Chapter 8 Microbial Metabolism:energy Production

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Energy production

Back 1

Nutrient molecules have energy associated with the electrons that form bonds between atoms
Various catabolic reactions concentrate this energy into the bonds of ATP
ATP has "high energy" bonds which allows the energy to be released quickly and easily

Front 2

General aspects of energy production (Figure 8.14)

Back 2

Oxidation-reduction (redox) reactions
Mechanisms of generation of ATP

Front 3

Oxidation-reduction reactions (Insight 2.2)

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Oxidation is the removal of electrons from a molecule
Reduction is the gaining of electrons by a molecule
Oxidation and reduction reactions are always coupled (redox reaction)

Front 4

Dehydrogenation reactions

Back 4

Many cellular oxidation reactions involve the removal of hydrogen atoms [i.e., not just an electron but also a proton (H+)]
Biological oxidation reactions are often dehydrogenation reactions

Front 5

Electron carriers (Figure 8.13)

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In catabolic reactions, energy is extracted from molecules in the form of electrons, which are transferred, along with H+ ions, to electron carriers like NAD. This energy is used to generate ATP

Front 6

ATP generation

Back 6

Cells use redox reactions in catabolism to extract energy from nutrient molecules
This energy is trapped by the generation of ATP by phosphorylation of ADP

Front 7

Mechanisms of ATP generation

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Substrate-level phosphorylation
Oxidative phosphorylation
Photophosphorylation

Front 8

Substrate-level phosphoryation (Figure 8.16)

Back 8

ATP is generated when a high-energy phosphate is transferred directly to ADP from a phosphorylated substrate

Front 9

Oxidative phosphorylation

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Electrons are transferred from organic compounds through a series of electron carriers to O2 or other oxidized inorganic or organic molecules
The sequence of electron carriers is called the electron transport chain
The transfer of electrons from one carrier to the next generates energy which is used to make ATP from ADP by chemiosmosis

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Photophosphorylation

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Only occurs in photosynthetic cells which contain light trapping pigment such as chlorophyll
Light causes chlorophyll to give up electrons
Energy released from the transfer of electrons (oxidation) of chlorophyll through a system of carrier molecules is used to generate ATP

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How do microbes generate energy?

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Metabolic pathways-sequence of enzymatically catalyzed reactions occurring in the cell
6 main nutrients-carbohydrate, protein, fat, vitamins, minerals and water
Most microorganisms oxidize carbohydrates as the major source of cellular energy
Energy can also be derived from the breakdown of fats and proteins

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Carbohydrate catabolism

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Microbes use two general processes to generate energy from glucose
Cellular respiration
Fermentation
Both start with glycolysis

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Glycolysis (Embden-Meyerhof-Parnas)

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Glucose is oxidized to pyruvic acid with ATP and energy-containing NADH produced Pyruvic acid is converted to acetyl CoA with NADH produced

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TCA cycle (Kreb's cycle)

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Acetyl CoA is oxidized to CO2 with ATP, NADH and FADH2 produced

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Electron transport chain

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NADH and FADH2 are oxidized through a series of redox reactions and a considerable amount of ATP is produced

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Glycolysis: preparatory stage

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2 ATPs are used
Glucose is split to form 2 molecules of Glyceraldehyde-3-phosphate

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Glycolysis: energy conserving stage (Figure 8.18)

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For each initial glucose molecule; 2 Glyceraldehyde-3-phosphate oxidized to 2 Pyruvic acid, 4 ATP produced, 2 NADH produced

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Summary of glycolysis

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Glucose (C6H12O6) is split and oxidized through a ten step pathway to two molecules of pyruvic acid (C3H4O3)
Net gain of 2 ATP molecules, 4 from energy conserving phase (by substrate level phosphorylation) minus 2 from preparatory phase
2 NADH molecules produced Pyruvic acid can now undergo either fermentation or respiration

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Alternatives to glycolysis

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Many bacteria have an alternative pathway to glycolysis for the oxidation of glucose
Phosphogluconate pathway
Entner-doudoroff reaction

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Phosphogluconate pathway (Hexose monophosphate shunt)

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Used simultaneously with glycolysis (several intermediates can reenter glycolysis)
Breakdown of 5-carbon sugars (pentoses) as well as glucose
Produces important intermediates in the synthesis of Nucleic acids (Ribose-5-phosphate)
Glucose from CO2 in photosynthesis (Ribulose 5-phosphate)
Certain amino acids (erythrose 4-phosphate)
Produces NADPH2 (for anabolic reactions) from NADP+
Yields only 1 ATP/glucose molecule
Acetobacter sp., Bacillus subtilis, and Cyanobacteria

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Entner-Doudoroff pathway

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Bacteria that have the enzymes for this pathway can metabolize glucose without glycolysis or the pentose phosphate pathway
Produces 2 NADPH2 and 1 ATP/glucose molecule
Used by obligate aerobes such as Pseudomonas sp. (Lack the phosphofructokinase glycolytic enzyme)
The ability to oxidize glucose by this pathway form the basis of tests sometimes used to identify Pseudomonas in clinical laboratories