron Transfer and Molecular Biology.

Abstract

INTRODUCTIONAerobic respiration, a process performed by all plants, involves the controlled oxidation of metabolites containing reduced carbon to produce carbon dioxide and water as the final products (Taiz and Zeiger, 1991). Severa1 types of reduced carbon compounds, including fatty acids, organic acids, and amino acids, can serve as the primary reducing substrates for plant respiration. However, the most common substrate used by plant tissues for respiration is carbohydrate (CHpO). The complete oxidation of a carbohydrate releases a large amount of free energy, much of which is coupled to the conversion of ADP and Pi to ATI? When sucrose (Cl2H=0,1) is the substrate, aerobic respiration can be divided into three distinct phases: glycolysis, the tricarboxylic acid (TCA) cycle, and the coupled processes of mitochondrial electron transfer and ATP synthesis. Except for glycolysis, all of the metabolic steps of aerobic respiration take place in the mitochondrion. Like mitochondria found in other eukaryotes, plant mitochondria are roughly spherical subcellular organelles that usually range from 1 to 2 pm in diameter. They are delimited by a mitochondrial envelope that consists of two phospholipid bilayers, known as the outer and inner mitochondrial membranes (Douce, 1985). The outer membrane is permeable to molecules of 3 kD or smaller (Mannella, 1985), whereas the inner membrane represents the primary physical barrier to the uptake of small molecules and ions by the organelle (Douce, 1985). The presence of two membranes separates the mitochondria into four metabolic compartments: the outer membrane, the volume between the two membranes (intermembrane space), the inner membrane, and thevolume within the inner membrane, known as the mitochondrial matrix.Glycolysis involves a series of soluble enzymes located in the cytosol that bring about the conversion of sucrose to four molecules of pyruvate and the net production of four molecules each of ATP and NADH. The pyruvate generated during glycolysis is transported from the cytosol into the mitochondria, where the enzymes of the TCA cycle oxidize it to three molecules of C02. For each molecule of sucrose oxidized by glycolysis and the TCA cycle, 12 molecules of COn, 20 molecules of NADH (16 in the mitochondria and four in the cytosol), and four molecules of FADH2 are produced. To allow continued operation of the two respiratory pathways, these two reduced coenzymes need to be reoxidized to NAD+ and FAD, respectively. This is accomplished by the mitochondrial To whom correspondence should be addressed.electron transfer chain (Figure l), which oxidizes the reduced coenzymes and transfers the resulting electrons to oxygen, producing water as the final product.For each molecule of oxygen reduced to water by the oxidation of two molecules of NADH, 435 kJ/mol is released. Roughly half of this free energy is lost as heat, but the remainder is coupled to the translocation of protons across the inner mitochondrial membrane to give rise to an electrochemical proton gradient, common...

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“…In plant mitochondria, there are two paths of respiratory electron transport from ubiquinol to O 2 (Day et al, 1980;Lambers, 1985;Siedow and Umbach, 1995). Electron transfer through the Cyt pathway is coupled (through the generation of a proton motive force) to ATP synthesis, and the terminal oxidase (Cyt oxidase) is inhibited by CN.…”

“…Therefore, it is clear that factors other than the AOX protein level and form are critical to ensure high AOX activity in vivo. Such factors include the level of pyruvate and the concentration and redox state of the ubiquinone pool (Siedow and Umbach, 1995;Ribas-Carbo et al, 1997).…”

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