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Respiration and Fermentation Overview

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Microbiology (Biological Sciences)
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RESPIRATION AND FERMENTATION OVERVIEW Respiration and fermentation are the two mechanisms microorganisms use to make ATP, the energy currency of biology. This section focuses on the energetics and metabolic pathways of fermentation and respiration (aerobic and anaerobic). Because there is a finite amount of ATP (and ADP) that can exist in the cell, in much the same way that the world’s gold supply is quite small, it is always more useful to think of exergonic reactions (catabolism) occurring in close proximity to endergonic reactions (anabolism). In other words, catabolic reactions drive anabolic reactions and these processes are tightly linked together. OBJECTIVES After attending lecture and reading this chapter you should be able to: 1) Discuss the difference in ATP production, use of membrane-associated electron transport chains, and use of electron acceptors between respiration and fermentation. 2) Know the difference between fermentation and anaerobic respiration. 3) Describe how electrons and protons are used to create proton motive force using membrane-associated protein complexes. 4) Understand the meaning of the proton motive force in terms of both structure (cell membrane) and function (generation of energy). 5) Understand the meaning of anabolic, catabolic, and anaplerotic reactions and recognize how the last type of reaction is advantageous to cells in terms of energy conservation. 6) Explain why glycolysis (our best-studied example of fermentation) is the fastest way to generate energy, but the least efficient. 7) Describe the difference in the fate of pyruvate between substrate-level phosphorylation and oxidative phosphorylation and why the latter process generates much more energy. 8) Understand the meaning and difference between oxidative phosphorylation and substrate level phosphorylation. 9) Explain why the TCA cycle is the most efficient at generating energy (respiration) and also its significance to anaplerotic reactions of all organisms. 10)Understand how polysaccharides and lipids are catabolized for energy. 11)Understand the significance of the pentose monophosphate pathway in anaplerotic reactions. CHAPTER OUTLINE I. Respiration vs. Fermentation These are the two mechanisms by which prokaryotes generate ATP. Respiration uses redox reactions and step-wise transfer of electrons to a number of carriers to generate proton motive force. The PMF is then used to make ATP, the energy currency of biology used to manufacture cellular components (e.g. proteins, nucleic acids, cell membranes, etc.) for growth and division. 1 Fermentation does NOT use electron carriers or PMF to generate ATP; this process involves direct transfer of high-energy phosphate to ADP by substrate-level phosphorylation. The best known example of substrate-level phosphorylation is glycolysis. Oxidative Phosphorylation: The passage of electrons in a step-wise fashion from carrier to carrier combined with the movement of protons from the inside to the outside of the cytoplasmic membrane to generate PMF. Select bacteria that have hydrogenases that channel H directly 2 into the electron transport chain (which is energetically advantageous since H has a 2ore negative redox potential than NADH). Organotrophic organisms such as E. coli (and mitochondria) lack the ability to metabolize hydrogen. Consequently, the electrons and protons produced from the oxidation of organic substrate are passed onto NAD during glycolysis and the TCA cycle. II . Glycolysis and Fate of Pyruvate Glycolysis is found in all three kingdoms of life Glucose, a 6-carbon sugar give rise to pyruvate molecules (3C), 4H /4e(as 2NADH/H ), and a + net gain of 2 ATP by substrate level phosphorylation. Oxidation of the NADH/H produced from glycolysis is crucial, and can occur via respiration or fermentation of pyruvate. In the absence of oxygen or any other ino+ganic electron acceptor, NADH is not oxidized by the electron transport chain, but NAD must be replenished for use in glycolysis. The cell has a very small amount of NAD and thus can not continue to accumulate reduced NADH. Fer+entation is the use of organic compounds as electron acceptors to oxidize NADH to NAD . It occurs only in the absence of O and does2not involve electron transport. Pyruvate is the central metabolite (starting product) of many fermentations. Fermentation products include alcohol, lactic acid, and formate, which is further fermented to H . 2 III. Respiration and the TCA cycle: Unlike fermentation in which NAD+ is only regenerated by formation of waste products, respiratory organisms regenerate NAD+ via the electron transport chain in the cytoplasmic membrane. Thus, pyruvate is decarboxylated to acetyl-CoA, which is then oxidized to CO via 2 the tricarboxylic acid (TCA) to generate the maximum amount of reduced fuel sources (NADH and FADH) for respiration. A full turn of the +CA cycle back to oxaloacetate results in the production of two CO , t2ree NADH + H , one FADH , and one NA2PH. The NADPH is usually used to provide electrons for biosynthetic processes. Some organisms like obligate fermenters and chemolithotrophs lack a complete TCA cycle, but they have most of the cycle enzymes to provide carbon skeletons for use in biosynthesis. The yield of ATP in glycolysis and aerobic respiration through the TCA cyle varies with each 2 organism but has a theoretical maximum of 38 molecules ATP per molecule of glucose catabolized. This works out to 2830 kJ / 38 ATP = 74.5 kJ /ATP. Anaerobic respiration yields less ATP, not because of differences in the TCA cycle, but because of the terminal reductase instead of the terminal oxidase in the electron transport chain. Thus, the TCA cycle can not be considered an aerobic pathway! A smaller difference in redox potential between the electron donor (NADH) and the electron acceptor (e.g. nitrate) in the electron transport chain means fewer protons are moved across the membrane. Nitrate respiration by E. coli results in a theoretical maximum of 28 molecules ATP per molecule glucose catabolized instead of the 38 made with oxygen as the electron acceptor. In contrast, glycolysis ending in fermentation to lactic acid produces only two ATP per glucose. IV. Catabolism of Molecules Besides Glucose Hydrolytic reactions normally do not produce usable energy for the cell: e.g. polysaccharides to monosaccharides, proteins to amino acids, lipids to fatty acids. Hydrolysis can be accomplished by exoenzymes, such as those produced by Gram positive bacteria. The monomers are transported into the cell for further processing. Lipids are catabolized to acetyl-CoA, which combines with oxaloacetate and enters the TCA cycle. Lipids are catabolized in 2 carbon units in a process called ß-oxidation, referring to the carbon ß to the acyl group of the fatty acid chain. V. Electron Transport Chain (ETC) NAD: Nicotinamide adenine dinucleotide is a universal electron acceptor in all forms of life. It represents the starting point of the electron transport system in organotrophic aerobic and anaerobic respiratory organisms. Its redox potential is more positive than that for H o2 formate. NADH is used for metabolism, whereas NADPH is used in biosynthetic reactions. Flavoproteins: The next universal carrier is FAD or FMN. These molecules accept both protons and electrons. They pass the protons through to the other side of the cytoplasmic membrane and pass the electrons inside the membrane to the next carrier; usually an iron-sulfur protein. Quinones. Soluble, hydrophobic electron carriers. Accepts
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